Core 2 CBLM (New)

USAT COLLEGE SAGAY CITY, INC.
Sagay City, Negros Occidental
COMPETENCY BASED
LEARNING MATERIALS (C B L M)
SECTOR: ELECTRONICS
QUALIFICATION TITLE: ELECTRONIC PRODUCTS ASSEMBLY AND

SERVICING NCII
UNIT OF COMPETENCY: Service consumer electronic products

and systems
MODULE TITLE: Servicing consumer electronic products

and systems
CBLM on
Electronic Date Develop:
Products Assembly Module:
June 25 2020
and Servicing NCII Servicing consumer electronic
products and systems
Plan Training Session Developed by:
Page |1
Joel S. Milan
ACKNOWLEDGEMENT
The competency based learning material Provide ELECTRONIC PRODUCTS ASSEMBLY

AND SERVICING core competency of ELECTRONIC PRODUCTS ASSEMBLY AND SERVICING NC II
preparation would not be possible without the help and support of the following people:
TESDA Provincial Director, Supervisors and Staff for the scholarship grant for the
trainers’ training in service to the community.
USAT School President, Vice President for Academic Affairs, Vice President for
Personnel
and Academic Affairs, Vice President for Finance and Administrators for the unwavering
support and inspiring the EPAS Administrators and faculty to excel in their field
of specialization.
La Salle Tech TM1 Trainers, facilitators, and staff for the patience and endurance in
coaching and mentoring the trainees for the completion of all the requirements.
Our beloved family for the encouragement, love and care for believing in us that
we can make it.
And finally to almighty God for the endless love, provision, strength and sustaining
and for making the impossible possible.
TO GOD BE THE GLORY!
Joel S. Milan
HOW TO USE THIS COMPETENCY- BASED LEARNING
MATERIALS
Welcome!
The unit of competency, Electronic Products Assembly and Servicing NCII is one of the
Competencies of Electronic Products Assembly and Servicing NC II, a course this
comprises the knowledge, skills and attitudes required for a TVET trainer to possess. The
module, Servicing Consumer Electronic Products and Systems , contains training
materials and activities related to identifying learner’s requirements, preparing session plan,
preparing basic instructional materials and organizing learning and teaching activities for
you to complete. In this module, you are required to go through a series of learning activities
in order to complete each learning outcome. In each learning Outcome are Information
Sheets, Self-Checks, Task Sheets and Job Sheets. Follow and perform the activities on
your own. If you have questions, do not hesitate to ask for assistance from your facilitator.
Remember to:
Read information sheets and complete Self-Checks. Suggested

References are included to supplement the materials provided in this
Module. Perform the Task Sheets and Job Sheets until you are confident that your
outputs conform to the Performance Criteria Checklist that
Follows the sheets. Submit outputs of the Task Sheets and Job Sheets to your facilitator
For evaluation and recording in the Accomplishment Chart. Outputs shall serve as
your portfolio during the Institutional Competency
Evaluation. When you feel confident that you have had sufficient
Practice, asks your trainer to evaluate you. The results of your
Assessment will be recorded in your Progress Chart and
Accomplishment Chart. You must pass the Institutional Competency Evaluation for
this Competency before moving to another competency. A Certificate of Achievement
will be awarded to you after passing the evaluation. You need to complete this module
before you can perform the module On Facilitating Learning Sessions.
Electronic Products Assembly and Servicing NCII
COMPETENCY-BASED LEARNING MATERIALS
List of Competencies
No. Unit of Competency Module Title
Code
1. Assemble Electronic Products Assembling Electronic Products ELC724335
Service consumer electronic Servicing consumer electronic

2. ELC724336
products and systems products and systems
Service industrial electronic Servicing industrial electronic

3. ELC724337
modules, products and systems modules, products and systems
Table of Contents
Cover page……………………………………………………………………………………………………………..…..1
Acknowledgement…………………………………………………………………………..…………………………….2
How to use this competency based learning materials…………………………….…………………………….3
List of competency …………………………………………………………………………...................................4
Table of Contents ………………………………………………………………………….……………………………..5
Module content ……………………………………………………………………………………………………………6
Summary of Learning Outcomes ……………………………………………………………………………………..7
Introduction………………………………………………………………………………………………………………..8
Information Sheet 3.1-1-Work Safety Requirements……………………………………………………………..9
Information Sheet 3.1-2-Application of 5S……………………………………………………………………10-11
Information Sheet 3.1-3-Use PPE and Equipment Clothing…………………………………………………..12
Self-Check 3.1-1…………………………………………………………………………………………………………13
Learning Outcome#2-Install consumer Electronic products and systems…………………………..14-15
Introduction
Information Sheets 3.2.1-Use and function of tool, equipment
And testing instruments…………………………………………………………………………………………16-21
Information sheets 3.2.2-using multimeter…………………………………………………………………22-31
Self-check 3.2.1……………………………………………………………………………………………………….32
Learning outcome#3-Diagnose faults and defects of consumer
electronic products and system……………………………………………………………………………………33
Introduction
Information Sheets 3.3.1-Electronic devices and symbols………………………………………………34-38
Information Sheets 3.3.2-Drawing and Interpreting Schematic Diagrams…………………………..39-41
Self-Checks 3.3.1………………………………………………………………………………………………………42
Learning Outcome#4 Maintain/repair consumer electronic products………………………………..43-45
Introduction
Information Sheets 3.4.1-Principles of Electrical Circuits……………………………………………….46-50
Information Sheets 3.4.2-Electronic Circuits……………………………………………………………….51-56
Information Sheets 3.4.3-Analysis of Troubles……………………………………………………………..57-58
Information Sheets 3.4.4-Microwave Oven Principles of Operation……………………………………59-66
Information sheets 3.4.5-Micro oven parts and Component Placement………………………………67-72
Self-Check 3.4.1………………………………………………………………………………………………………..73
Learning Outcome #5-Re-assemble and test repaired consumer electronics products……………74-75
Introduction
Information Sheets 3.5.1-Troubleshooting Components………………………………………………….76-81
Information Sheets 3.5.2-Microwave oven Troubleshooting……………………………………………...82-97
Job Sheets 3.5.1-Microwave Oven Disassembly…………………………………………………………….98-99
Job Sheets 3.5.2-Mirowave Oven Assembly………………………………………………………………100-101
Job Sheets 3.5.3-Discharge a Capacitor…………………………………………………………………..102-103
Job Sheets 3.5.4-Check turntable rotation……………………………………………………………………..104
Job Sheet 3.5.5-Troubleshooting Microwave Oven problems with circuit diagnosis………………….105
Job sheets 3.5.6-Conduct tests of high voltage components when little or no heat is
produced by an oven but all other operations appear normal………………………………….……106-108
Job sheets 3.5.7-Remove and Install Magnetron………………………………………………………...109-110
Job Sheets 3.5.8-Remove and disassemble a stirrer system………………………………………………...111
Institutional Assessment……………………………………………………………………………………….112-113
UNIT OF COMPETENCY: Service Consumer Electronic Products and Systems
UNIT MODULE: Servicing Consumer Electronic Products and Systems

MODULE DESCRIPTOR: This unit covers the knowledge, skills and attitudes required to
assemble electronic products and systems for customer/industrial uses/applications.
It consist of competencies in identifying and preparing electron is components and circuits,
preparing making printed circuit board (PCB) modules, mounting and soldering of components,
assembling electronic products and performing mechanical and electrical/electronic tests.
NOMINAL DURATION : 80 hours
QUALIFICATION LEVEL: NC II
INTRODUCTION:
This module contains information and suggested learning activities on Electronic

Products Assembly and Servicing NCII. It includes activities and materials on
Servicing Consumer Electronic Products and Systems.
Completion of this module will help you better understand the succeeding module on Service
consumer electronic products and systems.
This module consists of 5 learning outcomes. Each learning outcome contains

learning activities supported by each instruction sheets. Before you perform
the instructions, read the information sheets and answer the self-check and
activities provided to ascertain to yourself and your trainer that you have
acquired the knowledge necessary to perform the skill portion of the particular
learning outcome.
Upon completion of this module, report to your trainer for assessment to

check your achievement of knowledge and skills requirement of this module. If you
pass the assessment, you will be given a certificate of completion.
LO3. Diagnose faults and defects of consumer

electronic products and systems
SUMMARY OF LEARNING LO4. Maintain and repair of consumer electronic

OUTCOMES: products
Upon completion of this module, the
trainee/student must be able to: LO5. Re-assemble and test repaired consumer
electronic product
LO1. Prepare unit tools and

workplace for installation and
service
LO2. Install consumer electronic

products and systems
Qualification
Unit of Competency
Module Title
Learning Outcome # 1
Service Consumer Electronic Products and Systems
Electronic Products Assembly and Servicing Servicing Consumer Electronic Products and Systems
NC – II
Prepare unit, tools and workplace for installation and service
Assessment Criteria:
1. Electrical safety precautions are identified, enumerated and explained correctly

2. OHS in maintaining and repairing electronically controlled domestic appliances are defined
and explained
3. OHS in maintaining and repairing electronically controlled domestic appliances are
demonstrated in accordance to the set procedures/instructions
Resources:
 Learning elements and manuals

 Working area/bench
 PPE
LEARNING EXPERIENCES
Learning Outcome #1: Prepare unit, tools and workplace for installation and service
Learning Activities Special Instructions
1. Work Safety Requirements  Read Information Sheets 3.1.1

 Read Electrical Safety - Safety and Health
for Electrical Trades Student Manual
2. Application of 5S
 Read Information Sheets 3.1.2
 View “5S Application” CD
3. Use of Personal Protective Equipment and
 Read Information Sheets 3.1.3
Clothing  View “PPE” CD
 Answer Self Check 3.1.1
WORK SAFETY REQUIREMENTS
Objectives:
Upon completion of these information sheets, you will be able to:
Know basic Electrical Safety.
Electrical Safety
 Never use electrical tools on damp ground or around water

 Never place an antenna near power lines
 Keep a safe distance from pad mounted transformers
 Never insert anything (especially metal) into an electrical appliance (such as a toaster)
 Do not overload electrical outlets with too many electrical plugs. Buy one surge protector
with many outlets instead of ‘daisy-chaining’ smaller power splitters
 Wear rubber gloves and rubber boots when working near electrical components.
 Inspect tools and appliances for wear and damage prior to use
 Use electrical tape for power cord management, do not use staples
 Always use the correct size fuse, never use a fuse with a larger amperage allowance than
the original
 When working near power lines, use ladders made of wood instead of metal
 If you have a bad feeling about some work concerning electricity, stay away!
 Know where breakers and electrical boxes are in case of an emergency
 Label circuit breakers clearly
 Do not use electrical outlets or cords with exposed wiring
 Do not touch a person or electrical apparatus in the event of an electrical accident. Always
disconnect the current first.
 Do not clean tools with flammable or toxic solvents.
APPLICATIONS OF 5’S
Objectives:
Apply 5S
Phases of 5S
There are 5 primary phases of 5S: sorting, straightening, systematic cleaning, standardizing, and
sustaining. Additionally, there are two other phases sometimes included, safety & security.
Sorting (Seiri)
Eliminate all unnecessary tools, parts, and instructions. Go through all tools, materials, and so forth
in the plant and work area. Keep only essential items and eliminate what is not required, prioritizing
things as per requirements and keeping them in approachable places. Everything else is stored or
discarded.
Straightening or setting in order / stabilize (Seiton)
There should be a place for everything and everything should be in its place. The place for each
item should be clearly labeled or demarcated. Items should be arranged in a manner that promotes
efficient work flow. Workers should not have to bend repetitively to access materials. Each tool, part,
supply, or piece of equipment should be kept close to where it will be used – in other words,
straightening the flow path. Seiton is one of the features that distinguishes 5S from "standardized
cleanup". This phase can also be referred to as Simplifying.
Sweeping or shining or cleanliness / systematic cleaning (Seiso)
Keep the workplace tidy and organized. At the end of each shift, clean the work area and be sure
everything is restored to its place. This makes it easy to know what goes where and ensures that
everything is where it belongs. A key point is that maintaining cleanliness should be part of the daily
work – not an occasional activity initiated when things get too messy.
Standardizing (Seiketsu)
Work practices should be consistent and standardized. Everyone should know exactly what his or
her responsibilities are for adhering to the first 3 S's.
Sustaining the discipline or self-discipline (Shitsuke)
Maintain and review standards. Once the previous 4 S's have been established, they become the
new way to operate. Maintain focus on this new way and do not allow a gradual decline back to the
old ways. While thinking about the new way, also be thinking about yet better ways. When an issue
arises such as a suggested improvement, a new way of working, a new tool or a new output
requirement, review the first 4 S's and make changes as appropriate.
SAFETY
A sixth phase, "Safety", is sometimes added. While it is reasonable to assume that a properly
planned and executed 5S program will improve workplace safety, some argue that explicitly
including this sixth "S" ensures that workplace safety is given at least a secondary consideration.
Security
A seventh phase, "Security", can also be added. In order to leverage security as an investment
rather than an expense, the seventh "S" identifies and addresses risks to key business categories
including fixed assets (PP&E), material, human capital, brand equity, intellectual property,
information technology, assets-in-transit and the extended supply chain.
It is important to have continuous education about maintaining standards. When there are changes
that affect the 5S program such as new equipment, new products or new work rules, it is essential
to make changes in the standards and provide training. Companies embracing 5S often use posters
and signs as a way of educating employees and maintaining standards.
USE OF PERSONAL PROTECTIVE EQUIPMENT AND CLOTHING
Objectives:
Use Personal Protective Equipment’s (PPE)

YES NO
1. I can enumerate OHS policies and procedures in maintaining and
Personal protective
repairing equipment
electronically (PPE)
controlled refers to
domestic protective clothing, helmets, goggles, or other
appliances
garment designed to protect the wearer's body from injury by blunt impacts, electrical hazards, heat,
chemicals, and infection. The terms "protective gear" and "protective clothing" are in many cases
2. I can explain
interchangeable; OHS policies
"protective and procedures
clothing" is applied tointraditional
maintaining and
categories of clothing, and "gear" is a
morerepairing
generalelectronically controlled
term and preferably domestic
means appliances
uniquely protective categories, such as pads, guards,
shields, masks, etc.
3. I can apply OHS policies and procedures in maintaining and repairing
Goggles and safety glasses are forms of protective eyewear that usually enclose or protect the eye
electronically controlled domestic appliances
area in order to prevent particulates or chemicals from striking the eyes. They are used in chemistry
laboratories and in woodworking. They are often used in snow sports as well, and in swimming.
Goggles are often worn when using power tools such as drills or chainsaws to prevent flying
particles from damaging the eyes. Many types of goggles are available as prescription goggles for
those with vision problems.
Safety googles
Safety Mask
Protective masks have these functions:
 Providing a supply of breathable air or other oxygen-containing gas.
 Protecting the face against flying objects or dangerous environments, while allowing vision.
Self-Check 3.1.1
Instruction:. Tick
the box for your answer. Ask your instructor for evaluation afterwards.
Electronic Products Assembly and Servicing NC – II
Qualification :
Service Consumer Electronic Products and Systems
Unit of Competency :
: Servicing Consumer Electronic Products and Systems

Module Title
Install consumer electronic products/systems of
Learning Outcome # 2 :
Tools and testing instruments for maintaining and repairing
electronically controlled domestic appliances
1. Materials needed for maintenance and repair are identified and prepared and checked
according to the work instructions
2. Tools and equipment types and functions needed for maintenance and repair are identified
and demonstrated according to set standards
3. Testing devices and instruments operations needed for maintenance and repair are
explained and demonstrated according to instruction manual
4. Personal protective equipment are used in accordance with the Occupational Health and
Safety guidelines and policies
Resources:
 knife/stripper
TOOLS Wrenche  Pliers
 Long-nosed pliers s assorted, long
 Diagonal cutters assorted  nose, side
 Standard screwdrivers  Allen cutter
 Soldering iron, 30w wrench/k  Test jig
 De-soldering iron, 30w ey
 Screw driver assorted,  Utility
Phillips, slotted
FACILITIES
 Sufficient lighting and
 ventilation system
Learning materials
 Books and references
 Technical manuals
 Documentation forms
 Report forms
 Self-paced learning
elements
MATERIALS
 Solder lead
 Cleaning brush
 Lead free solder
 Resin core solder
 Wire stranded, #22,
(different colors)
 Silicon grease
 Resistors (different
values)
 Capacitors (different
values)
 Transform
Learning Outcome #2: Install Consumer Electronic Products Systems
1. Use and function of tools, equipment  Read Information Sheets 3.2.1
and  Read Information Sheets 3.2.2

testing instruments  View “Using Multimeter” CD
2. Using the Multimeter  Answer Self Check 3.2.1
USE AND FUNCTION OF TOOLS, EQUIPMENT AND TESTING INSTRUMENTS
Objective(s):
Know use and functions of tools, equipment and testing instruments
TOOLS
Screwdrivers
A screwdriver amplifies the hand's turning motion to the small, specially designed tip that is
inserted into the screw's head. There are numerous tip designs for special applications (such as
appliance assembly), but the two most common are standard (also called flat, flared, or straight) and
Phillips (X-shaped) tips. The screwdriver handle can be of plastic or wood, sometimes with a
rubberized cover to improve grip. Other tips include clutch (hourglass shape), Robertson (square),
and Torx (six slots). Some fasteners can be turned either by standard or Phillips tips.
Screwdrivers come in several designs, but the standard model

with a flat head is still among the most common.
Common screw driver tips

(a) Slotted, (b) Phillips, (c) Pozidriv, (d) Torx, (e) Hex,
(f) Robertson, (g) Tri-Wing, (h) Torq-Set, (i) Spanner
To ensure that the screwdriver is at good condition is to check if there is deformation on the
tip of the screwdriver and the shaft should be straight with good grip on the handle.
Wrenches
The purpose of a wrench is to turn a bolt head or nut. Selecting the appropriate wrench
depends on the fastener's design and size. It can also depend on how difficult the fastener is to
reach. Wrench types include open end, combination, adjustable, and Allen.
An Adjustable End wrench
Here are some of the different types of wrenches from which you can choose:
Box end. A box, or closed, end wrench is used where there is room to place the wrench mouth
around the fastener. Box end wrenches are available in 6- and 12-point versions to match the
number of sides on the fastener. Hexagon fasteners have 6 sides, or points, and are the most
popular.
Open end. This type of wrench is used for turning fasteners in locations where a box end wrench
cannot encompass the fastener.
Combination. A combination wrench has ends that perform specific tasks. One end may be open
and the other closed, one may be offset and the other straight, or the two ends might be of
fractionally different sizes.
Adjustable. An adjustable wrench can be used on a variety of fastener sizes. The disadvantage is
that it is less stable than a fixed-size wrench and can easily injure you or damage the fastener. An
adjustable wrench should be used only if the correct size wrench is not available.
Socket. Socket wrenches fit over the fastener, making removal easier and safer than with other
wrenches. Sockets come in standard and extended depth; extensions are available to make
removing fasteners easier. They are often purchased in sets by drive size.
Allen. Called by the Allen brand name, these are used on fasteners with a hexagonal hole in the
head. Allen wrenches are available with L- or T-shape handles.
Well-conditioned wrenches have a perfect jaw to fit on the bolt head or nut. Provides a good grip
when use.
Pliers
The primary purpose of the tool known as pliers is to grip objects firmly. The objects can
then be turned, bent, or otherwise manipulated. Pliers have parallel handles, a pivot where the
handles join, and parallel jaws that grasp the object. Special-use pliers may have additional
components and purposes, such as cutting pliers. Types of pliers include engineer's pliers for
gripping metal, flat-nosed pliers for grasping smaller objects, electrician's pliers for gripping
electrical wires, and round-nosed pliers for bending wire into loops. The most common are slip-joint
and plumber's pliers, both with slip-joint adjustments to change the width of the jaw grip. In addition,
locking pliers, sometimes known by the Vice-Grip brand name, are popular for firmly holding
objects.
Slip-joint pliers allow you to grip objects of varying sizes
Lineman's pliers
Needle-nose plier
Diagonal pliers
A plier in good condition has no deformations on its jaw and can provide good grip while handling
objects.
Soldering Iron/Soldering Gun
A soldering iron / soldering gun is a device for applying heat to melt solder for soldering
two metal parts together.
A soldering iron is composed of a heated metal tip and an insulated handle. Heating is often
achieved electrically, by passing a current, supplied through an electrical cord or a battery, through
a heating element.
For electrical work, wires are usually soldered to printed circuit boards, other wires, or small
terminals. Using a soldering iron
Soldering tools Soldering Gun
Desoldering tools
In electronics,desoldering is the removal of solder and components from a circuit for
troubleshooting and repair purposes. Electronic components are often mounted on a circuit board
and it is usually desirable to avoid damaging the circuit board, surrounding components, and the
component being removed.
Desolderi
ng Pump
Desoldering Braid
EQUIPMENT AND TESTING INSTRUMENTS
Multimeter or Multitester (VOM)
There are many types of and varieties of test and measuring equipment’s that are now used
in audio and video systems repair and diagnosis.
Most instruments and equipment’s are used to measure DC voltage, resistance, AC voltage
and DC current values. The most basic instrument that is still commonly used is the VOM or
multimeter.
However there are types of instruments under the category of multimeters that are being
used today because of their special functions and high accuracy such as the DVOM (digital VOM).
USE AND FUNCTION TOOLS OF TOOLS, EQUIPMENT AND TESTING INSTRUMENTS
Objective(s):
Operate testing instruments properly
How to use a Multimeter
- The basics or instructions of how to use a multimeter, including how to use an analog or analogue
multimeter, or a digital multimeter, DMM, and using multimeters their best advantage.
Multimeters are very cheap to buy and are one of the most commonly used pieces of electronics
test equipment. Although basic operational multimeter instruction may be given when the test meter
is bought, details of how to use the multimeter to test circuits and gain the maximum use from them
are not always available.
Although there are major differences between the internal circuits within analogue and digital
multimeters, the way in which they are used is comparatively similar. However separate sections
are given below with instructions on how to use a digital multimeter and how to use an analogue
multimeter.
How to use a digital multimeter
The operation of a DMM, digital multimeter, itself is normally very straightforward. With a knowledge
of how to make voltage, current and resistance measurements (see the "Related Articles" on the left
hand side of this page for further details) it is then a matter of putting the multimeter to use. If the
meter is new then it will obviously be necessary to install a battery to power it. This is normally
simple and straightforward and details can be found in the operating instructions for the DMM.
... apart from amps, volts, and ohms, many

DMMs can measure parameters including
frequency, capacitance, continuity, and
temperature....
When using the meter it is possible to follow a number of simple steps:
1. Turn the meter on
2. Insert the probes into the correct connections - this is required because there may be a number
of different connections that can be used.
3. Set switch to the correct measurement type and range for the measurement to be made. When
selecting the range, ensure that the maximum range is above that anticipated. The range on
the DMM can then be reduced as necessary. However by selecting a range that is too high, it
prevents the meter being overloaded.
4. Optimize the range for the best reading. If possible enable all the leading digits to not read zero,
and in this way the greatest number of significant digits can be read.
5. Once the reading is complete, it is a wise precaution to place the probes into the voltage
measurement sockets and turn the range to maximum voltage. In this way if the meter is
accidentally connected without thought for the range used, there is little chance of damage to
the meter. This may not be true if it left set for a current reading, and the meter is accidentally
connected across a high voltage point!
How to use an analogue multimeter
The operation of an analogue multimeter is quite easy. With a knowledge of how to make voltage,
current and resistance measurements (see the "Related Articles" on the left hand side of this page
for further details) it is only necessary to know how to use the multimeter itself. If the meter is new
then it will obviously be necessary to install any battery or batteries needed for the resistance
measurements.
... analogue multimeters have been available for

many years and they are very flexible in their
operation....
of different connections that can be used. Be sure to get the right connections, and not put
them into the ones for a low current measurement if a high voltage measurement is to be made
- this could damage the multimeter.
2. Set switch to the correct measurement type and range for the measurement to be made. When
selecting the range, ensure that the maximum for the particular range chosen is above that
anticipated. The range on the multimeter can be reduced later if necessary. However by
selecting a range that is too high, it prevents the meter being overloaded and any possible
damage to the movement of the meter itself.
3. Optimize the range for the best reading. If possible adjust it so that the maximum deflection of
the meter can be gained. In this way the most accurate reading will be gained.
measurement sockets and turn the range to maximum voltage position. In this way if the meter
is accidentally connected without thought for the range to be used, there is little chance of
damage to the meter. This may not be true if it left set for a current reading, and the meter is
accidentally connected across a high voltage point!
Measuring voltage with a multimeter

- an overview or tutorial about how to measure voltage with a digital multimeter (DMM) or an
analogue multimeter.
One the important measurements that it is possible to make with a multimeter (either and analog /
analogue multimeter) or a digital multimeter is that of voltage. Voltage measurements look at the
potential difference between two points. In other words they look at the difference in electric
pressure at the two points. In most cases the voltage is measured between a particular point and
the ground or zero volt line on a circuit. However this does not mean that the voltage cannot be
measured between any two points.
When making a voltage measurement with a multimeter, the first step is to switch the multimeter to
the voltage ranges. It is best to select a range higher than the expected voltage so that there is no
chance of the meter being overloaded and damaged. In addition to this check that the test leads are
plugged into the correct sockets. Many multimeters have different sockets for different types of
measurement so it is worth checking the correct ones have been chosen before making the
measurement. Usually a meter will be provided with two leads, one black, and the other red. The
black one is normally taken as the negative one. It is connected to the negative or "common" socket
on the meter. The red one is connected to the positive socket.
When making the measurement, the positive lead should be connected to terminal which is
expected to have the more positive voltage. If the leads are connected the wrong way round a
negative voltage will be displayed. This is acceptable for a digital multimeter (DMM) because it will
just display a negative sign. However for an analogue multimeter, the meter needle will move
backwards and hit a stop. If at all possible it is best not to allow this to happen.
With the multimeter connected, power can be applied to the circuit. The multimeter switches can
then be changed to reduce the value of the range. This is done until the largest deflection is seen on
the meter without it going over the top of the range. In this way the most accurate reading is
obtained.
How to measure current

- an overview or tutorial about how to measure electrical current with a digital multimeter (DMM) or
an analogue multimeter. This includes how to direct electrical current and how to measure ac
current with a multimeter.
It is often necessary to know how to measure current using a multimeter. Current measurements
are easy to make, but they are done in a slightly different way to the way in which voltage and other
measurements are made. However current measurements often need to be made to find out
whether a circuit is operating correctly, or to discover other facts associated with its current
consumption.
Current measurements can be made with a variety of test instruments, but the most widely used
pieces of test equipment for making current measurements is a digital multimeter. These items of
test equipment are widely available and at very reasonable prices.
Basics of current measurement
Current measurements are made in a different way to voltage and other measurements. Current
consists of a flow of electrons around a circuit, and it is necessary to be able to monitor the overall
flow of electrons. In very simple circuit is shown below. In this there is a battery, a bulb which can be
used as an indicator and a resistor. To change the level of current flowing in the circuit it is possible
to change the resistance, and the amount of current flowing can be gauged by the brightness of the
bulb.
A simple circuit in which to measure current
When using a multimeter to measure current, the only way that can be used to detect the level of
current flowing is to break into the circuit so that the current passes through the meter. Although this
can be difficult at times, it is the best option. A typical current measurement can be made as shown
below. From this it can be seen that the circuit in which the current is flowing has to be broken and
the multimeter inserted into the circuit. In some circuits where current may often need to be
measured, terminals with a shorting link may be added to facilitate the current measurement.
How to measure current using a multimeter
In order that the multimeter does not alter the operation of the circuit when it is used to measure
current, the resistance of the meter must be as low as possible. For measurements of around an
amp, the resistance of a meter should be much less than an ohm. For example if a meter had a
resistance of one ohm, and a current of one amp was flowing, then it would develop a voltage of
one volt across it. For most measurements this would be unacceptably high. Therefore resistances
of meters used to measure current are normally very low.
How to measure current with an analogue multimeter
It is quite easy to use an analogue meter to measure electrical current. There are a few minor
differences in way that current measurements are made, but the same basic principles are used.
of different connections that can be used. Be sure to get the right connections as there may be
separate connections for very low or very high current ranges.
2. Set switch to the correct measurement type (i.e. to measure current) and range for the
measurement to be made. When selecting the range, ensure that the maximum for the particular
range chosen is above that anticipated. The range on the multimeter can be reduced later if
necessary. However by selecting a range that is too high, it prevents the meter being overloaded
and any possible damage to the movement of the meter itself.
3. When taking the reading, optimize the range for the best reading. If possible adjust it so that the
maximum deflection of the meter can be gained. In this way the most accurate reading will be
gained.
measurement sockets and turn the range to maximum voltage position. In this way if the meter is
accidentally connected without thought for the range to be used, there is little chance of damage to
the meter. This may not be true if it left set for a current reading, and the meter is accidentally
How to measure current with a digital multimeter
To measure current with a digital multimeter it is possible to follow a few simple steps:
1. Turn the meter on
2. Insert the probes into the correct connections - in many meters there are a number of different
connections for the probes. Often one labeled common into which the black probe is normally
placed. The other probe should be entered into the correct socket for the current measurement to
be made. Sometimes there is a special connection for current measurements, and sometimes a
separate one for either low or high current measurements. Select the correct one for the current
measurement to be made.
3. Set main selector switch on the meter switch to the correct measurement type, (i.e. current) and
range for the measurement to be made. When selecting the range, ensure that the maximum range
is above the expected reading anticipated. The range on the DMM can then be reduced as
necessary. However by selecting a range that is too high, it prevents the meter being overloaded.
4. When the measuring the current, optimize the range for the best reading. If possible enable all
the leading digits to not read zero, and in this way the greatest number of significant digits can be
read.
measurement sockets and turn the range to maximum voltage. In this way if the meter is
accidentally connected without thought for the range used, there is little chance of damage to the
meter. This may not be true if it left set for a current reading, and the meter is accidentally
Following these steps it is very easy to measure current using any digital
multimeter.
How to measure resistance with a multimeter

- an overview or tutorial about measuring resistance with a digital multimeter (DMM) or an analogue
multimeter.
One important measurement that can be made with a multimeter is a resistance measurement. Not
only can these be made to check the accuracy of a resistor, or check it is functioning correctly, but
resistance measurements can be required in many other scenarios as well. It may be to measure
the resistance of an unknown conductor, or it may be to check for short circuits and open circuits. In
fact there are many instances where measuring resistance is of great interest and importance. In all
these cases a multimeter is an ideal piece of test equipment for measuring resistance
Basics of measuring resistance
When measuring resistance, all multimeters use exactly the same principle whether they are
analogue multimeters or digital multimeters. In fact other forms of test equipment that measure
resistance also use the same basic principle.
The basic idea is that the multimeter places a voltage at the two probes and this will cause a current
to flow in the item for which the resistance is being measured. By measuring the resistance it is
possible to determine the resistance between the two probes of the multimeter, or other item of test
equipment.
How to measure resistance with an analogue multimeter
Analogue multimeters are good at measuring resistance, although they are a few points to note
about the way in which it is done. The first point to note is that as the meter itself responds to
current flowing through the component under test, a high resistance which corresponds to a low
current appears on the left hand side of the dial, and a low resistance which corresponds to a higher
current appears on the right hand side of the dial as shown below. It will also be noticed that the
calibrations become much closer together as the resistance becomes higher, i.e. on the left hand
side of the dial.
Another aspect of using an analogue multimeter for measuring resistance is that the meter needs to
be "zero'ed" before making a measurement. This is done by connecting the two probes together so
that there is a short circuit, and then using the "zero" control to give full scale deflection on the
meter, i.e. zero ohms. Each time the range is changed, the meter needs to be zero'ed as the
position may change from one range to the next. The meter needs to be zero'ed because the full
scale deflection will change according to aspects such as the state of the battery.
There are a few simple steps required to make a resistance measurement with an
analogue
multimeter:
1. Select the item to be measured: This may be anything where the resistance needs to be
measured and estimate what the resistance may be.
2. Insert the probes into the required sockets Often a multimeter will have several sockets for the
test probes. Insert these or check they are already in the correct sockets. Typically these might be
labeled COM for common and the other where the ohms sign is visible. This is normally combined
with the voltage measurement socket.
3. Select the required range The analogue multimeter needs on and the required range selected.
The range selected should be such that the best reading can be obtained. Normally the multimeter
function switch will be labeled with the maximum resistance reading. Choose the one where the
estimated value of resistance will be under but close to the maximum of the range. In this way the
most accurate resistance measurement can be made.
4. Zero the meter: The meter needs to be zeroed. This is done by firmly palcing the two probes
together to give a short circuit and then adjusting the zero control to give a zero ohms (full scale
deflection) reading. This process needs to be repeated if the range is changed.
5. Make the measurement With the multimeter ready to make the measurement the probes can
be applied to the item that needs to be measured. The range can be adjusted if necessary.
6. Turn off the multimeter Once the resistance measurement has been made, it is wise to turn
the function switch to a high voltage range. In this way if the multimeter is used to again for another
type of reading then no damage will be caused if it is inadvertently used without selecting the correct
range and function.
Analogue multimeters are ideal pieces of test equipment for measuring resistance. They are
relatively cheap and they offer a reasonably good level of accuracy and general performance. They
normally provide a level of accuracy that is more than sufficient for most jobs.
How to measure resistance with an digital multimeter, DMM
Measuring resistance with a digital multimeter is easier and faster than making a resistance
measurement with an analogue multimeter as there is no need to zero the meter. As the digital
multimeter gives a direct reading of the resistance measurement, there is also no equivalent of the
reverse reading found on the analogue multimeters.
There are a few simple steps required to make a resistance measurement with a
digital multimeter:
1. Select the item to be measured: This may be anything where the resistance needs to be
measured and estimate what the resistance may be.
2. Insert the probes into the required sockets Often a digital multimeter will have several sockets
for the test probes. Insert these or check they are already in the correct sockets. Typically these
might be labeled COM for common and the other where the ohms sign is visible. This is normally
combined with the voltage measurement socket.
3. Turn on the multimeter
4. Select the required range The digital multimeter needs on and the required range selected.
The range selected should be such that the best reading can be obtained. Normally the multimeter
function switch will be labeled with the maximum resistance reading. Choose the one where the
estimated value of resistance will be under but close to the maximum of the range. In this way the
most accurate resistance measurement can be made.
5. Make the measurement With the multimeter ready to make the measurement the probes can
be applied to the item that needs to be measured. The range can be adjusted if necessary.
6. Turn off the multimeter Once the resistance measurement has been made, the multimeter can
be turned off to preserve the batteries. It is also wise to turn the function switch to a high voltage
range. In this way if the multimeter is used to again for another type of reading then no damage will
be caused if it is inadvertently used without selecting the correct range and function.
Digital multimeters are ideal pieces of test equipment for measuring resistance. They are relatively
cheap and they offer a high level of accuracy and general performance.
General precautions when measuring resistance
As with any measurement, when measuring resistance, there are some precautions to observe. In
this way damage to the multimeter can be prevented, and more accurate measurements can be
made.
* Measure resistance when components are not connected in a circuit: It is always advisable not
to measure the resistance of an item that is in a circuit. It is always best to make the measurement
of the component on its own out of the circuit. If a measurement is made in-circuit, then all the other
components around it will have an effect. Any other paths that will allow current to pass will affect
the readings, making them inaccurate to some degree.
* Remember to ensure the circuit under test is not powered on Under some circumstances it is
necessary to measure resistance values actually on a circuit. When doing this it is very important to
ensure the circuit is not powered on. Not only will any current flowing in the circuit invalidate any
readings, but should the voltage be high enough, the current resulting could damage the multimeter.
YES NO
1. I can identify tools, equipment and testing instruments
* Ensure capacitors in a circuit under test are discharged. Again when measuring resistance
values in a circuit, it is necessary to ensure that any capacitors in the circuit are discharged. Any
current that flows as a result of them will cause the meter reading to be altered. Also any capacitors
in the circuit that are discharged may charge up as a result of the current from the multimeter and as
a2.result
I canitexplain
may take
theauses
shortand
while for the of
functions reading
tools, to settle. and testing
equipment
instruments
* Remember diodes in a circuit will cause different readings in either direction When measuring
resistance in a circuit that includes diodes the value measured will be different if the connections are
reversed. This is because the diodes only conduct in one direction.
* Leakage path through fingers can alter readings in some cases. When making some
resistance measurements it is necessary to hold a resistor or component onto the multimeter test
probes. If high resistance measurements are being made the leakage path through the fingers can
become noticeable. Under some circumstances the resistance path through fingers can be
measured at just a few megohms, and as a result this can become significant. Fortunately the levels
of voltage used in most multimeters when measuring resistance is low, but some specialized meters
may use much higher voltages. It is wise to check.
Self-Check 3.2.1
Instruction. Tick
the box for your answer. Ask your
instructor for evaluation
afterwards.
Qualification : Electronic Products Assembly and Servicing NC – II
Unit of Competency : Service Consumer Electronic Products and Systems
Module Title Servicing Consumer Electronic Products and Systems
Learning Outcome # 3 Diagnose faults and defects of consumer electronic products

and system of symbols and block sections of schematic diagram
1. Electronic symbols are identified and selected according to the schematic diagram
2. Electronic parts value are read and matched correctly as required
3. Schematic diagram sections and functions are identified and explained
Resources:
 Drawing instruments and materials

 Sufficient lighting and ventilation system
Complete electronic supplies
Learning Outcome #3: Diagnose faults and defects of consumer electronic products and
systems
1. Electronic devices and symbols  Read Information Sheets 3.3.1
2. Resistor Color Code  View “Resistor Color Code” CD
3. Drawing and Interpreting Schematic  Read Information Sheets 3.3.2

Diagrams
 Answer Self Check 3.3.1
ELECTRONIC DEVICES AND SYMBOLS
Objective(s):
Identify electronic devices and symbols
The main components used in electronics are of two general types: passive (e.g. resistors and
capacitors) and active (e.g. transistors and integrated circuits). The main difference between active
and passive components is that active ones require to be powered in some way to make them work.
Active components can also be used to amplify signals.
CAPACITOR
COIL (Inductor)
FIXED
CRYSTAL
DIODE
FUSE
VARIABLE
LAMP
INTEGRATED CIRCUIT
LIGHT EMITTING DIODE
LOUDSPEAKER
METER
MICROPHONE
POTENTIOMETER
QUADRAC
RESISTOR
RELAY
SILICON CONTROLLED RECTIFIER (SCR)
THERMISTOR
TRANSFORMER
IF TRANSFORMER
TRANSISTOR
FIELD EFFECT TRANSISTOR
MOSFET
UNIJUNCTION TRANSISTOR (UJT)
ZENER DIODE
PHOTOTRANSISTOR
OPTICALLY COUPLED ISOLATOR

DRAWING AND INTERPRETING SCHEMATIC DIAGRAMS
Objective(s):
Draw and interpret schematic diagram
ELECTRONIC DIAGRAMS
Ideas in electronics are introduced in diagram form – called SCHEMATIC DIAGRAM.
It shows the components used and their interconnections. Each graphic symbol is also
accompanied with a reference designation to distinguish it from other similar symbols.
The reference designation is the letter and number nearest the graphic symbol. For
example, a section of a circuit is as follows:
The reference designations are R1, Q1, C1 and

SPKR. Their values or actual description are
given in the PARTS LIST like:
R1 – 10 K , ±5%, ¼ watt resistor
Q1 – 9013 NPN audio input transistor(TO-92)
C1 - 470 F 16 volts electrolytic
SPKR – 8-OHMS 0.5 Watt 2-inch diameter
loudspeaker
RULES AND CONVENTIONS IN ELECTRONIC DIAGRAMS
Electronic diagrams also follow some rules which are agreed upon by several associations
of electronic engineers.
Among the most common rules are the following:
1. Signal flow in a circuit should be from left

to right of a schematic diagram.
2. Voltage potentials are indicated with the

highest potential placed at the upper
portion of the diagram and the ground
(lowest) potential at the bottom.
3. When interesting lines are to be

connected a small solid circle should be
used.
4. When intersecting lines are not

electrically connected the circuit diagram
is drawn as shown on the left.
The meaning of a symbol does not

change with its position or orientation in a
diagram, its size or line width.
5. Connecting line linking a symbol should

be drawn horizontally or vertically but if
ever a connecting line is drawn at an
angle it implies the same meaning unless
otherwise specified.
6. The standard symbol for a terminal (O)

could be added to any symbol but should
not be considered as part of a symbol.
OTHER COMMON PRACTICES
YES NO
Interrupted Lines
1. I can identify electronic symbols and block sections of schematic
diagram
When a connecting line or group of lines
could notexplain
2. I can be directly continued
operation to its final
of electronic symbols and block sections of
destination, arrows
schematic diagram(brackets) with designation
of the destination could be implemented.
3. I can interpret operation of electronic symbols and block sections of
schematic diagram
Dashed Lines
Dashed lines may be used to indicate an
optionally connected component.
Dashed lines may be used indicate

component content in a single unit.
Dashed lines may be used to indicate

mechanical linkage of two or more components.
Self-Check 3.3.1
Instruction:. Tick
Electronic Products Assembly and Servicing NC – II
Qualification :
Unit of Competency : Service Consumer Electronic Products and Systems
Servicing Consumer Electronic Products and Systems

Module Title :
Learning Outcome # 4 : Maintain/Repair Consumer Electronic Products

1. Personal protective equipment are used in accordance with occupational health and safety
practices
2. Control settings/adjustments are checked in conformity with service-manual specifications
3. System defects/Fault symptoms are diagnosed and identified using appropriate tools and
equipment and in accordance with safety procedures
4. Identified defects and faults are explained to the responsible person in accordance with
enterprise or company policy and procedures
5. Customers are advised/informed regarding the status and serviceability of the unit
6. Results of diagnosis and testing are documented accurately and completely within the
specified time
Resources:
 Pliers
TOOLS assorted, EQUIPMENT
 Long-nosed pliers long Multimeter
 Diagonal cutters  Oscilloscope
 Standard screwdrivers  Function generator
 Soldering iron, 30w  Electronically controlled
Desoldering iron, 30w  unit(s)/appliance(s) and
 Screw driver assorted,  accessories
Phillips, slotted  ESD free work bench with
 Wrenches assorted mirror back-to-back/one
 Allen wrench/key sided
 Utility knife/stripper  High grade magnifying
glass with lamp  wire
MATERIALS  Stranded, #22, different
 Solder lead colors)
 Cleaning brush  Silicon grease
 Lead free solder  Resistors (different
 Resin core solder values)
nose, side cutter values)
 Test jig  Transformer
Learning materials
 Books and References
 Technical Manuals
 Documentation forms
 Report forms
Domestic appliances may include but not limited to the following:

 Washing machines and driers
 Vacuum cleaners and polishers
 Home food processing equipment
 Pressure and rice cooker
Learning Outcome #4: Maintain/Repair Consumer Electronic Products
1. Principles of Electrical Circuits  Read Information Sheets 3.4.1

 View “Ohm’s Law” CD
2. Electronic Circuits  Read Information Sheets 3.4.2
3. Analysis of Troubles  Read Information Sheets 3.4.3

4. Microwave Oven Principles of Operation  Read Information Sheets 3.4.4
5. Microwave Oven Parts and Component  Read Information Sheets 3.4.5
Placement
 Answer Self check 3.4.1
PRINCIPLES OF ELECTRICAL CIRCUITS
Objectives:
Understand Electrical Circuits
OHM’S LAW
Ohm's law states that the current through a conductor between two points is directly proportional to
the potential difference or voltage across the two points, and inversely proportional to the resistance
between them.
The mathematical equation that describes this relationship is:
where I is the current through the resistance in units of amperes, V is the potential difference
measured across the resistance in units of volts, and R is the resistance of the conductor in units of
ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the
current.
Series and parallel circuits
Components of an electrical circuit or electronic circuit can be connected in many different ways.
The two simplest of these are called series and parallel and occur very frequently. Components
connected in series are connected along a single path, so the same current flows through all of the
components. Components connected in parallel are connected so the same voltage is applied to
each component.
A circuit composed solely of components connected in series is known as a series circuit; likewise,
one connected completely in parallel is known as a parallel circuit.
In a series circuit, the current through each of the components is the same, and the voltage across
the components is the sum of the voltages across each component. In a parallel circuit, the voltage
across each of the components is the same, and the total current is the sum of the currents through
each component.
As an example, consider a very simple circuit consisting of four light bulbs and one 6 V battery. If a
wire joins the battery to one bulb, to the next bulb, to the next bulb, to the next bulb, then back to the
battery, in one continuous loop, the bulbs are said to be in series. If each bulb is wired to the battery
in a separate loop, the bulbs are said to be in parallel. If the four light bulbs are connected in series,
the same current flows through all of them, and the voltage drop is 1.5 V across each bulb and that
may not be sufficient to make them glow. If the light bulbs are connected in parallel, the current
flowing through the light bulbs combine to form the current flowing in the battery, while the voltage
drop is 6.0 V across each bulb and they all glow.
In a series circuit, every device must function for the circuit to be complete. One bulb burning out in
a series circuit breaks the circuit. In parallel circuits, each light has its own circuit, so all but one light
could be burned out, and the last one will still function.
Series circuits
Series circuits are sometimes called current-coupled or daisy chain-coupled. The current that flows
in a series circuit will flow through every component in the circuit. Therefore, all of the components
in a series connection carry the same current.
Resistors
Inductors
Capacitors
The working voltage of a series combination of identical capacitors is equal to the sum of voltage
ratings of individual capacitors. This simple relationship only applies if the voltage ratings are equal
as well as the capacitances. However, the division of DC voltage between the capacitors is
dominated by the leakage resistance of the capacitors, rather than their capacitances, and this has
considerable variation. To counter these equalizing resistors may be placed in parallel with each
capacitor which effectively add to the leakage current. The value of resistor chosen (perhaps a few
megohms) is as large as possible, but low enough to ensure that the capacitor leakage current is
insignificant compared to the current through the resistor. At DC, the circuit appears as a chain of
series identical resistors and equal voltage division between the capacitors is ensured. In high-
voltage circuits, the resistors serve an additional function as bleeder resistors.
Switches
Two or more switches in series form a logical AND; the circuit only carries current if all switches are
'on'. See AND gate.
Cells and batteries
A battery is a collection of electrochemical cells. If the cells are connected in series, the voltage of
the battery will be the sum of the cell voltages. For example, a 12 volt car battery contains six 2-volt
cells connected in series.
Parallel circuits
If two or more components are connected in parallel they have the same potential difference
(voltage) across their ends. The potential differences across the components are the same in
magnitude, and they also have identical polarities. The same voltage is applicable to all circuit
components connected in parallel. The total current I is the sum of the currents through the
individual components, in accordance with Kirchhoff’s current law.
Resistors
The current in each individual resistor is found by Ohm's law. Factoring out the voltage gives
To find the total resistance of all components, add the reciprocals of the resistances Ri of each
component and take the reciprocal of the sum. Total resistance will always be less than the value of
the smallest resistance.
For only two resistors, the unreciprocated expression is reasonably simple
This sometimes goes by the mnemonic "product over sum".
For N equal resistors in parallel, the reciprocal sum expression simplifies to:
and therefore to:
To find the current in a component with resistance Ri, use Ohm's law again:
The components divide the current according to their reciprocal resistances, so, in the case of two
resistors,
An old term for devices connected in parallel is multiple, such as a multiple connection for arc
lamps.
Inductors
Inductors follow the same law, in that the total inductance of non-coupled inductors in parallel is
equal to the reciprocal of the sum of the reciprocals of their individual inductances:
Capacitors
Capacitors follow the same law using the reciprocals. The total capacitance of capacitors in parallel
is equal to the sum of their individual capacitances:
The working voltage of a parallel combination of capacitors is always limited by the smallest working
voltage of an individual capacitor.
Switches
Two or more switches in parallel, form a logical OR; the circuit carries current if at least one switch
is 'on'. See OR gate.
Cells and batteries
If the cells of a battery are connected in parallel, the battery voltage will be the same as the cell
voltage but the current supplied by each cell will be a fraction of the total current. For example, if a
battery contains four cells connected in parallel and delivers a current of 1 ampere, the current
supplied by each cell will be 0.25 ampere. Parallel-connected batteries were widely used to power
the valve filaments in portable radios but they are now rare.
DC Circuits
A DC circuit (Direct Current circuit) is an electrical circuit that consists of any combination of
constant voltage sources, constant current sources, and resistors. In this case, the circuit voltages
and currents are constant, i.e., independent of time.
In electronics, it is common to refer to a circuit that is powered by a DC voltage source such as a

battery or the output of a DC power supply as a DC circuit even though what is meant is that the
circuit is DC powered.
AC Circuits
In alternating current (AC) the movement of electric charge periodically reverses direction. In direct
current (DC), the flow of electric charge is only in one direction.
AC is the form in which electric power is delivered to businesses and residences. The usual
waveform of an AC power circuit is a sine wave. In certain applications, different waveforms are
used, such as triangular or square waves. Audio and radio signals carried on electrical wires are
also examples of alternating current. In these applications, an important goal is often the recovery of
information encoded (or modulated) onto the AC signal.
ELECTRONIC CIRCUITS
Objectives:
Understand electronic circuits and components
Electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors,

capacitors, inductors and diodes, connected by conductive wires or traces through which electric
current can flow. The combination of components and wires allows various simple and complex
operations to be performed: signals can be amplified, computations can be performed, and data can
be moved from one place to another. Circuits can be constructed of discrete components connected
by individual pieces of wire, but today it is much more common to create interconnections by
photolithographic techniques on a laminated substrate (a printed circuit board or PCB) and solder
the components to these interconnections to create a finished circuit. In an Integrated Circuit or IC,
the components and interconnections are formed on the same substrate, typically a semiconductor
such as silicon or (less commonly) gallium arsenide.
Breadboards, per boards or strip boards are common for testing new designs. They allow the
designer to make quick changes to the circuit during development.
An electronic circuit can usually be categorized as an analog circuit, a digital circuit or a mixed-
signal circuit (a combination of analog circuits and digital circuits).
A circuit built on a printed circuit board (PCB)

Analog circuits
Analog electronic circuits are those in which current or voltage may vary continuously with time to
correspond to the information being represented. Analog circuitry is constructed from two
fundamental building blocks: series and parallel circuits. In a series circuit, the same current passes
through a series of components. A string of Christmas lights is a good example of a series circuit: if
one goes out, they all do. In a parallel circuit, all the components are connected to the same
voltage, and the current divides between the various components according to their resistance.
A circuit diagram representing an analog circuit, in this case a simple
amplifier.
The basic components of analog circuits are wires, resistors, capacitors, inductors, diodes, and
transistors. (Recently, memristors have been added to the list of available components.) Analog
circuits are very commonly represented in schematic diagrams, in which wires are shown as lines,
and each component has a unique symbol. Analog circuit analysis employs Kirchhoff's circuit laws:
all the currents at a node (a place where wires meet) must add to 0, and the voltage around a
closed loop of wires is 0. Wires are usually treated as ideal zero-voltage interconnections; any
resistance or reactance is captured by explicitly adding a parasitic element, such as a discrete
resistor or inductor. Active components such as transistors are often treated as controlled current or
voltage sources: for example, a field-effect transistor can be modeled as a current source from the
source to the drain, with the current controlled by the gate-source voltage.
When the circuit size is comparable to a wavelength of the relevant signal frequency, a more
sophisticated approach must be used. Wires are treated as transmission lines, with (hopefully)
constant characteristic impedance, and the impedances at the start and end determine transmitted
and reflected waves on the line. Such considerations typically become important for circuit boards at
frequencies above a GHz; integrated circuits are smaller and can be treated as lumped elements for
frequencies less than 10 GHz or so.
An alternative model is to take independent power sources and induction as basic electronic units;
this allows modeling frequency dependent negative resistors, gyrators, negative impedance
converters, and dependent sources as secondary electronic components.
Digital circuits
In digital electronic circuits, electric signals take on discrete values, to represent logical and numeric
values [3]. These values represent the information that is being processed. In the vast majority of
cases, binary encoding is used: one voltage (typically the more positive value) represents a binary
'1' and another voltage (usually a value near the ground potential, 0 V) represents a binary '0'.
Digital circuits make extensive use of transistors, interconnected to create logic gates that provide
the functions of Boolean logic: AND, OR, NOT, and all possible combinations there of. Transistors
interconnected so as to provide positive feedback are used as latches and flip flops, circuits that
have two or more metastable states, and remain in one of these states until changed by an external
input. Digital circuits therefore can provide both logic and memory, enabling them to perform
arbitrary computational functions. (Memory based on flip-flops is known as SRAM (static random
access memory). Memory based on the storage of charge in a capacitor, DRAM (dynamic random
access memory) is also widely used.)
Digital circuits are fundamentally easier to design than analog circuits for the same level of
complexity, because each logic gate regenerates the binary signal, so the designer need not
account for distortion, gain control, offset voltages, and other concerns faced in an analog design.
As a consequence, extremely complex digital circuits, with billions of logic elements integrated on a
single silicon chip, can be fabricated at low cost. Such digital integrated circuits are ubiquitous in
modern electronic devices, such as calculators, mobile phone handsets, and computers.
Digital circuitry is used to create general purpose computing chips, such as microprocessors, and
custom-designed logic circuits, known as Application Specific Integrated Circuits (ASICs). Field
Programmable Gate Arrays (FPGAs), chips with logic circuitry whose configuration can be modified
after fabrication, are also widely used in prototyping and development.
Mixed-signal circuits
Mixed-signal or hybrid circuits contain elements of both analog and digital circuits. Examples include
comparators, timers, PLLs, ADCs (analog-to-digital converters), and DACs (digital-to-analog
converters). Most modern radio and communications circuitry uses mixed signal circuits. For
example, in a receiver, analog circuitry is used to amplify and frequency-convert signals so that they
reach a suitable state to be converted into digital values, after which further signal processing can
be performed in the digital domain.
Logic gates
Type Distinctive Shape Rectangular Boolean algebra Truth Table

Shape between A & B
INPUT OUTPUT
A B A AND
INPUT B
OUTPUT
AND AB
NOT 0 0 0
A NOT A
0 1 0
0 1
1 0 0
1 0
1 1 1
INPUT OUTPUT
INPUT OUTPUT
A B A NAND B
NAND 0 0
A B A OR B
OR A+B 1
00 0 1 0
1
01 1 0 1
1
1 0 1
1 1
1 1 1 0
INPUT OUTPUT
1
A B
NOR 1
0 0
0 1
0
A 1 0
1 NOR
B 0 0
INPUT OUTPUT
A B A XOR B
XOR
0 0 0
0 1 1
1 0 1
1 1 0
INPUT OUTPUT
A B A XNOR B
0 0 1
0 1 0
1 0 0
1 1 1
All other types of Boolean logic gates (i.e., AND, OR, NOT, XOR, XNOR) can be created from a
suitable network of NAND gates. Similarly all gates can be created from a network of NOR gates.
Historically, NAND gates were easier to construct from MOS technology and thus NAND gates
served as the first pillar of Boolean logic in electronic computation.
The 7400 chip, containing four NANDs. The two additional pins supply power (+5 V) and connect
the ground.
ANALYSIS OF TROUBLES
Objectives:
Analyze troubles
THREE MAJOR STEPS IN ELECTRONIC SYSTEM REPAIR
Three major steps are observed in most electronic system repair procedures:
1. Evaluation of trouble symptoms, with preliminary diagnosis of equipment, malfunction.

2. Testing of logical conclusions.
3. Repair of defect and verification of normal operation.
The first step involves careful observation of equipment response to input signals and
equipment reaction to variation of operating and maintenance controls. In turn, this preliminary
evaluation and diagnostic procedure will often lead the troubleshooter to probable or possible
causes of malfunction.
The second step comprises definitive tests of various logical conclusions that have been
established. These tests often include quick checks such as click tests and noise injection, as
detailed subsequently. Modules may be substituted. Systematic signal tracing or signal substitution
tests may be made. Eventually, all but one of the possible causes for malfunctions will be eliminated
– by specialized troubleshooting procedures, if necessary.
Then, the third step is taken: repair of defect(s) and final verification of system operation.
MICROWAVE OVEN PRINCIPLES OF OPERATION
Objectives:
Explain how a microwave oven operates
How a microwave oven works
The operation of a microwave oven is really very simple. It consists of two parts: the controller and
the microwave generator.
A schematic diagram of the microwave generating circuitry and portions of the controller is
usually glued to the inside of the cover.
The controller is what times the cooking by turning the microwave energy on and off. Power
level is determined by the ratio of on time to off time in a 10-30 second cycle.
The microwave generator takes AC line power. steps it up to a high voltage, and applies this
to a special type of vacuum tube called a magnetron - little changed from its invention during World
War II (for Radar).
Controller
The controller usually includes a microcomputer, though very inexpensive units may simply
have a mechanical timer (which ironically, is probably more expensive to manufacture!). The
controller runs the digital clock and cook timer; sets microwave power levels; runs the display; and
in high performance ovens, monitors the moisture or temperature sensors.
Power level in most microwave ovens is set by pulse width control of the microwave
generator usually with a cycle that lasts 10-30 seconds. For example, HIGH will be continuous on,
MEDIUM may be 10 seconds on, 10 seconds off, and LOW may be 5 seconds on, 15 seconds off.
The power ratios are not quite linear as there is a 1 to 3 second warmup period after microwave
power is switched on.
However, some models use finer control, even to the point of a continuous range of power.
These are typically "inverter" models which use a more sophisticated type of power supply than the
simple high voltage transformer, capacitor, rectifier, system described below. However, there have
been some back in the 1970s that did this with a 1 second or so pulse width modulated cycle, fast
enough to have the same effect as continuous control for all practical purposes.
The operating voltages for the controller usually are derived from a stepdown transformer.
The controller activates the microwave generating circuitry using either a relay or triac
Sensors
More sophisticated ovens may include various sensors. Most common are probes for
temperature and moisture. A convection oven will include a temperature sensor above the oven
chamber.
Since these sensors are exposed to the food or its vapors, failures of the sensor probes
themselves are common.
Cooling fans
Since 30 to 50 percent of the power into a microwave oven is dissipated as heat in the
Magnetron, cooling is extremely important. Always inspect the cooling fan/motor for dust and dirt
and lubricate if necessary. A couple of drops of electric motor oil or 3-in-One will go a long way. If
there are any belts, inspect for deterioration and replace if necessary.
An oven that shuts off after a few minutes of operation could have a cooling problem, a
defective overtemperature thermostat, a bad magnetron, or is being operated from very high AC line
voltage increasing power to the oven.
One interesting note: Since 30 to 50 percent of the power goes out the vents in the back as
heat, a microwave oven is really only more efficient than conventional means such as a stovetop or
gas or electric oven for heating small quantities of anything. With a normal oven or stovetop, wasted
energy goes into heating the pot or oven, the air, and so on. However, this is relatively independent
of the quantity of food and may be considered to be a fixed overhead. Therefore, there is a
crossover point beyond which it is more efficient to use conventional heat than high tech
microwaves.
Microwave generator
This is the subsystem that converts AC line power into microwave energy. The majority of
microwave ovens use a brute force approach which consists of 5 parts: high voltage (HV)
transformer running off the AC line, HV rectifier diode, HV capacitor, magnetron, waveguide to oven
chamber. (A few employ solid state inverter in place of the simple HV transformer. These will be
discussed later.)
The most common microwave generator consists of the following:
 High Voltage Transformer. Typically has a secondary of around 2,000 VRMS at 0.5 to 1 amp
- more or less depending on the power rating of the oven. There will also be a low voltage
winding for the Magnetron filament (3.3 V at 10 A is typical).
You cannot miss this as it is the largest and heaviest component visible once the cover is
removed. There will be a pair of quick-connect terminals for the AC input, a pair of leads for
the Magnetron filament. and a single connection for the HV output. The HV return will be
fastened directly to the transformer frame and thus the chassis.
These transformers are designed with as little copper as possible. The primary for 115 VAC
is typically only 120 turns of thick wire - thus about 1 turn per volt input and output (this is
about 1/4th as many turns as in a "normal" power transformer. (It's usually possible to count
the primary turns by examining how it is wound - no disassembly required!) So there would
be about 3 turns for the magnetron filament and 2080 turns for the high voltage winding for
the transformer mentioned above. The reason they can get away with so few turns is that it
operates fully loaded about 90 percent of the time but is still on the hairy edge of core
saturation. The HV components are actually matched to the HV transformer characteristics.
Performance will suffer if the uF value of a replacement HV capacitor is not close to that of
the original.
There is also generally a "magnetic shunt" in the core of the transformer. This provides some
current limiting, possibly to compensate for various magnetron load conditions. However, it's
not enough to provide any reduction in the likelihood of electrocution should you come in
contact with the HV winding!


Rectifier - usually rated 12,000 to 15,000 PRV at around 0.5 amp. Most commonly, this will
be rectangular or cylindrical, about 0.5 inch long with wire leads. Sometimes, it is a box
bolted to the chassis. One end will be electrically connected to the chassis.
Capacitor - 0.65 to 1.2 uF at a working voltage of around 2,000 VAC. Note that this use of
'working voltage' may be deceiving as the actual voltage on the capacitor may exceed this
value during operation. The capacitor is metal cased with quick-connect terminals on top
(one end). Always discharge the capacitor as described below before touching anything
inside once the cover is removed.
Magnetron - the microwave producing tube includes a heated filament cathode, multiple
resonant cavities with a pair of permanent ceramic ring magnets to force the electron beams
into helical orbits, and output antenna. The magnetron is most often box shaped with cooling
fins in its midsection, the filament/HV connections on the bottom section, and the antenna
(hidden by the waveguide) on top. Sometimes, it is cylindrical in shape but this is less
common. The frequency of the microwaves is usually 2.45 GHz.
When salvaging parts from dead microwave ovens, save the HV components (transformer,
capacitor, and diode) as a group (assuming all are still good). Then, if a repair is needed to another
oven it may be better to replace all 3 both because this eliminates uncertainty if more than 1 part
failed or is marginal, and they will have been designed to have the best compatibility.
Magnetron construction and operation
A. Most magnetrons are constructed as a series of cylinders:

1. The outer cylinder is a permanent magnet
2. The middle cylinder is an anode with vanes or ridges in it that create “resonant
cavities”
3. The inner cylinder is a cathode with heating filament in the center
B. As high voltage is supplied to the magnetron from the transformer, the filament heats the
cathode
C. As the cathode is heated, it gives off electrons whose negative charges are attracted to the
anode which operates at a positive potential
D. The magnetic field around the anode repels the electrons so that they travel in a circular
path to reach the anode instead of traveling in a straight line as electrons normally would
E. As the electrons moved in a rapid orbit from cathode to anode, they travel past the resonant
cavities in the anode and cause the cavities to vibrate at high frequency
F. The pulsating 2450 MHz frequency generated by the magnetron is picked up by the antenna
and directed into a waveguide and on to a stirrer which distributes the microwave into the
oven cavity
Microwave generator circuit diagram
Nearly all microwave ovens use basically the same design for the microwave generator. This
has resulted in a relatively simple system manufactured at low cost.
The typical circuit is shown below. This is the sort of diagram you are likely to find pasted
inside the metal cover. Only the power circuits are likely included (not the controller unless it is a
simple motor driven timer) but since most problems will be in the microwave generator, this
schematic may be all you need.
Note the unusual circuit configuration - the magnetron is across the diode, not the capacitor
as in a 'normal' power supply. What this means is that the peak voltage across the magnetron is the
transformer secondary + the voltage across the capacitor, so the peaks will approach the peak-peak
value of the transformer or nearly 5000 V in the example above. This is a half wave voltage doubler.
The output waveform looks like a sinusoid with a p-p voltage equal to the p-p voltage of the
transformer secondary with its positive peaks at chassis ground (no load). The peaks are negative
with respect to the chassis. The negative peaks will get squashed somewhat under load. Take
extreme care - up to 5000 V at AMPs available! WARNING: Never attempt to view this waveform on
an oscilloscope unless you have a commercial high voltage probe and know how to use it safely!
The easiest way to analyze the half wave doubler operation is with the magnetron
(temporarily) removed from the circuit. Then, it becomes a simple half wave rectifier/filter so far as
the voltage acrtoss the capacitor is concerned - which will be approximately V(peak) = V(RMS) *
1.414 where V(RMS) is the output of the high voltage transformer. The voltage across the HV
rectifier will then be: V(peak) + V where V is the waveform out of the transformer. The magnetron
load, being across the HV diode, reduces the peak value of this somewhat - where most of its
conduction takes place.
Note that there is a difference in the labels on the filament connections of the magnetron.
Functionally, it probably doesn't matter which way they are connected. However, the typical
schematic (as above) shows FA going to the node attached to the Anode of the HV diode, while F
goes to the lone Filament terminal on the HV transformer.
WARNING: What this implies is that if the magnetron is not present or is not drawing power
for some reason - like an open filament - up to V(peak) will still be present across the capacitor
when power is removed. At the end of normal operation, some of this will likely be discharged
immediately but will not likely go below about 2,000 V due to the load since the magnetron does not
conduct at low voltages.
Other types of power supplies have been used in a few models - including high frequency
inverters - but it is hard to beat the simplicity, low cost, and reliability of the half wave doubler
configuration.
There is also usually a bleeder resistor as part of the capacitor, not shown. HOWEVER: DO
NOT ASSUME THAT THIS IS SUFFICIENT TO DISCHARGE THE CAPACITOR - ALWAYS DO
THIS IF YOU NEED TO TOUCH ANYTHING IN THE MICROWAVE GENERATOR AFTER THE
OVEN HAS BEEN POWERED. The bleeder may be defective and open as this does not effect
operation of oven and/or the time constant may be long - minutes. Some ovens may not have a
bleeder at all.
In addition, there will likely be an over-temperature thermostat - thermal protector -
somewhere in the primary circuit, often bolted to the magnetron case. There may also be a thermal
fuse or other protector physically elsewhere but in series with the primary to the high voltage
transformer.
Other parts of the switched primary circuit include the oven interlock switches, cooling fan,
turntable motor (if any), oven light, etc.
Interlock switches
Various door interlock switches prevent inadvertent generation of microwaves unless the
door is closed completely. At least one of these will be directly in series with the transformer primary
so that a short in the relay or triac cannot accidentally turn on the microwaves with the door open.
The interlocks must be activated in the correct sequence when the door is closed or opened.
Interestingly, another interlock is set up to directly short the power line if it is activated in an
incorrect sequence. The interlocks are designed so that if the door is correctly aligned, they will
sequence correctly. Otherwise, a short will be put across the power line causing the fuse to blow
forcing the oven to be serviced. This makes it more difficult for an ignorant consumer to just bypass
the door interlocks should they fail or to run the oven with an open door as a room heater - and
protects the manufacturer from lawsuits. (That interlock may be known as a "dummy switch" for
obvious reasons and is often not even mentioned in the schematic/parts manifest.) Of course,
should that switch ever actually be used, not only will the fuse blow, but the switch contacts will
likely be damaged by the high initial current! This also means it probably wouldn't be a bad idea to
replace the interlock switch which might have been affected if your oven fails with a blown fuse due
to a door problem.
Failed door interlocks account for the majority of microwave oven problems - perhaps as
high as 75 percent. This is not surprising considering that two of the three switches carry the full
oven current - any deterioration of the contacts results in increased resistance leading to their
heating and further deterioration. And, opening the door to interrupt a cook cycle results in arcing at
the contacts. Complete meltdowns are not unusual! If any defective door switches are found, it is
probably a good idea to replace all of them as long as the oven is already apart.
The typical door switches and their function:
 Door Sensing: Input to the microcontroller to indicate the state of the door.
 Interlock Monitor: Shorts out the AC line (and blows the main fuse) should the Primary
Interlock not open due to incorrect sequencing of the door switches or a failed switch.

Primary Interlock: In series with the high voltage (magnetron) power supply so cuts power
when the door is open.
Note that if the Door Sensing switch should malfunction, peculiar behavior may occur (like the fan or
turntable operating at the wrong time) but should never result in microwaves being generated with
the door open.
MIROWAVE OVEN PARTS AND COMPONENT PLACEMENT
Objective(s):
Upon completion of these information sheet(s), you will be able to:
Identify and locate different oven components.
2 1
3 5
1. Fuse
2. Cavity thermal protector
3. Cavity light
4. Blower fan blades
5. Magnetron
8
6 7
6. Interlock switches
7. Blower fan motor
8. Transformer
9. Capacitor
10
10. Digital control board and components

11
11. Diode
12
13
14
12. Front see through glass

13. Turntable tray
14. Digital control pads
YES NO
1. I can apply principles of electrical/electronic circuits
2. I can apply microwave oven principles of operation
3. I can identify microwave oven parts
4. I can analyze microwave oven troubles
15
15. Turntable motor
Self-Check 3.4.1
Instruction.: Check
Qualification Electronic Products Assembly and Servicing NC – II
:
Unit of Competency
: Service Consumer Electronic Products and Systems
Servicing Consumer Electronic Products and Systems

:
Re-Assemble and Test Repaired Industrial Electronics Products
Module Title
:
Learning Outcome #5
1. Materials, tools and instruments needed in troubleshooting repair and cleaning are selected
and checked in accordance with established procedures
2. Isolation of troubles are performed with proper Personal Protective Equipment (PPE) and
following the Occupational Health and Safety (OHS) practices
3. Troubles are isolated by following the systematic procedures and using proper instruments
in accordance with the prescribed instructions
4. Defective parts/components are replaced with identical or recommended appropriate
equivalent ratings and soldered/mounted in accordance with the current industry standards
5. Control settings/adjustments of repaired unit are performed in conformity with service-
manual specifications
6. Cleaning of unit is performed in accordance with standard procedures
7. Periodical tests of the repaired unit are maintained and documented according to standard
procedures
8. Repaired units are reassembled completely and waste materials are disposed of in
accordance with environmental require
9. Results of troubleshooting and repair are documented properly within the specified time.
Resources:
 Long- pliers
TOOLS nosed  Diagonal
cutters
 Standard screwdrivers EQUIPMENT
 Soldering iron, 30w Multimeter
Desoldering iron, 30w  Oscilloscope
 Screw driver assorted,  Function generator
Phillips, slotted  Electronically controlled
 Wrenches assorted unit(s)/appliance(s) and
 Allen wrench/key accessories
 Utility knife/stripper  Audio unit(s) and
 Pliers assorted, long accessories
nose, side cutter  Video unit(s) and
 Test jig accessories
 ESD free work bench with
mirror back-to-back/one
sided
 High grade magnifying
glass with lamp
 TV pattern generator
 High voltage probe
MATERIALS
 Solder lead
 Cleaning brush lead
free solder
 Resin core solder
 Wire stranded, #22,
different colors)
 Silicon grease
 Resistors (different
values)
values)
 Transformer
Learning materials
 Books and References
 Technical manuals
Domestic appliances may include but not limited to the following:
 Washing machines and driers
 Vacuum cleaners and polishers
 Home food processing equipment
 Pressure and rice cooker
 Blender, coffee maker
 Toaster, waffle maker
 Microwave oven
 Electronic clock
 Flat irons and presses
 Rechargeable light
 Electronic controlled light
 Home security equipment
TROUBLESHOOTING COMPONENTS
Objectives:
Troubleshoot electronic components
When a defective has been isolated the next problem is to pinpoint the specific component
or parts presumably defective or totally inoperable.
The following are components together with the corresponding faults and how they are
tested.
Resistor
Faults Incurred/Indication Test Procedure/Applied Remedy
Burnt body loose end caps broken body Faults are usually visible to the eye and no test
equipment is needed. equipment is needed.
- use a VOM only for assurance
- replace resistor with identical rating
Resistance goes out of tolerance Check resistance with a VOM

Replace resistance goes beyond the tolerable
Limit
Three common forms of resistor failure
Potentiometer
Intermittent Continuity Test nominal value of potentiometer using VOM.

Rotate shaft clockwise to determine the steady
increasing or decreasing value of resistance.
If there is jarring movement of the needle the pot
contact area is defective.
- Replace with a new identical value
a) Ohmmeter should indicate he nominal value of the pot; b) Rotating the shaft clockwise will
cause the resistance reading to drop.
Capacitor (electrolytic)
When you observed carefully the meters needle movement. If it deflects toward the right side
scale (zero-scale) then return to its initial position, then return to its initial position, the capacitor
under test is diagnosed to be good.
Open circuited If no needle deflection is observed at all.
Short circuited If the needle deflects near at the zero ohm scale
Leaky If the needle deflects toward the right sidescale

but does not return fully to its initial position
Note: The VOM used here is analogue type. For digital type meter, open, short, or leaky
capacitance is presented figures. Skills proficiency in interpreting numerical values needed.
Accurate values of capacitance are determined more easily using a digital capacitance
meter.
Erroneous interpretation can also happen if the probes polarity is incorrectly positioned when
testing polarized capacitor.
A capacitor can be determined if it is leaky, open, or shorted by using a VOM as shown.

Diode
Semiconductor diodes are commercially available in wide variety of packaging style

designed to suit different specific requirements. Regardless of their appearance, however, they
function in very much the same way, hence can be tested in the same manner.
The ohmmeter function of the VOM is again utilized. If the ohmmeter probe is connected
with the diode (a), a low resistance reading is obtained. If connected (b), a very high resistance
should be obtained. In most cases, no needle deflection should be observed at all. Otherwise, the
diode could be leaky. (Remember, don’t touch any exposed part of the probe tip.) Note that
germanium diodes exhibit small amount of leakage current which is normal for this type of diodes.
Light Emitting Diode (LED)
A light emitting diode is a derivative of the semiconductor diode, hence, it is tested in very
much the same way. There is just one additional thing which must be noted when testing a LED –
the brightness of light it emits. If the VOM used has an open circuit terminal voltage of at least 3
volts and can source at least 25 mA current, it will light the led as well while being tested as shown
below.
Zener Diode
It is a special type designed to operate on its reverse breakdown region. Zener diodes are
also tested out of circuit in the same manner as in the semiconductor diode.
A better method of testing zener diode is by actually measuring the voltage that appears
across it while in circuit. This voltage should match with the nominal rating of the zener diode, give
or take the maximum allowable tolerance.
Note: Not all circuits use a zener diode in constant breakdown mode. In which case the voltage
across the diode will not correspond with its nominal rating.
Transistors
A transistor can be viewed as two diodes connected back to back. In fact, you can test one
out-of-circuit using the diode test procedure.
Testing a transistor out of circuit is a 6-step process in which each of the “diode” junction is
tested for its forward and reverse resistance. A good transistor should give the readings as
indicated.
Note: The collector – emitter junction must give a very high resistance in both test polarity.
Some power transistors, particularly germanium types, exhibit a relatively low resistance in
the collector emitter junction in one or both probe polarity. This is because of the moderate amount
of collector emitter leakage current of these devices. (Consult with the manufacturer specification of
the transistor to determine the allowable collector emitter leakage current.) The reading may be low
but it should not be too close to the zero-ohm mark. If it hits the zero-ohm mark , the transistor is
shorted.
In circuit testing of transistors is also possible, often times more practical. As shown in the
figure, the transistor can be checked by noting the voltage appearing across each terminals. A
transistor biased in its linear region (i.e. amplifiers) should yield voltage readings as shown.
Still another method of in-circuit transistor testing is shown in the figure below. If the base
emitter junction of the transistor will switch off, causing the collector-emitter voltage to swing
towards the VCC level. If a 220-ohm resistor is shunted across the collector base junction of the
transistor, the transistor will be driven into full conduction, causing the collector-emitter voltage to
drop near zero-volt. Caution: Never perform this test procedure on power transistors and similar
power circuits. If the transistor does not respond to the tests as indicated, check first its associated
components, particularly the electrolytic capacitors. If the symptoms are all good, the transistor is
diagnosed to be defective and should be replaced. Finally, it should be noted that these in-circuit
test procedure can be applied only when the transistor is operated in its linear operating region.
Silicon Controlled Rectifier (SCR)
SCR is another special type of diode used mainly for power control circuits. Small to medium
power SCRs can be tested with an ohmmeter. Initially, (a) no resistance reading should be noted at
all as the ohmmeter’s probe is attached to the SCR’s A-K terminals; (b) shorting the gate G terminal
with the anode A terminal will cause the SCR to conduct, in effect, a near-zero ohm resistance
reading. Even as the shorting wire is removed (c), the ohmmeter should remain on its present
reading.
Repeat the procedure with the probe polarity reversed. No resistance reading should be
observed in any part of this test.
Note: Ohmmeter short circuit current must be greater than 100 mA.
Ohmmeter setting:Rx1
Triac
A triac is equivalent to two SCRs connected back to back. This means the procedure used in
testing the SCR also applies here.
Testing a Triac
Perform the test procedure as in the SCR, with the probe applied to the MT1 and MT2
terminals of the triac.
You should observe similar result as in the SCR test. Reverse the probe and polarity and
repeat the test. Unlike with the SCR, the triac should act the same as when the probe polarity was
not yet reverse.
MIROWAVE OVEN TROUBLESHOOTING
Objective(s):
Upon completion of these information sheet(s), you will be able to:
Troubleshoot microwave oven problems.
The most common problems occur in the microwave generating portion of the system,
though the controller can be blown by a lightning strike or other power surge. Bad interlock switches
probably account for the majority of microwave oven problems. Also, since the touchpad is exposed,
there is a chance that it can get wet or damaged. If wet, a week or so of non-use may cure keys that
don't work. If damaged, it will probably need to be replaced - this is straightforward if the part can be
obtained, usually direct from the manufacturer.
The interlock switches, being electromechanical can fail to complete the primary circuit on an
oven which appears to operate normally with no blown fuses but no heat as well. Faulty interlocks or
a misaligned door may result in the fuse blowing as described above due to the incorrect
sequencing of the door interlock switches. Failed interlocks are considered to be the most common
problems with microwave ovens, perhaps as high as 75% of all failures.
General system problems
The following problems are likely power or controller related and not in the microwave generator
unless due to a blown fuse or bad/intermittent connections:
 Totally dead oven.

 No response to any buttons on touchpad
 Oven runs when door is still open.
 Oven starts on its own as soon as door is closed.
 Oven works but display is blank.
 Whacked out controller or incorrect operation.
 Erratic behavior.
 Some keys on the touchpad do not function or perform the wrong action.
 Microwave oven does not respond to START button.
First, unplug the microwave oven for a couple of minutes. Sometimes, the microcontroller will get
into a whacko mode for some unknown reason - perhaps a power surge - and simply needs to be
reset. The problem may never reoccur.
Note: when working on controller related problems, unplug the connection to the microwave
generator (HV transformer primary) from the power relay or triac - it is often a separate connector.
This will prevent any possible accidental generation of microwave energy as well as eliminating the
high voltage (but not the AC line) shock hazard during servicing.
If this does not help, there is likely a problem with the controller circuitry or its power and you will
have to get inside the oven.
Totally dead oven
1. Check power to the outlet using a lamp or radio you know works. The fuse or circuit breaker
at your service panel may have blown/tripped due to an overload or fault in the microwave
oven or some other appliance. You may just have too many appliances plugged into this
circuit - microwave ovens are high current appliances and should be on a dedicated circuit if
possible. If you attempt to run a heating appliance like a toaster or fryer at the same time,
you *will* blow the fuse or trip the circuit breaker. A refrigerator should never be plugged into
the same circuit for this reason as well - you really don't want it to be without power because
of your popcorn!
If you find the fuse blown or circuit breaker tripped, unplug everything from the circuit to
which the microwave is connected (keep in mind that other outlets may be fed from the
same circuit). Replace the fuse or reset the circuit breaker. If the same thing happens again,
you have a problem with the outlet or other wiring on the same branch circuit. If plugging in
the microwave causes the fuse to blow or circuit breaker to trip immediately, there is a short
circuit in the power cord or elsewhere.
The microwave oven may be powered from a GFCI outlet or downstream of one and the
GFCI may have tripped. (Removing a broken oven lamp has been known to happen.) The
GFCI outlet may not be in an obvious location but first check the countertop outlets. The
tripped GFCI could be in the garage or almost anywhere else! Pushing the RESET button
may be all that's needed.
2. Try to set the clock. With some ovens the screen will be totally blank following a power
outage - there may be nothing wrong with it. Furthermore, some ovens will not allow you
perform any cooking related actions until the clock is set to a valid time.
Assuming these are not your problems, a fuse has probably blown although a dead
controller is a possibility.
If the main fuse is upstream of the controller, then any short circuit in the microwave
generator will also disable the controller and display. If this is the case, then putting in a new
fuse will enable the touchpad/display to function but may blow again as soon as a cook cycle
is initiated if there is an actual fault in the microwave circuits.
Therefore, try a new fuse. If this blows immediately, there may be a short very near the line
cord, in the controller, or a defective triac (if your oven uses a triac). Or, even a shorted oven
lamp - remove and inspect the light bulb and socket.
If it does not blow, initiate a cook cycle (with a cup of water inside). If the oven now works,
the fuse may simply have been tired of living. This is common.
If the fuse still blows immediately, confirm that the controller is operational by unplugging the
microwave generator, power relay, and/or triac from the controller. If a new fuse does not
now blow when a cook cycle is initiated - and it appears to operate normally - then one of the
components in the microwave generator is defective (shorted).
Some models have a thermal fuse as well and this may have failed for no reason or a
cooling fan may not be working and the oven overheated (in which case it probably would
have died while you were cooking something for an important guest - assuming you would
use a microwave oven for such a thing!).
Other possible causes: bad controller power supply or bad controller chip.
Totally dead oven after repair
On some microwave ovens, there is at least one cabinet screw that is slightly longer than all the
others. This engages a safety interlock which prevents the oven from receiving power if the correct
screw is missing or in the wrong hole. Check the length of all the screws and locate the interlock
switch behind one of the screw holes. I don't know how common this practice is but have heard of it
on some Sharp models.
Of course, any number of other pre-existing or induced problems can result in the oven playing
dead after it has been "repaired".
Dead Controller
The most common way that the controller circuitry can be harmed is by a power surge such as from
a lightning strike. Hopefully, only components on the primary side of the power transformer will be
affected.
 Check the primary of the power transformer - if it is open, there may be a fuse/thermal fuse
underits outer insulation. If not, the transformer will need to be replaced. There is a good
chance that the surge didn't propagate beyond the transformer and thus the rest of the
controlled should be unaffected.
 In some cases, circuit board traces may have been vaporized (but repair may still be
possible by simply jumpering across the crater). Some of these thin traces may be there
specifically to act as fuses - and there may even be spares to use for just this situation!
 Assuming that the main fuse and power transformer primary checks out, then check the
power supply for the controller next.
 As always, also check for bad solder connections.
If the controller power supply is working and there is still no sign of life (dead display and no
response to buttons) the microcontroller chip or some other part may be bad. It could be a simple
part like a capacitor or diode, but they would all need to be tested. At this point, a schematic of the
controller board will be needed - often impossible to get - and replacement controller or even just
the main chip may be nearly as expensive as a complete new oven.
No response to any buttons on touchpad
There can be many causes for this behavior (or lack of behavior):
 Door is not closed - on many ovens, there will be no response to any buttons - even setting
the clock - unless the door is securely closed.
 You waited too long - some models (like Sharp) have a timeout. If you close the door but don't
proceed to activate any functions with a couple of minutes, they will require you to open and
close the door to reset their pathetic brains.
 Controller is confused - a power surge or random non-reproducible action of the universe may
have resulted in the controller's program ending up in an infinite loop. Pull the plug for a
minute or two to reset it.
 Defective interlock switches - this can result in the controller thinking the door is open and
ignoring you.
 Faulty controller or its power supply - a power surge may have damaged the electronics.
Other than checking for bad connections and obviously bad power supply components,
diagnosing this will be tough without a schematic (and possibly much more).
 Touchpad or controller board contaminated by overenthusiastic cleaning - if you recently
power washed the oven (or even if you only use some spray cleaner), some may have gotten
inside and shorted out the touchpad or controller.
 Defective or damage touchpad - physical abuse is not a recommended technique for getting a
microwave oven to cooperate. If there is any visible damage to the touchpad - the outer film is
broken - it will probably need to be replaced.
Oven runs when door is still open
WARNING: Needless to say, DO NOT operate the oven with the door open! While extremely
unlikely, the microwave generator could be running!
For microwaves to actually be generated with the door still open would require the failure of all 3
interlock switches. The only way this could really happen would be for the 'fingers' from the door that
engage the interlocks to break off inside the oven keeping the interlocks engaged. In this case, the
controller would think the door was always closed.
Where no such damage is evident, a failure of this type is extremely unlikely since power to the
microwave generator passes through 2 of the 3 interlock switches. If both of these failed in the
closed position, the third switch would have blown the fuse the last time the door was opened.
Another more benign possibility is that one or more fans are running as a result of either a defective
sensor or normal operation to maintain air flow until all parts have cooled off.
Oven starts on its own as soon as door is closed
If the oven starts up as soon as the door is closed - regardless of whether a cook cycle has been
selected, the cause could be a shorted triac or relay or a problem with the controller or touchpad.
1. Unplug the oven for a couple of minutes to try to reset the controller.
If this doesn't help, put a cup of water into the oven and let it run for a minute to check for
heating. (You could also note the normal sound change or slight dimming of lights that
accompanies operation of the magnetron.) Much more must be enabled to actually power
the magnetron so this might point more to the controller as being faulty but not always.
Oven works but totally dead display
If all functions work normally including heating but the display is blank (assuming you can issue
them without being able to see the display), the problem is almost certainly in the controller or its
power supply.
Try pulling the plug for a minute or two - for some reason the display portion of the controller may
have been sent out to lunch by a power surge or alpha particle. It wouldn’t be the first time.
Check for bad connections between the display panel and the power supply and solder joints on the
controller board.
With everything else operational, a bad microcontroller chip is not that likely but is still a possibility. If
the oven was physically abused, the display panel may have fractured though it would take quite a
bit of violence. In this case, more serious damage to the door seals may have resulted as well which
would be a definite hazard.
Whacked out controller or incorrect operation
The following are some of the possible symptoms:
 All the display digits may have come on, EEEE or FFFF, or be displaying in Greek.
 The end-of-cooking cycle or key press tone may be wailing away continuously. (By 'tone' I
mean from the controller (not a low buzzing or humming when attempting to cook which would
indicate a microwave generator power problem like a shorted magnetron).
 Pressing a button on the touchpad may result in a totally incorrect action such as entering the
time resulting in the oven starting to cook. However, for the special case where pressing
START results in erratic behaviors.
 The oven may start cooking (or at least appear to) as soon as the door is closed. Pressing
buttons on the touchpad may or may not have any effect. (This could also be a shorted triac or
power relay).
First, try unplugging the oven for a couple of minutes - perhaps the controller is just confused due to
a power surge, lightning strike or the EMP from a nearby nuclear detonation because it wanted
attention.
If you recently cleaned the oven, some liquid may have accidentally gotten inside the touchpad or
even the controller circuitry (though this is less likely).
If the oven seems to have a mind of its own - running a cycle you didn't think you programmed, are
you sure a previous cook cycle was not interrupted and forgotten? Try to recreate the problem using
a cup of water as a load.
Assuming this does not apply, it sounds like a controller problem - possibly just a power supply but
could also be the controller chip.
Erratic behavior
There are three different situation:
 Whenever the oven performs unexpectedly both during setup and the cook cycle, suspect the
controller power supply or bad connections.
 Where problems only occur when entering or during the cook cycle, suspect a power relay or
mechanical timer (if used) with dirty or worn contacts, or (less likely) the power surge from
energizing the microwave generator or microwave (RF) leakage into the electronics bay
affecting the controller.
The filter capacitor(s) in the controller's power supply may be dried up or faulty. Check with a
capacitor meter or substitute known good ones. Prod the logic board to see if the problem comes
and goes. Reseat the flex cable connector to the touchpad.
For mechanical timers, the timing motor could be defective or require lubrication. The contacts could
be dirty or worn. There may be bad connections or loose lugs.
The primary relay may have dirty or burnt contacts resulting in erratic operation. If the oven uses a
HV relay for power control, this may be defective.
If the times and power levels appear on the display reliably but then become scrambled when
entering the cook cycle or the oven behaves strangely in some other way when entering the cook
cycle, there are several possibilies:
 The power surge caused by the cook cycle starting is resulting in changes to the settings or
else the microcontroller is not interpreting them properly. This may be due to a faulty part of
bad connections in the controller or elsewhere. As with intermittent problems, a thorough
search for loose ground and other connections and bad solder joints may locate the source of
the difficulty.
 Microwave (RF) leakage into the electronics bay due to an faulty joint between the magnetron
and the waveguide or structure failure of the magnetron may be interfering with the operation
of the microcontroller. Unless the oven was dropped or 'repaired' by an butcher, this sort of
failure is unlikely. If you suspect either of these, inspect the integrety of the magnetron-
waveguide joint and make sure the RF gasket is in place. Unfortunately, this is sometimes
difficult to pinpoint because unless there is obvious mechanical damage, the 'problem' may
disappear once the cover is removed for testing.
 On rare occasions, the main fuse may become intermittent rather than failing completely. The
surge or vibration of starting can jiggle the element open or closed. It is easy to try replacing it!
Some of the keys on the touchpad do not function or perform the wrong action
Touchpads are normally quite reliable in the grand scheme of things but can fail as a result of
physical damage (your spouse threw the roast at the oven), liquid contamination (from overzealous
cleaning, for example), or for no reason at all.
Look carefully for any visible signs of damage or spills. The touchpads often use pressure sensitive
resistive elements which are supposed to be sealed. However, any damage or just old age may
permit spilled liquid to enter and short the sensors. A week or so of drying may cure these problems.
If there is actual visible damage, it may be necessary to replace the touchpad unit, usually only
available from the original manufacturer. Also, check the snap type connector where the touchpad
flex-cable plugs into the controller board. Reseating this cable may cur a some keys dead problem.
Some people have reported at least temporary improvement by simple peeling the touch pad off of
the front panel and flexing it back and forth a few times. Presumably, this dislodges some bit of
contamination. I am skeptical as this could just be a side effect of a bad connection elsewhere.
With a little bit of effort (or perhaps a lot of effort), the internal circuitry of the touchpad can be
determined. This may require peeling it off of the front panel). Then, use resistors to jumper the
proper contacts on the flex cable connector to simulate key presses. This should permit the
functions to be verified before a new touchpad is ordered.
Caution: unplug the microwave generator from the controller when doing this sort of experiment!
If the problem was the result of a spill into the touchpad, replacement will probably be needed.
However, if you have nothing to lose, and would dump it otherwise, remove the touchpad entirely
and wash it in clean water in an effort to clear out any contamination, then do the same using high
purity alcohol to drive out the water, and then dry it out thoroughly. This is a long shot but might
work.
Microwave oven does not respond to START button
While all other functions operate normally including clock, cook time, and power setting, pressing
START does nothing, including no relay action and the timer digits do not count down. It is as
though the START button is being totally ignored.
If there is an alternate way of activating the cook cycle, try it. Use this to confirm the basic controller
logic and interlock circuitry. If it works, then the problem may indeed be a faulty START button. If it
is also ignored, then there may be a bad interlock or some other problem with the controller.
Check for bad interlocks or interlocks that are not being properly activated.
Next confirm if possible that the START touch pad button is not itself faulty. If you can locate the
matrix connections for this button, the resistance should go down dramatically (similar to the other
buttons). The START button does, after all, sees quite a lot of action!
Assuming it is not the touch pad, it sounds like the controller is either not sensing the start command
or refusing to cooperate for some reason - perhaps it thinks an interlock is open. Otherwise, the
timer would start counting. Testing the relay or triac control signal will likely show that it is not there.
Check that there are no missing power supply voltages for the controller and bad connection.
Microwave generator problems
Failures in the microwave generator can cause various symptoms including:
 No heat but otherwise normal operations.

 Fuse blows when closing or opening door.
 Loud hum and/or burning smell when attempting to cook.
 Arcing in or above oven chamber.
 Fuse blows when initiating cook cycle.
 Fuse blows when microwave shuts off (during or at end of cook cycle).
 Oven heats on high setting regardless of power setting.
 Oven immediately starts to cook when door is closed.
 Oven heats but power seems low or erratic.
 Oven heats but shuts off randomly.
No heat but otherwise normal operation
If the main power fuse is located in the primary of the high voltage transformer rather then at the line
input, the clock and touchpad will work but the fuse will blow upon initiating a cook cycle. Or, if the
fuse has already blown there will simply be no heating action once the cook cycle is started. There
are other variations depending on whether the cooling fan, oven light, and so forth are located down
stream of the fuse.
Some models may have a separate high voltage fuse. If this is blown, there will be no heating but no
other symptoms. However, high voltage fuses are somewhat rare on domestic ovens.
A number of failures can result in the fuse NOT blowing but still no heat:
 Bad connections - these may be almost anywhere in the microwave generator or the primary
circuit of the HV transformer. A common location is at the crimp connections to the magnetron
filament as they are high current and can overheat and result in no or intermittent contact.
 Open thermal protector - usually located on magnetron case. Test for continuity. It should read
as a dead short - near zero ohms.
 Open thermal fuse - some ovens have one of these in the primary circuit. It may be in either
connection to the HV transformer or elsewhere. Test for continuity. It should read as a dead
short - near zero ohms.
 Open HV capacitor - A shorted HV capacitor would likely immediately blow the fuse.
 Open HV diode
 Open magnetron filament - This failure may also be due to loose, burnt, or deteriorated press
(Fast-on) lugs for the filament connections and not an actual magnetron problem.
 Open winding in HV transformer
 Defective HV relay. A few models use a relay in the actual high voltage circuitry (rather than
the primary) to regulate cooking power. This may have dirty or burnt contacts, a defective coil,
or bad connections
 Shorted HV diode
 Short or other fault in the magnetron.
 Short in certain portions of the HV wiring.
A shorted HV diode, magnetron, or certain parts of the HV wiring would probably result in a loud
hum from the HV transformer but will likely not blow the main fuse. (However, the HV fuse - not
present on most domestic ovens - might blow.)
Depending on design, a number of other component failures could result in no heat as well including
a defective relay or triac, interlock switch(s), and controller.
Fuse blows when closing or opening door
This means that the main fuse in the microwave (or less commonly, the fuse or circuit breaker for
the power outlet) pops when the microwave oven door is closed or opened. This may be erratic,
occurring only 1 out of 10 times, for example.
The cause is almost certainly related to either the door interlock switches or the door itself. Marginal
door alignment, broken 'fingers' which operate the switches, dislocated parts in the interlock
mechanism, or a defective interlock switch may result in either consistent or erratic behavior of this
type.
On some ovens, this can happen at any time regardless of the control panel settings or whether the
oven is in the cook cycle or not. On others, it can only happen when interrupting the cook cycle by
opening the door or when initiating the cook cycle from the front panel (if the switches are in the
wrong state).
The rational for this basic design - some form of which is used in virtually all microwave ovens - is
that a defect in the interlock switches or door alignment, which might result in dangerous microwave
radiation leakage, will produce a hard permanent failure. This will prevent the oven from being used
until it is inspected and repaired.
 As noted, one of the interlock switches is actually across the power line. If the switches are
activated in the wrong sequence due to a misaligned door, that switch will not turn off before
the other switches turn on shorting the power line. Similarly, if its contacts are welded closed,
the power line will be shorted when the other switches close.
 Inspect the door, its mounting, and the plastic 'fingers' which operate the interlock switches as
well. Again, if the sequence is not correct, the power line will be shorted blowing the fuse. If
the oven was dropped, then such damage is quite likely. Look for broken or dislocated parts,
warpage, and other indications of problems with the door and interlock mechanism Of course,
if the oven was dropped, there could be much more extensive internal damage as well.
Loud hum and/or burning smell when attempting to cook
A loud abnormal hum is an indication of a short somewhere. The sound may originate from the HV
transformer vibrating and/or from within the magnetron depending on cause. There may be a burnt
odor associated with this behavior:
 Shorted HV diode
 Shorted magnetron (filament to anode) or other internal fault in the magnetron. Arcing within
the Magnetron case (visible through ventilation holes in the bottom section) is usually an
indication of a bad magnetron.
Note that a short on the load side of the HV capacitor will likely result in the actual wattage drawn
from the power line being much lower than under normal conditions. Although there will be a high
current flowing in the HV transformer secondary through the HV capacitor (which is what causes the
hum orbuz), the real power consumed will be reduced since the current and voltage will be out of
phase (due to the series capacitor) and the power factor will be low. A reading on an AC line
wattmeter of 300 W compared to the normal 1,200 to 1,500 W would be reasonable.
 Other short resulting from frayed insulation or wires touching in the microwave generator.
 Shorted HV transformer
 Short resulting from burnt on food (usually) in or around the waveguide. If the odor is coming
from the oven chamber.
The following procedure will quickly identify the most likely component if the
problem is not
food/spills/carbon related:
1. Discharge HV capacitor! (If there is a short it is doubtful if it has any charge but never hurts
to be safe).
2. Remove one end of the lead from the HV capacitor to the transformer.
3. Start the oven.
 Hum gone? If so, it is the HV circuitry, go to step 4.
 If it still hums you probably have a faulty HV Transformer. (Not uncommon.)
4. Discharge the HV capacitor again, reconnect wire and disconnect the 2 wires to the
magnetron.
5. Restart oven.
 Hum Gone? If so, magnetron is shorted. Replace or get a new oven.
 Hum still there? If so, go to step 6.
6. You have either
 Shorted HV capacitor,
 Shorted HV Diode,
 Shorted clamp diode across the HV Cap terminals (if one is present, about 30% of
microwave ovens use these). (The oven will run 100% without this protection for the HV
capacitor but it should be replaced if possible.)
Arcing in or above oven chamber
There is often a simple cause:
 Arcing in the oven chamber with a normal load (a cup of water, for example), often just
indicates that a thorough cleaning of the oven chamber is needed, particularly around and
inside/above the waveguide cover. Any food that gets trapped here will eventually burn and
carbonize resulting in a focal point for further arcing. Usually, the waveguide cover is designed
to be removable without taking the (cabinet) cover off of the oven. However, burnt food and
carbon often make this difficult so that some disassembly will be required. Clean the
waveguide cover and clean inside the waveguide as well. Trim to fit with a pair of heavy duty
scissors, metal snips, or a paper cutter. The oven will work fine without it but replacement will
prevent contamination of the waveguide with food vapors or splatters which can lead to more
expensive damage. Take extra care to cover all food (which you should do anyhow) until the
waveguide cover is replaced.
 Any sharp metal edges may also result in arcing or sparking. However, the only way such
damage could occur as part of the oven (not added knives or forks!) would be through
physical abuse.
 If your oven uses a stirrer above the oven chamber (no turntable), it may be stuck. The result
will be an uneven distribution of microwave energy and localized heating, arcing, and possibly
melting plastic or metal.
 Flashing and sparking may also result from the stirrer/fan blades contacting the metal
surrounding it due to the motor/bearings becoming loose or dislodged.
Fuse blows when initiating cook cycle
The fuse may only blow when actually attempting to cook but depending on design, triacs and/or
door switches may always be live and may result in a blown fuse at any time when plugged in or
when the door is opened or closed.
The following can cause the fuse to blow (in approximate order of likelihood):
 Defective interlock switches or misaligned door. At least one of the interlock switches is
across the power line and will blow the fuse if not activated in the correct sequence. See the
sections: "Fuse blows when closing or opening door" and "Testing and replacing of interlock
switches".
 Shorted HV capacitor.
 Shorted HV diode (see note below).
 Shorted magnetron (filament to anode - see note below).
 Defective triac (shorted or partially shorted).
 Old age or power surge. Fuses sometimes blow for no apparent reason.
 Defective HV transformer shorted windings.
 Shorted wiring due to vibration or poor manufacturing quality.
Note that a shorted magnetron or shorted HV diode - which you would think should blow the fuse -
probably will not do so because current will be limited by the impedance of the HV capacitor
(assuming it is not shorted as well). However, there will likely be a loud hum from the HV
transformer as it strains under the excess load. Such a sound in conjunction with no heat is a likely
symptom of a shorted magnetron or HV diode. If your oven has a separate high voltage fuse -
somewhat rare in domestic ovens - it may certainly blow due to a fault in any of the HV components.
Fuses also die of old age. The types of fuses used in microwave ovens are subjected to a heavy
load and you may find that all that is needed is to replace the fuse with one with equivalent ratings.
(but check for shorts first). There could be an intermittent problem as well which will only show up at
some random time in the future. A poorly timed power surge (as opposed to the well timed variety)
could also weaken the fuse element resulting in eventual failure.
The fuses used in microwave ovens are usually ceramic 1-1/4" x 1/4" 15 or 20 A 250 V fast blow
type. Replace with exactly the same type and rating.
Another possible cause of a blown fuse is a partially bad triac. Some ovens use a triac rather than a
relay to control the main power to the high voltage transformer. One type of failure of a triac is for it
to be totally shorted causing the oven to come on whenever the door is closed. Alternatively, the
gate may be defective preventing the triac from ever turning on. A third, and most interesting
possibility, is that one half of the triac is bad - shorted or open, or doesn't turn on or turn off reliably.
Recall that a triac is in effect a pair of SCRs in parallel in opposite directions. If one side is defective,
the main fuse will blow due to transformer core saturation since the triac will act as a rectifier and
transformers really do not like DC.
Fuse blows when microwave shuts off (during or at end of cook cycle)
This could be due to a number of faults including shorting wires or defective relay. However, a
common cause that might not be obvious is that the triac used to switch power to the high voltage
transformer is faulty. What is probably happening is that only one half of the triac (recall that a triac
is controlled for both polarities of the line voltage/current) is turning off completely resulting in DC to
the HV transformer, core saturation, and excessive current which blows the fuse. Drive to the triac
could also be marginal but the bad triac is more likely.
Exactly how a bad relay could result in these symptoms unless it was actually arcing and shorting is
unclear. However, there is anecdotal evidence to suggest that inspecting the relay contacts and
cleaning them if necessary may cure it in some cases.
Oven heats on high setting regardless of power setting
Power levels in a microwave oven are controlled by cycling the microwave generator on and off with
a variable duty cycle - kind of like slow pulse width modulation. For 'HIGH', it runs continuously; for
low, it may run 10% on and 90% off; other settings are in between.
When the oven always seems to be stuck at high power, it is likely to be due to one of two possible
causes - a faulty relay or Triac, or controller. The relay or triac may have failed in the on state. This
will probably show up with ohmmeter tests (with the oven unplugged!) but not always.
Replacements should be readily available. If the problem is is the controller, it will be more difficult
to diagnose as schematics for the controller are usually not readily available. However, it could be
something simple like a bad connection or dirty connector.
Oven heats but power seems low or erratic
Some considerations are how old the oven is and did the problem happen suddenly or did it just
gradually weaken over the years.
First, are you sure the problem is real? Perhaps you are just a little less patient than you used to be.
Perform a water heating test or try to pop a bag of popcorn using you usual time setting.
 If you are subject to brownouts or are running on your own generator, the line voltage may be
low. Power output is quite sensitive to the AC input - there is no regulation. A 10% drop in line
voltage is likely to reduce microwave power output by more than 20%.
 Magnetrons, like other vacuum tubes, can weaken with age and use. An oven that sees daily
use may indeed weaken over the course of several years. It is unlikely that any other
electronic components could change value in such a way as to significantly affect power
output. However, a failure of the controller or sensor (if you have one) could result in short
cycling.
Testing on HIGH will eliminate this possibility. Make sure the magnetron is powered
continuously and it is not cycling. You can often tell by listening for the relay clicks and/or by
observing the oven light/other lights dimming as the magnetron kicks in. 50% power should
result in approximately equal on and off times.
 If you run the oven on HIGH, can you tell if it is actually heating continuously or rather it thinks
you want LOW? Many microwave ovens make a clicking sound as they use a relay to switch
microwave power on and off - check if you can hear this. Alternatively, lights on the same
circuit or the oven light may dim slightly when the magnetron kicks in. There should not be any
cycling on HIGH - the microwave power should stay on continuously while it is cooking. If it is
cycling, there may be a problem with the controller or you may unknowingly be in a low power
mode - check it.
 Mechanical problems are also possible. Where a spinning paddle wheel is used to 'stir' the
microwave energy (often where there is no turntable), its failure to rotate can result in hot and
cold spots. Thus, you may see an unexplained variation in cooking times. The paddle is often
accessible by unclipping a plastic cover above the oven cavity. Check for bearing failure,
binding, broken or lose belt if direct driven, etc. Note that some are rotated by air flow from the
cooling fan and require that cover to be in place to rotate. Therefore, it is not really possible to
inspect for correct operation with the cover removed. However, you can put a microwave
power indicator (NE2 neon light bulb with its leads twisted together) in the oven (with a cup of
water for a load) and observe it through the window. You should see a periodic variation in
intensity as the paddles do their job.
 There could be intermittent connections to the magnetron filament, thermal protector, or
elsewhere. But, these would likely show up as erratic operation - no heat at all sometimes -
not just a weak oven.
Inspect and clean and tighten (if necessary) all connections in the microwave generator
including the magnetron filament, HV transformer, HV Diode, HV capacitor, and thermal
protector. Be sure to unplug the unit first and discharge the HV capacitor before touching
anything!
 The thermal protector may be intermittent. Test by clipping a light bulb across it or monitoring
with a multimeter on AC voltage.
Oven heats but shuts off randomly
Everything operates normally, but the oven shuts off after varying amounts of time. This could be a
faulty magnetron, bad cooling fan (or just built up dust and grime block ventilation grilles), bad
thermal protector, faulty controller, some other intermittent component, or bad connections.
 If resetting it allows cooking to resume immediately, if even for a few seconds, I would not
suspect the magnetron or thermal problem as no cool down time is required. It could be bad
connections in the controller or elasewhere, a marginal door interlock switch, or a controller
problem. Jiggle the door to see if this will cause it to shut off.
 If the magnetron was overheating, you would not be able to resume cooking until it cooled and
the thermal protector reset. If it just stopped working (i.e., the filament opened), everything
would appear normal but there would be no heating. If the magnetron were shorting, there
would likely be a loud hum associated with the periods where there was no heat.
 If it is not possible to resume cooking for a few minutes indicating that something needs time
to cool off, then the magnetron could be faulty but check for the obvious cooling problems first:
blocked or dirty ventilation grill. Determine if the magnetron cooling fan is operating by
listening for its sound or looking through the ventilation opening in the back of the oven. If it is
not, there could be a broken or weak belt, gummed up or lack of lubrication, other mechanical
problems, a bad motor, or bad connections.
 Extremely high power line voltage may also result in overheating on a poorly designed or oven
where the components are marginal.
 Make sure the stirrer fan is turning normally. Should it gets stuck, some models may sense
this and shut down/restart.
Oven makes (possibly erratic) buzzing noise when heating
Assuming operation is normal otherwise, this is most likely either a fan or other motor vibrating on
its mounts, fan blades hitting something, or some sheet metal or the high voltage power transformer
laminations vibrating. There may be something stuck under the turntable or above the waveguide
cover interfering with the stirrer.
Something may have loosened up with age and use.
If the noise is caused be simple vibrations, no damage is likely to result. However, if the main
cooling fan is on its way out and it stops or gets stuck, parts will overheat quite quickly at which
point the oven will shut down (hopefully) and there could be damage to the magnetron or other
components. Therefore, at least identifying the cause is probably a good idea.
The solution may be as simple as tightening a screw or weging a shim between two pieces of
vibrating sheet metal.
Oven light does not work
If the oven light no longer works, a burned out light bulb is likely.
Light bulbs may be typically located in any of 3 places:
1. Oven chamber - it may be behind a mesh grill requiring a screw or snap to be removed. This
is the easiest.
2. Rear - the bulb may be in a recessed compartment accessible by removing a screw or two
on the back of the oven.
3. Inside - it may be behind a non-removable grille requiring the removal of the cover.
These are typically not your usual vanilla flavored appliance bulbs either.
Bad connections are also possible but not that likely.
Fans or turntables that do not work
There are up to 4 motors in a microwave oven:
 Magnetron cooling fan - always present.

 Mechanical timer (on inexpensive non-touchpanel or older units).
 Turntable.
 Convection air circulation (combo units only).
When any of these do not operate properly, the most likely causes are:
 Gummed up lubrication/dry bearings. Check for free rotation of the affected part(s). Clean and
lubrication as needed. Also confirm that there are no other mechanical problems (e.g.,
turntable improperly installed).
 Loose or broken belt. Confirm that belt is properly installed. Test to determine if it is worn and
flabby - stretch it by about 25%. It should return to its relaxed length instantly. Clean and/or
replace if needed.
 Bad motor. Disconnect one wire and check for continuity with an ohmmeter. If open, winding
is bad but check for break at terminal which you can resolder.
 Bad thermostat. Where a fan only runs when the oven is hot as in a microwave/convection
oven, the thermostat or controller could also be at fault. Locate the thermostat and jumper
across its terminals with power off. Plug the oven in and see if the fan now runs all the time or
at least when the appropriate mode(s) are entered.
 Bad connections - trace wiring and check continuity (unplugged, capacitor discharge) to motor
terminals.
Note that the opposite problem - a turntable and/or fan that runs after the cook cycle is completed
may be normal for your oven. This is a "cool-down" function designed to allow the heat to equalize.
What to do if the door handle breaks off

Usually this happens at the places where the handle is screwed to the door.
Plastic is generally tough to glue where a strong bond is needed and where the joint is subject to
abuse. However, depending on the type of plastic, one or more of the following may work:
semiflexible adhesive like windshield sealer, plastic cement (the kind that fuses the plastic, not
model cement), Duco cement, PVC (pipe) cement, or even superglue (though it seems not all
brands are equally effective). Make sure the surfaces to be glued are perfectly clean (remove any
residual library paste if you tried that!) and provide a means of clamping the pieces until the bond
sets up (adhesive tape and/or rubber bands may be all you need). Consider providing some
reinforcements around the joint (i.e., plastic splints or sisters depending on your profession) for
added durability.
Replacement door handles and/or entire doors may be available from the manufacturer of the oven.
Here are the door disassembly instructions from the Amana service manual. Many others are
similar:
1. Pry out the inner door trim with a small screwdriver on the latch side of the door.
2. Remove two screws securing the latch assembly and door handle to the outer panel (this
may be all that's needed to replace the handle).
3. Remove six screws and release 4 spring fingers that secure the choke to the outer panel.
WARNING: A microwave leakage test must be performed any time a door is removed, replaced,
disassembled, or adjusted for any reason.
Crack or other damage to door window
If the metal screen/mesh is behind and separate from the glass, there is no danger. In this case, the
function of the glass is mostly cosmetic and a small crack should not be a problem.
However, if the screen is inside the glass and now broken as well, there could be microwave
leakage. Even if it is not actually broken at this time, future failure is possible. Therefore, the glass
panel or entire door should be replaced.
Also, any break large enough to allow something to touch the metal screen is a hazard because
during cooking, there could be shock hazard due to microwaves inducing current in the screen. And,
poking something metallic through the screen would make is susceptible to microwave pickup as
well.
However, damage to the inner plastic is probably not a cause for concern as that is only there to
keep the screen and inside of the door glass clean.
Repairing damage to the oven interior
If spilled food - solid or liquid - is not cleaned up soon after the oven is used, it will tend to harden
and carbonize. Not only will this be much more difficult to remove, but hot spots may develop and
result in possible sparking, arcing, and damage to the interior paint.
If this happens in the vicinity of the mica waveguide cover, it may be damaged as well. In addition,
sometimes splatters may find their way above the waveguide cover and cause problems above the
roof of the oven chamber in the waveguide.
Needless to say, clean up spills and food explosions as soon as possible. Not only will it be easier,
the chance of future expensive problems will be minimized.
To prevent arcing and sparking, the interior needs to be smooth. Sharp edges and hard carbon in
particular creates places where electric field gradients can become great enough to cause
problems. Thus the warning not to use any metal utensils in a microwave.
Once damage occurs - paint blisters and peels, or totally hardened impossible to remove carbon
deposits - more drastic action is called for:
 Assuming cleaning does not work on the carbon - even after repeated attempts, carefully
scrape it off with a blunt knife or other suitable tool. This will probably damage the paint. Use
fine sandpaper to completely smooth out the metal and feather the edges of the paint in the
immediate area.
Special microwave oven cavity paint is available but any common gloss enamel will work just
as well (and costs about 1/10th as much). Use touch-up paint (with a small brush) or spray
paint. The typical color is beige, almond, or some other form of off-white - just match it to your
oven (if you care).
Until you can obtain the paint, the oven will work fine but since the chamber is made of sheet
steel, rust will set in eventually. So, do paint it.
 If the waveguide cover is damaged seriously - such that it no longer will prevent splatters from
entering the waveguide, obtain replacement material, cut to fit. Leaving it larger than
necessary is fine as well. Use a suitable bit in a hand drill to make holes in the mica for the
mounting screws or plastic snaps.
Alternatives to mica which can stand the elevated temperatures in a microwave oven may also
be acceptable. Possible choices include plastic or fiberglass laminate but not all materials will
allow microwaves to pass without some heating - check it out. Heat a cup of water and the
candidate material on high for a couple of minutes. If the material doesn't heat up, it should be
fine. Of course, it must also not have any metal coating (don't use a piece of one of those
'browning disks' :-). Mica is also non-flammable which is may not be the case with other
materials.
 If the interior of the door is damaged seriously such that either it will not longer seal around
the edge properly or that the mesh screening is breeched, a replacement will be required to
assure continued safety with respect to minimizing microwave emissions.
Microwave oven cavity paint, waveguide cover mica sheets, and even some replacement doors are
available from the parts suppliers listed at the end of this document. For most ovens, parts like
doors will need to be obtained direct from the manufacturer, however.
Microwave/convection oven problems
In addition to the microwave components, these ovens also include an air circulating fan and an
electric heating element as well as a temperature sensing thermistor. Any of these can fail.
 A convection oven which shuts down after a couple of minutes during the preheat cycle with
the temperature display (if any) stuck at LOW (even though the oven is hot when opened)
may have a bad thermistor temperature sensor.
 The over temperature protection sensor (rather than the normal temperature sensor) is
shutting the oven down. The thermistor will usually be accessible after removing the oven
cover. It will be located centrally just above the oven ceiling duct or elsewhere in the
convection air flow. It is a two terminal device that may look like a tiny resistor or diode and
may be mounted on a metal header fastened with a couple of screws. Remove and test with
an ohmmeter. An infinite reading means it is bad. As a test, jumper a 50 K ohm potentiometer
in place of the thermistor. During preheat, as you lower the resistance of the pot you should
see the temperature readout climb. The oven will then indicate READY when the simulated
temperature exceeds the set point. Replacement thermistors are available from the oven
manufacturer - about $20. Cheaper alternatives may be possible but you would need to know
the exact specifications and it is probably impossible to obtain this information.
 If the convection preheat cycle never completes and the oven is cool when opened, then
either the heating element is bad (test with an ohmmeter) or the relay controlling the heating
element or the controller itself is bad. If the circulating fan runs off of the same relay and it is
operating, then the problem must be the heating element.
 The heating element will be either a Calrod type (GE trade name?) which is a steel tube
enclosing a Nichrome wire coil embedded in ceramic filler or a coiled Nichrome element
strung between ceramic insulators. The former is probably only available from the oven
manufacture, though it is worth trying an appliance parts distributor or a place like MCM
electronics first. It may be possible to find a replacement Nichrome coil and form it to fit. Make
sure the wire gauge and length are identical.
 The circulating fan is probably driven by a belt, which may break or deteriorate. Inspect the
belt. If it is loose, cracked, or does not return to its normal length instantly after being
stretched by 25% replace it. Check the fan motor and fan itself for adequate lubrication. Check
the fan blades for corrosion and damage.
Sensor problems
Fancier microwave or microwave/convection ovens include various probes that can be used to shut
off the oven when the food is supposedly done or maintain it at a preset temperature.
When problems develop with these automatic features, the sensor and the probe cable are the
primary suspects. However, it is possible that the electronic circuitry could also be affected by a
damaged or defective probe unit.
 Check for bad connections where the probe plugs in as well as broken wires inside the cable
particularly near the ends where it gets flexed.
 Temperature probes may use a thermistor similar to one that controls the convection portion
of a microwave/convection oven. Steam/humidity probes may also behave similarly.
 If you have never tried the probe before, check your user’s manual. It may only be active in
certain modes, etc.
The best test of the probe unit is to substitute a known good one. Of course, this is generally not
convenient.
 There should be some resistance when measuring between the signal conductors of the
probe cable. It may be high (hundreds of K ohms) but probably should not be open. A very low
value (a few ohms or less) might indicate a short in the cable or sensor.
 Testing to determine if the controller is responding to the input from the sensor can be done in
a similar manner except that access must be from inside the electronics bay while the oven is
running (the probe normally plugs in inside the oven chamber). Substitute a fixed or variable
resistor and see if you can get the oven to shut off (or stay on) as a function of resistance.
CAUTION: Don't forget to put a cup of water in as a load if you are testing microwave
operation.
If the resistor test determines that the controller is responding, than a bad probe unit is likely.
If the probe checks out or substituting a known good one makes no difference in behavior, look for
corrosion or other deterioration of the socket in the oven chamber as well as bad connections.
Faulty circuitry in the controller is also possible.
MICROWAVE OVEN DISASSEMBLY
A. Tools and Materials
1. Microwave oven as selected by instructor

2. Screwdriver with insulated handle
3. Safety glasses
B. Procedure
1. Put on safety glasses

2. Unplug the oven cord
(CAUTION: Perform the rest of the procedure ONLY IN THE PRESENCE OF AND UNDER
THE DIRECT SUPERVISION OF YOUR INSTRUCTOR.)
3. Remove two screws at the right side of the oven.
screws
4. Remove five screws from the back side of the oven.
screws
5. Remove top cover to see inside of the oven.
Have your instructor check your work
6. Clean up are and return tools and materials to proper storage, or prepare for next job sheets
as directed by your instructor.
MICROWAVE OVENREASSEMBLY

3. Safety glasses
B. Procedure

2. Prepare the parts for re-assembly and put the top cover of the oven.
3. Replace the screws at the back side of the oven.
screws
4. Replace the screws at the side of the microwave oven.
screws
5. Remove top cover to see inside of the oven.
6. Clean up are and return tools and materials to proper storage, or prepare for next job sheets
DISCHARGE A CAPACITOR

2. Service manual for selected oven
4. 20kΩ resistor with jumper type leads
5. Needle nose pliers with insulated handles
6. Safety glasses
B. Procedure

2. Unplug the oven cord
3. Remove the oven wrap or access panels as required to reach the capacitor. (SEE JOB
SHEETS 3.3.1 FOR MICROWAVE DISASSEMBLY)
Removing capacitor from its fastener
1. Remove screw that fastens the capacitor to 2. Carefully remove the capacitor away from the
oven body. oven.
3. Remove screw fastening the diode. 4. Expose the capacitor terminals for
discharging.
4. Position the resistor leads so that they are equal to the distance between the capacitor
terminals
5. Hold the resistor at mid-point with a pair of pliers with insulated handles
6. Move the resitor and leads to a point where the leads contact the capacitor terminals and
discharge the capacitor
(NOTE: A capacitor can always be discharge by barring across the two capacitor
terminals with a screwdriver that has an insulated handle, and you ask your instructor
whether or not to practice that procedure.)
7. Clean up area and return tools and materials to proper storage, or prepare for next job
sheets as directed by your instructor.
CHECK TURNTABLE ROTATION
1. Microwave oven (SHARP R-240F)

3. Safety glasses
B. Procedure

2. Plug AC cord to outlet.
3. Set the oven in demonstration mode. Press the CLOCK pad and the number pad 0, then
PRESS INSTANT COOK/START pad and hold for 3 seconds. dISP will appear in the
display.
4. Press COOK/START pad and the display will show 1.00 and count down to zero at ten
times the speed faster than normal. Observe the tray if it rotates. If not replace turntable
motor.
Turntable
5. Cancel demonstration mode. Press CLOCK pad, the number 0 and the STOP/CLEAR
pad.
6. Unplug the AC cord from the outlet
7. Clean up area and return tools and materials to proper storage, or prepare for next job sheet
TROUBLESHOOTING MICROWAVE OVEN PROBLEMS WITH CIRCUIT DIAGNOSIS
1. Microwave oven
3. Microwave safe container with two cup capacity
4. Pencil and paper
5. Safety glasses
B. Procedure

2. Conduct a preliminary inspection of the oven to assure that it is safe to service
3. Place a little less than two cups of water in a container, place it in the oven, and set the oven
for full power (cook condition).
4. Close the oven door, set the timer for two minutes, and start the oven
5. Check blower motor is operating by feeling gush of air through the ventilation duct at the
back of the oven, and if it is, move on to Step 7.
6. Establish that the blower motor is not working and complete the following procedure in order:
a. Check for a bad fuse.
b. Check for an open thermal protector for the magnetron or the cavity
c. Check for a timer assembly
d. Check for a defective or out of adjustment interlock switch module
e. Check for a defective or binding motor with standard troubleshooting procedures
7. Check to see if turntable is rotating, and if they do, move to Step 9
8. Establish that the turntanle is not rotating and correct the defective turntable system.
9. Check to make sure timer is advancing properly, and if it is, move on to step 11
10. Establish that the timer is not advancing properly and troubleshoot for a defective or binding
timer assembly
11. Open the oven door
12. Check to see if the water you placed in the cavity has heated, and if it has, the cook
operation is normal and you should move on to Step 15
13. Establish whether or not the water is heating slowly by performing power tests that covers:
a. Problems with line voltage less than 220V AC

b. Problems with a defective power transformer, a defective capacitor, or a defective
magnetron
14. Establish that the water is not heating at all by performing the no heat tests that covers:
a. Defective diode
b. Defective capacitor
c. Defective power transformer
d. Defective magnetron
15. Write a brief summary of your findings complete with the date and model and serial numbers
of the microwave oven
16. Clean up area and return tools and materials for proper storage, or prepare for next job
sheets as directed by your instructor.
CONDUCT TESTS OF HIGH VOLTAGE COMPONENTS WHEN LITTLE OR NO HEAT
IS PRODUCED BY AN OVEN BUT ALL OTHER OPERATIONS APPEAR NORMAL
1. Microwave oven
3. Ohmmeter with a range of x10,000 or greater
4. 9-V battery AN OVEN
6. Pencil and paper
7. Safety glasses
B. Routine #1 – Testing the capacitor

2. DISCHARGE THE CAPACITOR
3. Set the ohmmeter on its highest resistance scale
(NOTE: The following test will not work with a capacitor with a built-in bleeder resistor, so
that type of capacitor should be checked with a capacitor analyzer.)
4. Remove the wires from the capacitor terminals and connect ohmmeter leads to the capacitor
terminals
5. Check the meter to make sure it momentarily deflects toward zero and then returns to infinite
a. If no deflection occurs, replace the capacitor
b. If continuous deflection occurs, replace the capacitor
6. Check between each terminal to the capacitor case for infinite resistance, and if it is not
present, replace the capacitor
C. Routine #2 – Testing the Diode
DISCHARGE THE CAPACITOR

1. Set the ohmmeter to the highest resistance scale
2. Remove the leads that come from the capacitor and connect with the diode terminals
3. Place the ohmmeter leads on the diode terminals
4. Check for an infinite resistance of either 50,000 or 200,000 ohms and record your finding
5.
(NOTE: Diodes on other models ovens may not give these readings.)
6. Reverse the ohmmeter leads on the diode terminals and check for either 50,000 or 200,000
ohms on the second testing and record your finding
(NOTE: If the first reading was 50,000, the second reading should be 200,000 or the
reverse.)
7. Replace the diode if it fails to pass the infinite resistance tests

D. Routine #3 – Testing the magnetron

2. Remove the wires from the magnetron and connect ohmmeter to its terminals
3. Check for a reading of less than 1 ohm between terminals
(CAUTION: Set your VOM at highest scale for the following check.)
4. Check for infinite resistance between each magnetron terminal and ground
(NOTE: This test is not conclusive, and if all other components tests good and the oven still
does not heat, replace the magnetron.)
E. Routine #4 – Testing the power transformer
2. Check primary winding first by removing wires from the terminals marked 1 and 2 and
connecting the ohmmeter leads to thoise terminals and recording your findings
3. Check between each terminal and ground
a. On all three types of power transformers, the primary winding check should read less
than 1 ohm
b. On all three types of power transformers, the terminal to ground should read infinite
4. Check high voltage winding next by removing the wire from the secondary terminal marked
HV
5. Connect ohmmeter between the HV terminal and the oven chassis
a. HV to ground on a Type 1 transformer should read 54 to 64 ohms
b. HI to ground on a Type 2 transformer should read 66 to 74 ohms, and LO to ground on
Type 2 transformer should read 63 to 71 ohms
6. Record all findings
7. Check the filament winding next by removing the wires 3 and 4 on the transformer and
connecting the ohmmeter between these terminals
8. Check between each filament terminal and ground
a. Step 7 should give a reading of less than 1 ohm
b. Step 8 should show infinite resistance
9. Record all findings

10. Replace power transformer if findings indicate problems
as directed by your instructor
REMOVE AND INSTALL A MAGNETRON
1. Microwave oven
3. Standard tool set
5. Pencil and paper
6. Electrical Tape
7. Safety glasses
B. Procedure (Figure 1)

2. Unplug the power cord and remove the access panel
4. Remove the blower motor as previously outlined
5. Remove the diode
6. Remove the magnetron mounting nuts and lower the magnetron to remove it
7. Check magnetron RF gasket to make sure it is in good shape and properly placed before
reinstalling the magnetron
8. Reinstall magnetron, tighten mounting nuts, and check for secure and even alignment of the
magnetron
9. Install the magnetron air duct and thermo protector
10. Install the high voltage transformer and reconnect magnetron filament leads and transformer
leads
11. Install diode
12. Install blower, blower housing duct, and magnetron support bracket
13. Place a container of cool water in the cavity and check the oven for proper operation
14. Check the oven for RF leakage with an approved RF test meter and procedure
directed by your instructor
1
REMOVE AND DISASSEMBLE A STIRRER SYSTEM
1. Microwave oven
4. Standard tools
5. Sealant M13D2 as required
6. Safety glasses
B. Procedure (Figure 1)

2. Unplug the power cord and remove the access panel
4. Remove the access panel
5. Remove the stirrer cover by pulling forward to release the molded-in button that holds it in
place
(NOTE: On early production ovens the rear poly buttons have to be removed.)
6. Remove the stirrer bracket support and lower the support assembly and stirrer blade
7. Remove the left and right hand air baffles as required
8. Keep the stirrer insert intact with the bearing and set it aside
9. Inspect the stirrer blade and baffles for evidence of damage
10. Inspect stirrer insert and bearing for excessive wear
11. Reassemble as directed and use sealant as indicated in Figure 1

directed by your instructor.
TR
ASSESSMENT MAINTAIN AND REPAIR N.C. LEVEL : II

ELECTRONICALLY-CONTROLLED
SHEETS CORE MODULE: 2
DOMESTIC APPLIANCES
TAINEE’S NAME ______________________________________
EVALUATOR’S NAME ___________________________________
SELF-ASSESSMENT
DATE _______________
ATTEMPT NO. ________
Instruction: Tick the box for your answer. Ask your instructor for evaluation afterwards.
Instructions: When you are ready to perform this task, ask your instructor to observe the procedure
and complete this form. All items listed under “Process Evaluation” must receive a “Yes” for you to
receive an overall performance evaluation.
PROCESS EVALUATION
EVALUATOR NOTE: Place a check mark in the “Yes” or “No” blanks to designate whether or not
the trainee has satisfactorily achieved each step in this procedure. If the trainee is unable to achieve
this competency, have the trainee review the materials and try again.
The trainee YES NO

1. Checked out proper tools and materials. ______ _______
2. Properly used static protection. ______ _______
3. Checked in/put away tools and materials. ______ _______
4. Cleaned the work area. ______ _______
5. Identified correctly defects/faults in the appliance.
6. Used proper tools correctly. ______ _______
7. Performed steps in a timely manner. (____hrs. ____min. _____sec.) ______ _______
8. Practiced safety rules throughout procedure. ______ _______
9. Provided satisfactory responses to questions asked. ______ _______
EVALUATOR’S COMMENTS:
________________________________________________________________
________________________________________________________________
________________________________________________________________
________________________________________________________________
ASSESSMENT MAINTAIN AND REPAIR N.C. LEVEL : II
ELECTRONICALLY-CONTROLLED
SHEETS CORE MODULE: 2
DOMESTIC APPLIANCES
Tools and Equipment

 Microwave oven
 Star screwdriver
Multimeter
Performance
1. Disassemble completely the microwave oven and label each part and component correctly.
(Time allotted: 30 minutes)
2. Based on your diagram, identify the cause(s) of the following symptoms:
a) When the door opens, the cavity light illuminates but the turntable and blower motor still
operates.
I. ______________________
II. ______________________
III. ______________________
b) At cook condition, the cavity light illuminates, turntable and blower motor operates but
after two minutes the water in the cavity did not heat up.
I. ______________________
II. ______________________
III. ______________________
c) At cook condition all parts and components acted ok, but after 10 minutes the unit
suddenly died. You attempted to power up the unit again but still it’s dead.
I. ______________________
II. ______________________
III. ______________________

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USAT COLLEGE SAGAY CITY, INC.

Sagay City, Negros Occidental

QUALIFICATION TITLE: ELECTRONIC PRODUCTS ASSEMBLY AND

UNIT OF COMPETENCY: Service consumer electronic products

MODULE TITLE: Servicing consumer electronic products

The competency based learning material Provide ELECTRONIC PRODUCTS ASSEMBLY

TO GOD BE THE GLORY!

Read information sheets and complete Self-Checks. Suggested

No. Unit of Competency Module Title

1. Assemble Electronic Products Assembling Electronic Products ELC724335

Service consumer electronic Servicing consumer electronic

Service industrial electronic Servicing industrial electronic

UNIT OF COMPETENCY: Service Consumer Electronic Products and Systems

UNIT MODULE: Servicing Consumer Electronic Products and Systems

NOMINAL DURATION : 80 hours

This module contains information and suggested learning activities on Electronic

This module consists of 5 learning outcomes. Each learning outcome contains

Upon completion of this module, report to your trainer for assessment to

LO3. Diagnose faults and defects of consumer

SUMMARY OF LEARNING LO4. Maintain and repair of consumer electronic

LO1. Prepare unit tools and

LO2. Install consumer electronic

Prepare unit, tools and workplace for installation and service

1. Electrical safety precautions are identified, enumerated and explained correctly

 Learning elements and manuals

Learning Activities Special Instructions

1. Work Safety Requirements  Read Information Sheets 3.1.1

Upon completion of these information sheets, you will be able to:

Know basic Electrical Safety.

 Never use electrical tools on damp ground or around water

Upon completion of these information sheets, you will be able to:

Straightening or setting in order / stabilize (Seiton)

Sweeping or shining or cleanliness / systematic cleaning (Seiso)

Sustaining the discipline or self-discipline (Shitsuke)

Upon completion of these information sheets, you will be able to:

Use Personal Protective Equipment’s (PPE)

: Servicing Consumer Electronic Products and Systems

Learning Outcome #2: Install Consumer Electronic Products Systems

Learning Activities Special Instructions

1. Use and function of tools, equipment  Read Information Sheets 3.2.1

and  Read Information Sheets 3.2.2

Upon completion of these information sheets, you will be able to:

Know use and functions of tools, equipment and testing instruments

Screwdrivers come in several designs, but the standard model

Common screw driver tips

An Adjustable End wrench

Slip-joint pliers allow you to grip objects of varying sizes

Soldering Iron/Soldering Gun

Soldering tools Soldering Gun

EQUIPMENT AND TESTING INSTRUMENTS

Multimeter or Multitester (VOM)

Upon completion of these information sheets, you will be able to:

Operate testing instruments properly

How to use a Multimeter

How to use a digital multimeter

... apart from amps, volts, and ohms, many

1. Turn the meter on

How to use an analogue multimeter

... analogue multimeters have been available for