Gallien-Krueger ML Series Amplifier
Gallien-Krueger ML Series Amplifier
Gallien-Krueger ML Series Amplifier
ML Series
Table of Contents
Operating Instructions
Troubleshooting Instructions
Output Disabling
Preamp Description
Power Supply Start-up
ML/E Turn-On Procedure
ML/S Turn-On Procedure
Schematics
Engineering Change Orders (ECOs)
ML/S Bill of Materials
ML/E Bill of Materials
3
8
10
12
15
19
21
27
38
53
60
Disassembly Procedure
1) Remove the bottom cover first by removing the six # 6 screws on the bottom and the three # 6 screws on each side. One side has an extra
screw which may require a # 4 screwdriver. Note that the center screw on each side is a machine screw and will need to be returned to the
tapped hole in the side of the heat sink when reassembling the unit. On a combo unit, removing only the side screws will allow the
electronics to be lifted away from the speaker compartment. The wires connecting the speaker to the electronics should then be detached
from the power amp.
2a) ML-S: The ML-S preamp removes easily by removing the three # 6 flat head screws attaching the front panel to the heat sink.
2b) ML-E: The ML-E preamp requires the same three screws to be removed. Also, at the rear panel, remove the two # 2 screws that
secure the RFG4 jack and the two # 6 screws holding the balanced outs in place. Using a 15mm nut driver, remove the plastic nuts from the
Stereo Aux. In, Return, and Send jacks. Pull the preamp board out and away from the rear panel being careful not to lose the fiber washers
from the jacks.
3) The two separate parts- the power amp/power supply attached to the heat sink, and the preamp board- can be set on a flat surface with the
component sides facing up and the wires remaining connected. The required troubleshooting can be done.
Problems with the Amp- Where to Look for Help
This short troubleshooting guide will attempt to analyze problems to quickly determine whether preamp, power amp, or power supply related.
Use the power supply/amp troubleshooting guide or the preamp circuit descriptions wherever applicable to fix the problem.
1) Front panel LEDs do not light (no power): Possible blown fuse or other power supply problem; see power supply troubleshooting
procedure.
2) LEDs work but no output from power amp: Could be preamp or power amp problem. First, turn power switch off for 3 seconds, then
back on without input signal to clear shutdown feature. Set all knobs to 10 and put in a signal at 5mVrms. If there is still no output, look
at either end of the red and white wires going between the preamp and power amp with a scope probe. If there is a signal there but no
output, then there is most likely a problem with the power amp. If there is no signal, the problem is in the preamp. Check the wires to
make sure there is a good connection at either end.
3) Distortion at the Output: Follow procedure from #2 above (may want to start with output level on 0 and gradually increase it). This
time, note whether the signal is a clean sine wave like the input, or if it is clipping off part of the signal or has any distortion. If the signal
is clean, the problem is most likely from the power amp. If the signal is cut in half, check the 15V supplies (any LF353- pins 8 & 4 should
have +/- 15VDC). If those supply voltages are not there, or are not clean DC levels, refer to the power supply/amp troubleshooting
procedure. If there isnt a problem with the 15V supplies but the signal is still distorted, the problem is most likely preamp related.
4) Excessive noise or humming: If the problem goes away when the output level is set to 0, it is probably a preamp problem- often a loose
part or bad jack. If it doesnt, the power amp/supply has the problem.
5) Amp works OK except for some individual features: Problem is usually in the preamp, but check the footswitch as well.
Scope
This document lists some steps to follow to quickly troubleshoot the microamplifier series of power supplies and power amps. Only common
failures for Rev. E power amps / supplies will be addressed. Previous power supplies used TIP50 transistors, which have different failure
modes. For more detail, this procedure should be used along with the assembly line power supply / amp start-up procedure, GK Document
#420-0071.
NOTE: The following instructions are given with the heat sink on the bench and the rear panel away from the technician.
Fuse Inspection
Lightly blown- Very often an amp in service has a blown fuse, and is one of the first things a service tech checks. Sometimes, much can be
determined from the condition of a blown fuse. With the amp series, it is possible that the amp is OK and the fuse is the only problem. a
lightly blown fuse may indicate merely an overload condition that did not trip the overload protection circuit, or a weak
Troubleshooting Procedure (contd.)
fuse. In this case, the element is open in the center, and the ends are intact and visible. If it is suspected that the overload circuit was affected,
refer to the test in the start-up procedure. Also, make sure the unit is equipped with a spare fuse.
A failure in either the +15V or 15V supply may also cause a lightly blown fuse. If the amp had been operating normally for some
time, a preamp problem may exist. To check, remove the wires connecting to the preamp and check the power supply waveform. If OK, the
problem may be in the preamp and a prime suspect may be a TL604, if it has one. If a problem still exists, check the 15V supplies on the
power amp board.
Mildly blown- If the fuse is mildly blown, the power amp may be blown. This type of failure will often result in a fuse with a black band in
the center. Refer to Sec. 3.
Violently blown- If the fuse is violently blown, most of the fuse will look black, the glass body may be cracked an break upon removal. This
usually indicates a blown power supply. See Sec. 5.
Power Amp
Caution! - The power amp must always be operated with the outside output transistors, Q824 and Q759, firmly clamped to the heat sink with
screws through the brass heat clips with A06s inside. These latter transistors, Q823 and Q758, provide feedback for the biasing of the outputs.
Failure to clamp these parts may result in thermal runaway, as the bias current becomes several amps and the transistors operate outside their
safe operating area. Mono amps do not have Q759, but the board needs to be positioned at each end anyway. Always screw down all power
transistors to the heat sink when providing output power for any length of time.Also, the ground must be connected, which is completed by
screwing a 4-40 having a captive lock washer into the hole directly between Q824 and Q808. Make sure the lock washer is not touching the
board. Do not use a flat washer. The heat sink for the hole is not anodized.
Ohm Check of Outputs Failed output transistors usually measure zero or low for collector to emitter. This turns out to be the two leftmost
terminals. Check all TIP33s and TIP34s. bad transistors will usually have a cracked or broken plastic body. Replace both outputs if one
measures bad. Usually, transistor replacement is all that is necessary to fix the amp, unless some other problem caused the amp to fail. These
other problems are not within the scope to this document. Note that the outputs can be disengaged by moving the clips near the transistor
terminals. The rule is, the transistor is engaged when the clip is closest to the .33 ohm emitter resistor of the transistor.
Auto Shutoff Feature The unit may be in service to correct the problem of nuisance tripping of the output, usually caused by other
equipment on the same power circuit or by radiating equipment in close proximity. See GK Document #450-0062-01 for an explanation of the
shutdown feature and how to disable it.
Power Supply
Caution! The two power MOSFETs, (BUK456-800A), must be firmly clamped to the heat sink when powering the amplifier. It is OK to run
them without the heat sink under no load conditions.
Visual Inspection Look the board over for obvious problems. Severe power supply failures will sometimes result in burn marks around
R521. If so, R521 and the following may need to be changed: control chip oscillator U506, zener diode D507, and possibly Q515 and R509.
Power MOSFET Ohm Check Power supply failure usually means failure of the power FETs. When failed, they will usually measure short
from drain to source, the two leftmost terminals. Usually the gate measures short to drain and source as well. A good transistor will measure
either open or body diode resistance from drain to source, depending on the orientation of the probes. Always replace both BUKs. Power
supply failure often results when the power amp fails. Simply replace the BUKs when this happens; no setup of the power supply is necessary,
since the characteristics of the transistors are very uniform. When replacing BUKs, cut off some length on the leads and flair the outputs
slightly. Always install the BUKs with the board firmly in place to assure proper alignment of the heat sink holes. When bringing up a power
supply that has a blown power amp, it mat be necessary to disconnect the outputs using the clips mentioned before (see Power Amp-Ohm
check of Outputs).
Oscillator Circuit
Visually inspect U506 for cracks. Whenever replacing IC s, it is always a good idea to install the new piece in a socket. Check that U515 is
OK by measuring diode drops from base to emitter and collector. R509 sometimes opens up under severe power supply failure, check that it is
low ohms. After failed parts are replaced, bring the supply up slowly with the variac as noted in the Start Up Procedure.
Microamplifier Series
Circuit Description of Output Disable
GK Document # 450-0062-01
A. Mihalka, 2-13-91, Rev. 9/10/99-SW
Scope
This document describes the shutoff feature of the microamplifier series. Some boards may be stereo, but all boards work on the same
principle. There are 4 conditions whereby the output is disabled, each of which are described below (see Circuit Descriptions).
Disable Modes
Pop Elimination with Power Switch
The first type of output disabling is for the elimination of speaker pops when the unit is either turned on or off. In the case of turn-on, the
output is clamped for a couple of seconds and then released. At turn-off, the output is disabled and clamped until the entire power supply
decays. This takes about 10 seconds, after which a low thump may be heard if listening close to the speakers.
Output Overload
The second type of output disabling occurs when an unsafe output operating condition exists. A circuit senses overload conditions by
comparing output voltages and current. The unit is able to drive 4ohms per side stereo or 2ohms mono without tripping off. Some conditions
that will cause output disable are shorted cords, blown speakers, heavy loads, and plugging into the output jack while the unit is on. If an
overload condition is detected, the outputs will be clamped indefinitely. In this condition, the LEDs on the front panel will remain on, and the
preamp and its outputs will function, but no output is heard. The power switch must be turned off for 3 seconds before the unit can be turned
back on and again produce output power. Both outputs of a stereo unit are clamped if the condition is detected in one side.
Thermal Shutdown
Finally, if the heat sink becomes too hot the unit will shut off. This is also considered an unsafe operating condition and will disable the
outputs indefinitely. A circuit senses the temperature of the case of the output transistors. Both sides of a stereo unit will be disabled if one
side detects a problem. The power switch must be turned off for 3 seconds before output power is again enabled.
Circuit Descriptions
Output Disabling
The outputs are disabled by Q648 and Q647, an A06 and A56 respectively, which are either both on (disabled) or on (enabled). When on,
these transistors effectively clamp the output drivers. When off during normal operation, the base of Q648 should be low and that of Q647
should be high. The bases are driven by U605 (LM324), a low speed quad op amp. If the unit has no output, U605/7 will be low and U605/14
will be high. If these points are not OK, check the power supplies for U605. Pin 4 should be should be about + 5.3V and pin 11 5.3V. Q722
and Q723 clamp the other side of a stereo unit.
Turn-On Pop
U605 is powered from the rails by 6.8K resistors and 5V zeners. This IC starts operating at about q3 volts. When the unit is powered up, pin 5
follows the + 15V on its way up to 3.14V. Meanwhile, pin 6 is climbing up to 4.8V, but is slowed by C821. Therefore, the output of the op
amp, pin 7, is initially high, disabling the outputs. Note that pin 14 is an inversion of pin 7 and drives the opposite polarity transistor. After a
delay the voltage on pin 6 exceed 3.14V, forcing pin 7 low and enabling the outputs.
Turn-Off Pop
Pin 5 is connected to pin 3 and is 3.14V. Pin 2 is 3.05V. Note that these voltages are obtained with 1% resistors, plus a 470j to give a higher
voltage on pin 3. The output pin 1 is always high when the outputs are enabled, and D802 is back-biased to enable pin 6 to function. When
the unit is turned off, the + 15V supply drops and pin 3 follows, but pin 2 lags because of C803, a 10uF cap. This causes pin 1 to go low,
forward-biasing D802 and pulling down pin 6. Also, R801 provides feedback to clamp pin 3 low and keep
The overload circuit is developed around U604, another slow, quad LM324 op amp. The voltage and current are summed to determine if a safe
operating point exists. The unit is allowed to operate down to 2 ohms total load (4 ohms each side). When an additional 8 ohms is paralleled,
giving 1.6 ohms, after a second or two the unit shuts off indefinitely. This protects the outputs from shorted cabinets and from driving too
many speakers. This condition may also occur while the speaker jacks are moved in and out.
The following circuit works for positive waveforms only. The voltage is obtained from a divider on the output, R784 and R777. Current is
derived from R604 and R605, and is inverted and amplified at U604/1. These signals are now out of phase and subtract from each other at pin
9. When current is too large for a given voltage, pin 9 will tend to go more negative. This is because pin 1 is negative during a positive
waveform excursion, and large loads keep the voltage from going high and demand more current. When pin 9 goes lower than the 2.5 2.65V
reference on pin 10, the output pin 8 will go high. D766 is now forward-biased and starts to charge up C741, a 10uF cap, through R743, a
220K. When this unsafe condition persists long enough to charge C741 and turn on Q742, U605/3 is momentarily pulled down. This causes
U605/1 to go low, and R801, a 100K, provides hysteresis to keep pin 1 low by providing enough current to keep pin 3 lower than pin 2. The
outputs will now be turned off indefinitely. The unit must be turned off for 3 seconds to allow the system to bleed down before output power
can resume.
Since the overload sensing circuit is not made from precision resistors (current sense resistors are 10%) and is somewhat sensitive, occasionally
a 1K resistor must be paralleled with R777. Also the original boards did not have a spot for R741A, which bleeds C741. Without this
bleederC741 may charge up slowly through excess solder flux around the pins of Q742. Whenever output is inhibited for no apparent reason,
check Q742 and find out which of the diodes feeding its base through R743 is responsible.
Thermal Overload
The final method of output disable is provided by U602, which happens to be a fast LF353 dual op amp, although there is nothing fast about
the circuit. Q807 is situated on top of one of the output transistors. As Q807 heats up with the output, pin 2 will drop from about 2.75V. Pin 1
is low until pin 2 drops below the 2.06V reference on pin 3. Pin 1 will go high and stay high enough for the outputs to be disabled by the same
mechanism using Q742 described above. Once again, the LEDs will remain lit and the preamp will continue to function while the output is
off. During this time the heat sink will continue to cool. To resume power at the instant of shutoff, not only must the power switch be turned
off for 3 seconds, but the outputs must cool off as well. Q782 senses the temperature of the other channel in a stereo unit.
Defeating the Disable Circuit
There will be situations where the auto shutoff feature of the microamplifier will be triggered by the noise from other equipment that is either
connected to the same AC line or is operated in close proximity to the amp. An easy way to prevent this circuit from tripping is described.
This method also disables the thermal trip circuit; however the thermal trip circuit does not fire under most operating conditions.
Procedure: Remove the bottom cover of the amp. Look through the side of the amp with the heat sink down and the front panel on the left.
On the left side of the lower circuit board you will see an MPSA06 transistor labeled Q742. Directly to the right of Q742 and also on the board
edge is capacitor C741. To defeat tripping, solder a wire across C741.
Introduction
The following is a brief description of the operation of the circuits found in the ML Series amplifiers. It should provide enough information to
get a basic understanding of the circuits in order to perform any repairs. When calling attention to a component with the use of a reference
designator, the r.d. will be that of the E preamp board. If the two boards (E & S) share the same number then only one number will be shown.
However, if they are different, the S reference designator will be shown in these brackets [ ].
Input Stage
The input stage is very low noise and is built around the JFET-Q9 (J113). In the clean mode it has a gain of just over four and should swing
approximately 22 Vp-p. The 1K trimpot, R5, is used to adjust the DC bias voltage at the source of Q9 which in turn sets the output (drain of
Q9) for the maximum voltage swing. U1 is a unity gain buffer used to drive both the lead and clean channel circuitry. Its output (pin 1)
should be about 2V DC for the maximum possible voltage swing out of Q9. When the channel switch is engaged, the capacitor C15 [C16] is
switched into the circuit. Its presence increases the input gain to nearly six at 100Hz.
Channel Switching
The Channel Select switch, S1, selects the clean channel in the out position and the lead channel in the in position. When out, the C/L line
is grounded and the clean LED lights. This puts a 14Vdc signal on the collector of Q30 [Q26], which in turn shuts off Q20 [Q19]. This gives
the lower gain condition at the input stage. The 14Vdc at the base of Q36 [Q35] turns on the FET Q40 [Q36], grounding out the signal
through the lead channel so it wont bleed through. (The 4.7K resistor, R35 [R41], also keeps the FET on for less noise in the lead channel
when there is nothing plugged in at the input jack.) The C/L line also controls the analog switch control pin, U6-2 [U3-2], that switches
between the clean and lead circuitry. A low voltage there selects pin 4 and clean voicing. When S1 is in, the C/L line gets pulled up to 2Vdc
and selects the switch at pin 3 of U6 [U3] for the lead channel output. This also lights the lead LED and puts a 0.63Vdc signal at the collector
of Q30 [Q26]. This puts the FETs in the opposite state, giving higher gain at the input and letting the signal go through the lead channel.
When the footswitch is inserted, the channel select switch needs to be in the in position so that it is floating. This allows the footswitch to
perform in the same way as the front panel switch did.
Lead Channel
The overdrive of the lead channel is generated by the high gain of three FETs. The first stage, with Q50 [Q48], has a gain of 9 at 100Hz and a
fixed bias of about 7.3Vdc. The next stage, with Q67 [Q59], has a gain of 10 (adjustable with R68 [R70]) and has a bias set around 4.2Vdc
with R66 [R63]. The stage, Q78 [Q69], has a gain of 8 and a bias set around 6Vdc with R87 [R86]. The signal at each of those stages should
clip softly and therefore have rounded rather than sharp corners when driven into the rails. The gain throughout all these stages can be
controlled by the Dual Gain pot, R38 [R29], and the Boost pot, R18 [R17]. The boost pot is normally in the circuit with a 15Vdc signal at U22 (2Vdc with a footswitch), the switch between pin 3 and pin 7 is selected. But it can be bypassed when the control pin U2-2 becomes
grounded via the footswitch and selects the switch between pins 4 and 6. Finally, the Shape control pot, R74 [R61], provides a variable
midrange cut just before the lead channel Master pot.
Clean Channel
The first stage of the clean channel is a notch filter leading into an op-amp, U1 pins 5-7, with a gain of 3. This buffers the signal before it goes
to the clean volume control (assuming the compressor is not selected) and into the FET stage. Q121 [Q87] provides a gain of about 20 and its
drain is biased at about 7.6Vdc. When the signal hits the rails, it should again clip softly as described above.
Compressor (ML-E only)
The compressor circuit provides 2:1 compression and consists of half of U4 and half of U5. Correct operation of a compressor means that as
the input signal level increases, the gain of a circuit decreases, so that a change in level at the input doesnt result in an identical change in level
at the output. Therefore, for a 2:1 compressor, if the input increases by 6dB, the output only increases by 3dB. The attack and release voltages
are set by C106 (335). There is a 1.8Vdc reference voltage that appears at pins 2,3,5,6, and 8 of U5 and pins 2 and 3 at U4. The voltage at U5/
1 is a rectified DC voltage and increases with increasing
Tone Controls
The tone controls are active and provide boost and cut at four preset frequencies. The Hi-Mid and Lo-Mid controls are peaking filters and
provide boost and cut of 16dB at 4kHz and 18dB at 800Hz, respectively. The treble and bass controls are shelving filters. The treble filter
provides a boost and cut of up to 12dB at 10kHz, while the bass gives 15dB or cut at 80Hz.
Footswitch Circuit
The footswitches themselves consist of two (RF2) or four (RFG4) SPST switches each wired across a green LED that has its cathode tied to a
common ground. When attached to the unit through the proper connecting wire, the internal resistors to +15V will light the LED when its
corresponding switch is open. When the switch gets closed, the LED becomes grounded on both sides and no longer lights. The RF2 is meant
for use with the ML-S and controls channel switching and the boost function. It should be connected with a standard 3-conductor stereo
cable. The RFG4 should be connected to the ML-E through a 5 conductor DIN cable. This looks like a standard MIDI cable but it must have
all five wires inside rather than just three, which is very common. The RFG4 controls channel switching, boost/compressor, reverb and chorus.
In both cases, the front panel select switch must be in for the function to work properly from the footswitch. The same goes for the chorus
feature of the RFG4. Further operational information on the footswitches (with the functions they control) is covered in the individual circuit
descriptions described herein.
Effects Loop (and Stereo Aux. in on ML-E)
The effects loop consists of the Send and Return jacks. The return is set up in such a way that plugging into it will disconnect the signal that
was previously going to it from the preamp. The E-preamp has a mono return so that it may later be split into stereo with the use of the stereo
effects (chorus and reverb). If a stereo return is desired, the Stereo Auxiliary In can be used to mix in a stereo signal without breaking the
straight signal coming through. It mixes in at U15, which is where the chorus and reverb signals also get mixed together into stereo. The Spreamp has a Stereo-Mono switch with its return to allow for putting stereo effects directly in line.
Headphone Outputs
The level at the output of the headphone jack is controlled by the Output Level knob. U11 and U12 [U8, 7] are LM386 chips and can provide
about W each into headphones of 8 ohms minimum. The chips run off of ground and 15Vdc so there is a DC voltage of about 7V on pin 5
of each. The LM386s also have a gain of about 20 so the resistor dividers of R182/R179 [R159/R149] and R194/R190 [R151/R150] provide
attenuation of the signal from the output level that makes the headphone outputs clip at about the same time that the power amp reaches full
power into a load.
Chorus (ML-E only)
After the return, the signal goes through a unity gain inverting op amp, U27. R377 and C376 pass the low frequencies straight through to the
summing amplifier U15. The high frequencies are sent, via the filter of C378, R379, and R380, to the chorus input compressor, U14, and the
chorus select analog switch, U16. The compressor and expander around the chorus circuitry provides for low noise operation. The
compressor is built around one half of U14 and works very much like the compressor circuit described above. Pins 11, 12, 14, and 15 should
all have the 1.8Vdc offset. Pin 16 again has a DC voltage that changes with the level of the input signal. The internal op amp is used in this
case so the output is pin 10 and it is biased to 7.3Vdc for operation between ground and +15V. This goes into a 2-pole lowpass filter, with a
cutoff at 3.6kHz, built around U27. The 1N747 diodes limit the signal so that it doesnt overload the Bucket Brigade Delay chip, U24. The
center position of the trim-pot R356 is generally an acceptable DC bias setting of around 6.3Vdc at U24/3, but if the output of U24 appears to
be distorting early, it can be adjusted to eliminate that problem. The MN3101, U25, takes a single 0-to-15V square wave clock pulse from a
LM355 timer chip, U26, and generates the two, out-of-phase clock pulses used by the BBD for its sampling. These square wave signals are 0
to 15V and have a constantly changing period of 34 to 44 microseconds (with depth at max). The voltage that defines the frequency of the
LF355 comes from an oscillator circuit around U10. R172 (rate) adjusts the frequency of the oscillation ad R178 (depth) adjusts the amplitude
of the triangle wave that becomes the control voltage at U26/5. After the signal has been sampled, the delayed version appears at pins 7 & 8 of
U24. The two signals are summed together through R369 and R365 and the must be low-pass-filtered to remove the sampling frequency
element. It then passes
Scope
This procedure is for bringing up Rev. C power supplies and Rev. D power amps for the assembly line. ( It can also be used for
troubleshooting and technical purposes.) This procedure is written with the board oriented in such a way so that the heat sink is on the bench
and the rear panel is facing from the technician, so that the channel on the technicians left is labeled the Right Channel. There are various
combinations of units (heads and combos) that will be turned on with this procedure.
Preparation
Equipment. The equipment list includes a 35MHz oscilloscope connected to the output through the load box, a 100MHz scope and probe for
looking at the supply, a variac w/ current meter, a VOM, a Panasonic VP-7201A oscillator, a load box with toggle switches and monitoring
features for 2 each 4 ohm and 8 ohm loads, and an 8.2 ohm load on a jack.
Line Jumper. For 117V and 100V operation, the jumper on JU528 should be towards the rear panel. For 230V operation, this jumper should
be away from the rear panel.
Transformer Jumpers. For 117V and 230V operation, JU524 and JU527 should be towards the right side of the board. For 100V operation,
they should be towards the left side as noted on the silkscreen.
Power Amp Output Jumpers. The power amp is provided with jumpers that disconnect the output transistor emitters. Move them when
there is a power amp problem- there will be no output current. The unit should then have the emitters connected unless there is a problem. To
disconnect them, move P612A, P655A, and P683A away from the rear panel and P602A towards the center of the board.
Floating Instruments. All instruments, including the oscilloscope, must float with respect to the chassis. Use a three-pronged adapter for this
purpose, and also float the loads.
NOTE: The voltages listed in this procedure are for a 117V ac line.
CAUTION
Primary common (transformer T523/7) floats at 160Vdc with respect to the chassis and must be distinguished from
the secondary common, which is referenced to the chassis. Never look at primary and secondary voltages
simultaneously on the same scope or a scope that is referenced to earth, but always check that loads are floating when
looking at the primary. The following procedures do not require monitoring primary voltages. If it is necessary to do
so, the scope must be floating and referenced to primary common. Do NOT touch the primary, which is 640V square
wave at 100KHz, and readily burns flesh.
Power Supply Start Up
Visual Check
Position the unit with the heat sink on the bench and the rear panel away from you. Check that all the jumpers are in the proper positions, as
described above. Visually check the solder connections near the center of the board. The components to check are (parentheses denote parts
not applicable for mono): Q823, 824, 827, 808, 783, (Q782 and 758), Q759, VR601 and VR602. Also check Q742 for a build-up of solder
flux. Inspect the board edges for holes not filled with solder.
Scope Setup
Connect the 100MHz scope probe to the cathode of D537, which is the secondary for the main negative supply. Hook onto the left leg
between the body of the part and the board. Secondary common is available on the bottom of the rightmost of the two power supply 2200uF
filter caps on the power amp board, or on the topmost pin of the connector between the two boards. Also, the power supply shield and rear
panel may be used for common when the rear panel is assembled. Set the 100MHz scope to 20V/div and 2 usec/div, with zero at the
centerline.
Adjust Reset
Caution: The following procedure calls for increasing the variac from 25Vac to 117Vac in less than 5 seconds. The startup circuit is designed
as such that Q518 turns on when line voltage is below 90V. This causes it to dissipate a large amount of power. Q518 will fail after about 10
seconds if line is set at 50Vac. During normal turn-on, when power is switched with 117Vac applied to the line, the transistor will dissipate
for only a few milliseconds. Set the variac to zero. Connect the power cord and turn power on. Set the voltmeter to the 100V scale. With an
insulating adjuster inserted into R508, slowly raise the variac to 25V. The line current should be less than 200mA. Compare the waveform
with Fig. 1 (As of 9/99- picture available only as poor copy, see accompanying wave form pages- Ed.). The top part of the waveform should
be starting to break over and droop towards the centerline. Adjust R508 until the scope looks like the photo. Counterclockwise increases
droop.
Adjust Period
At this stage the period is set such that the bottom flat part of the waveform is 4usec/ wide, +or-0.2usec. This may cause the period to be about
10.5usec, which can be adjusted by connecting or omitting C503, 504, & 505 with the shorting bock JU502. As a shorting clip is added from
the board edge towards the interior,, the capacitance added to the oscillator increases, and the period will increase. Always turn the unit off
when adding or removing clips. Adjust the reset as above as necessary. The waveform should now be similar to Fig. 2. The period and droop
will be finalized below. Leave the unit on.
IMPORTANT! Only a polyester or polypropylene capacitor may be used for the oscillator, which is used for temperature stability. If it
is necessary, change one of the caps to get a waveform like Fig. 2. However, the caps provided will usually be sufficient.
Continuity Test
With the DVM check that there is a short circuit between the grounding screw and the rear panel. The grounding screw is between Q824 and
Q808, and its washer must be touching the board. If an open circuit is measured, check that the screw hole in the heat sink is not anodized. It
is crucial that this connection is made.
Adjust Bias
With the loads open and the DVM on the 200mV DC scale, adjust R652 so that 8.5mV (+ or 0.5mV) appears between the emitters of Q808
and Q824. (The emitter is the leftmost pin on TIP33 and TIP34). This voltage will increase as the unit warms up, and 8.5mVis recommended
when the unit is cool. For a stereo amp, repeat this procedure with R711, Q759 and Q783. For a mono amp, check above Qs to see if base
connections have been made. Usually, a smaller voltage will appear between the left channel emitters of a mono power amp.
Load Tests
Open Circuit Test
Set the voltmeter to 30V full scale. Set the 35MHz scope to 10 div @ 200usec/div. Connect a 1kHz signal from the oscillator into the Stereo
Line Out (stereo), Right Line Out (mono), or Line In (power amp ML/P). This jack is next to the output jack on the right. Use a mono-tostereo adapter from the oscillator. With the loads open circuited, look at the output signal on each output jack with the 35MHz scope. The
waveform should look sinusoidal at low input levels (-16dB) and distorted at high input levels
Resonance Check
With the loads at 8 ohms and a sine wave input, raise the input signal to 0dB and note that line current is 2A, which should send the output into
the rails. View the cathode of D537 with the 100MHz scope as mentioned above (Scope Setup). The S-curve due to resonant current should
stop about 0.5usec (deadtime) before the waveform rises, as shown in Fig.6. If this is not the case, the period must be readjusted (see Adjust
Period). The condition of not enough deadtime may be the result of a transformer core having too high a relative permeability, which makes
the magnetizing inductance too high. Therefore, the magnetizing current will be less, which will increase the ring-up time. In this case, add
more capacitance. If there is too much deadtime, decrease the period by decreasing capacitance, making sure deadtime is still at a safe level.
This will increase the operating frequency to a value closer to the transformer deign goal. Once the period is adjusted with respect to the
resonance, adjust the droop to 10V above zero. Securely glue the pot (R508).
Thermal Lift
With both sides connected to 8 ohms, set the input signal to be a 6dB, 1kHz sine wave. While observing the left output signal, short JU789
from the left outside pin to the middle with a screwdriver. After a short delay the output should go to zero. For a stereo unit, cycle the power
switch, waiting 2 seconds before turning back on. Repeat the above procedure for the right side output, this time shorting the right side of
JU789 to the center pin.
Overload
NOTE: in the following sections the amp will automatically shut off. To reset, the power switch must be turned off for 2 seconds before the
unit can again produce output when turned on.
Stereo- With the input set to 0dB, set both loads to 4 ohms. The amp should not trip off. Switch in the 8.2ohm load on the left side. The
automatic shutoff circuit should force the output to go to zero after a short delay (a few seconds max.). If this does not happen, R777 must be
adjusted. This component must be decreased in value (from 470 ohms) by paralleling it with either a 2.2k or a 1k resistor. If the unit still stays
on, R777 must be changed to a lower value. Settle on the value that trips first. Repeat the shutoff test on the right side, switching in the
8.2ohm load and adjusting R761 if necessary. Remove the 8.2 ohm load and switch the load box to 8 ohm on either side.
Mono-Remove the right jack from the unit and plug it into the left side of the load box, paralleling the loads. With a 0dB sine wave input and
both loads set to 4 ohms, the unit should not trip off. Switch in the 8.2ohm load. The output should go to zero after a short delay. If this does
not happen, R777 should be adjusted. This may be adjusted by paralleling a 2.2k or a 1k resistor, as noted in the above section. If this doesnt
work, change R777 to a lower value, settling on the value that trips first. Switch loads back to 8 ohms and connect each output jack to a load.
Microamplifier Series-Lead
Standard Preamp Turn-On Procedure
GK Document #420-0073-A
Preamp Board # 206-0073-D- ML/S board
Model #s- ML120S, ML260S-All Options
5/18/90 Rev. 9/15/99-SW
NOTE: Italics refer to stereo power units.
SETUP
Power switch off- Connect power cord.
1) Connect power 3-pin connector from power amp to P1 on board. Colors should read (L-R): Black, Violet, Brown. P1 is furthest to the
back.
2) Connect signal 3-pin connector from power amp to P2. These colors should read (L-R): White, Black, Red. P2 is furthest the front.
3) Connect outputs to load box- Left to Load A, Right to Load B.
4) Resistance loads open (switch in center).
5) Load box to scope-B and AC-VM.
6) Set scope switch on load box to look at load A (down).
7) Probe (1:1) to scope-A and DVM. (No gnd lead required).
8) Connect oscillator to unit.
9) Set oscillator on 100Hz sine wave @ 1.6Vrms (+4dBV).
10) DVM on 20V DC range.
11) AC Voltmeter on 30V range.
12) Scope-A on 5V/cm, scope-B on 10V/cm.
13) Scope sweep on 1ms/cm, scope trigger on A.
14) Output level to 0, switches out.
15) Boost, shape, gain, lead master to 0.
16) Clean volume, tones to 10.
FOOTSWITCH TEST
1)
2)
3)
4)
5)
6)
NOISE TEST
1)
2)
3)
4)
5)
6)
7)
8)
9)
Microamplifier Series-Lead
Effects Preamp Turn-On Procedure
GK Document # 420-0063 A
Model #s: ML120E, ML260E- All options
Board #: 206-0063-E (ML/E preamp)
5/18/90 Rev.9/15/99-SW
NOTE: Italics refer to stereo power units.
SETUP
1) Power switch off- Connect power cord.
2) Connect power 3-pin connector from power amp to P1 on board. The colors should read (bottom-top): Black, Violet, and Brown. P1 is
closest to front.
3) Connect signal 3-pin connector from power amp to P2 on board. The colors should read (bottom-top): White, Black, and Red. The colors
are clearly marked on the board for both connectors.
4) Connect outputs to load box- left to load-A, right to load-B.
5) Resistance loads open (switch in center).
6) Load box to scope-B and AC-VM.
7) Set scope switch on load box to look at load-A (down).
8) Probe (1:1) to scope-A and DVM. (No gnd lead required.)
9) Connect oscillator to input.
10) Oscillator on 100Hz sine wave @ 1.6Vrms (+4dBV).
11) DVM on 20V DC range.
12) AC-VM on 30V range.
13) Scope-A on 5V/cm, Scope-B on 10V/cm.
14) Scope sweep on 1ms/cm, trigger on A.
15) Boost, gain, lead master, shape, and output level to 0.
16) Clean volume, tones 10, all switches out.
17) Reverb, rate, and depth to 0.
Set front panel knobs to center (12 oclock) except: boost, shape reverb, rate and depth to 0, gain to 10, switches out.
Set scope-B to 2V/cm @1ms/cm.
Set oscillator to 100Hz square wave at 46dBV.
Look at output, compare to Fig. 1.
5) One at a time, turn control knobs and compare outputs to figures below (resetting after finishing):
A) Treble to 10 Fig.2; treble to 0- Fig. 3.
B) Hi-mid to 10- Fig.4; hi-mid to 0- Fig.5.
C) Lo-mid to 10- Fig.6; lo-mid to 0- Fig.7
D) Bass to 10- Fig.8; bass to 0- Fig.9.
FOOTSWITCH TEST
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
13)
NOISE TEST
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)