Manson Ep-925 Modifications: EP-925 and PALSTAR PS-30M

1/16/2021 EP-925 modifications
MANSON EP-925 MODIFICATIONS

Also Conrad, Daiwa, ITG, Palstar, Stabo, Velleman, Voltcraft power supply.
18-jul-2016 The resitor in TR7's collector is not always installed!
EP-925 and PALSTAR PS-30M

On the Internet circulates an unclear copy of this supply's diagram and one is regularly asked for a better drawing. As
owner of this device I also wanted a good readable diagram. So I draw a compact one using the Internet's example but it
turned out the example had a lot of errors. Therefore it was again drawn according to a MANSON ENGINE RING
INDUSTRIAL LTD's Jun 99 redrawn schematic (tnx PAØKV).
Click on Manson for more types of Single Output DC regulated power supply.
The Palstar below is almost similar except two red coloured components.
https://pa0fri.home.xs4all.nl/Diversen/EP925/ep925eng.htm 1/6
SHORTCOMINGS
This power supply («fig) is

also for sale as EP-925 by
Conrad, Daiwa, ITM, Manson,
Palstar, Stabo, Velleman,
Voltcraft and others. The unit
is used in many shacks but it
has some shortcomings, which have resulted in failures.
Fortunately the device can be improved by some
modifications.
OVERVOLTAGE
Remember that a surge protector is not present. If one
transistors (TR1-TR5) blows, the entire unregulated
voltage (~ 24V) is connected to your set and it will
usually not survive. Such protection is still not in my EP-925 installed. You know the routine, the plan exists but it need
not be directly because I have other power supplies available. When use the EP-95 with a transceiver I always connect a
small battery in parallel.
At right (fig.1») is the principle of a (15

V) overvoltage protection system. It is
almost a standard "brute force" circuit
with a Zener diode driving a thyristor,
which short-circuit the overvoltage, a
fuse blew and the set is no longer
energised. Details of such security can be
found on the Internet.
A less crude system (fig.2) is very similar. After turning the main switch and press the start/reset button, the relay
connects and the power supply is on. If the voltage at the terminals is too high, the thyristor is conducting and the voltage
is reduced to a safe value by the large current through the 0.3 Ohm resistor. Simultaneously, the voltage across the relay
is almost zero and disconnects the load. This is not a brute force system and the thyristor is less affected.
G3MWO wrote me: "With the crowbar applied, then there is a path from the auxillary smaller power supply up through
the TIP31C (TR6) and back through the combined path through the base/collector junctions of all the 2N3055 transistors
in parallel" (or via the extra diode #PAØFRI)
In order to protect TR6 install a 3 Amp fast fuse in series with TR6's collector.
BRIDGE RECTIFIER
In my EP-925 only a 20 amperes bridge rectifier (DR1) was
mounted, but the factory fitted also a 25 Amp type.
Both rectifiers are only suitable for continuous use with

about 12 A or 15 A and not to the specifications of a 25 A
continuous current or 30 A peak current. One has
experienced that a sustained large current will develop a
fault.
I did not believe the factories specifications and therefore I

tested the supply thoroughly. The rectifier collapsed, but
that was partly due to lack of cooling. The cause: the
mounting screw and nut were lose and there was no
thermal paste applied between bridge rectifier and heat
sink. It is advisable to check all the fixing and thermal paste
because my transistors were providing with almost nil paste.
After a 50 A rectifier bridge was installed the power supply
specifications were right and the unit has not failed since
1994.
SERIE TRANSISTOR
During the
mentioned test a
2N3055-power
transistor had
gone short circuit
also by lose fixing
and nil paste.
Again, check your
power supply for
essential fixings
and sufficient
thermal paste.
VOLTAGE REMAINS
When the power is switched off, the voltage remains for some time. For a faster discharge a load resistor (fig») was
installed parallel to the unregulated voltage across capacitor (C19).
EXTRA DIODE
If the supply is used for battery charging it may happen that the power is switched off before the battery is removed or
the battery is connected before the unit is on. Temporarily components can have a reverse polarity and failure. A large
diode (fig») mounted from the positive terminal to the plus pole of C19 can avoid this.
DIODE EN EXTRA FUSE

If accidentally the poles of a battery are reversed, diode D8 (1N4001) conducts and not survives. One can installed the
known reverse protection circuit by replacing a stronger diode type (> 20A) and an additional output (25A) fuse. In case
of wrong connection the diode conducts and the fuse blow.
OTHER FAN
The original 12 V fan was too noisy and did not sufficiently cooled at maximum load. That
is mainly because the insufficient mounting of all components. One resistor («fig) was
removed or short circuit and a powerful fan type was replaced: a 24 V PAPST Multifan
8314. Now the airflow is sufficient with less noise.
The resistor in TR7's collector is not always installed in this type of power supply!
G3MWO cured the overheating by running the fan slowly all the time - but also added a
series of 6 mm holes in the bottom of the case just in front of the heatsinks for air flow through the lower heatsinks.
PAØKV's MODIFICATIONS
PAØKV has designed some interesting

changes. He was not satisfied with the
voltage stabilization at maximum load. In
examining the cause he discovered a number
of weaknesses of the design. In his
enthusiasm to proceed, he thought the
changes might be too far. However the result
was a safe, quiet and stable 13.8 V supply
for its transceiver. His findings are given
below
STABILITY
The LM723 (IC1) voltage regulator is not
working properly despite feeding with a
separate winding on the transformer. The output voltage varies considerably due to the heavy load of the system to the
other secondary winding and probably by thin wire on the primary windings.
To improve, he replaced D13 (1N4001) with an additional 24 V stabilizer (7824) mounted on the heat sink and a track was
interrupted. Since C17 was not installed in his EP-925 he mounted a 100 nF capacitor.
The changes are marked in red. The circuit for the fan is simplified.
The circuit of the LM723

(IC1) voltage regulator could
be improved. NPN TR6
(TIP31C) was replaced by a
PNP transistor TIP42C with
increased HFE that contribute
to the stabilization. A track
(fig») below IC1 was cutted.
Further pin 12 (V+) was
connected to 7824, pin 11
(Vc) to the base of TIP42C,
pin 10 (Vout) to the emitter
of TIP42C. Resistor R7 (1.8 k)
replaced by 680 ohms and R8
(22 Ohm) and D5 (1N4001) removed.
The inputs of the LM723 were designed for lower voltages (up to 9 V) to the outputs. He chose for 13.8 V, R14 was
reduced to 680 ohms. He decreased the maximum voltage across VR3 (5 kOhm) to 14 V with a 1.2 kOhm resistor in
parallel of VR3.
SAFE OVERVOLTAGE
For maximum 14 V voltage at the output terminal he applied a 16 V overvoltage
protection. A thyristor (BTV24/1400R) was mounted in parallel with (fig») an
installed bridge rectifier (50 A/120 V). A zener diode (ZD2) and resistor (100
ohms) across the output trigger the action.
FAN
In his opinion the circuit for the fan is overly complicated: a thermostat, two
opamps and a transistor to switch a fan! I am agreeing! The reason is probably
the use of an NC (Normal Closed) thermostat.
The fan was replaced by a PAPST 844414NG and mounted to suck the hot air out
of the casing. Thus reduces noise considerably. To increase the airflow through
the side slits, he sealed the air gaps («fig) of the lid with tape.
The original fan's supply system was

superfluously because the thermostat was
replaced by an N-type (Normal Open) in series
with the fan. With a (fig») 220 Ohm/5 W resistor
parallel to the thermostat the fan runs quietly at
half speed and constantly cooled the heat sinks.
It's never happened that the fan was running at full capacity.
LY3BG MODIFICATIONS
Left: R7 (6k8) is a thermistor, FAN = silent fan; Right: R1-R5 = 0.1 Ohm.

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1/16/2021 EP-925 modifications

MANSON EP-925 MODIFICATIONS

EP-925 and PALSTAR PS-30M

This power supply («fig) is

At right (fig.1») is the principle of a (15

in parallel" (or via the extra diode #PAØFRI)

Both rectifiers are only suitable for continuous use with

I did not believe the factories specifications and therefore I

DIODE EN EXTRA FUSE

PAØKV has designed some interesting

The circuit of the LM723

The original fan's supply system was

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