ICOM PS-125 modification
Several amateurs have experienced ICOM PS-125 Power supply as very noisy and will never let the equipment remain in their shack - it is designed in an awkward way caused by the way to control the fan.
A temperature measuring circuit starts the fan at approx. 50 deg. and switch off at approx. 40 deg. The fan has two modes on and off - when it's on the voltage over the fan is 13V and it's not quiet but rather noisy.
My proposal is to let the fan run with a lower voltage 6 - 7 volt all the time except when the temperature measuring circuit takes over (high temp inside the unit).
OBS: Remove mains cable from the unit!
To disassemble the cabinet:
Remove the 10 black screws and the 2 black cover parts.
Remove the aluminum cover with ventilation holes - 12 self tapping screws.
Loosen front and rear - 2 x 3 screws.
Remove 4 screws fastening the PCB then you can carefully tilt it 7 - 8 mm to make better room for soldering the zener.
The operation is very simple: connect a 6V2/1W zener over the switch transistor Q14 - voila!
Now the fan will run very quiet and let a mild breeze flow through the unit - you can hardly hear it.
You will find Q14 close to the connector for the fan wires (red/black).
Connect the zener cathode to Q14's collector or the fan connector CN15, pin 3 (black wire to fan) and the zener anode to ground (ground terminal in the PCB corner).
Check the circuit for short-circuits and remount all the disassembled parts in opposite order.
OBS: Lethal voltages inside the unit.
Connect the unit to mains and enjoy the quiet fan and the breeze through the unit.
During normal SSB-service the cabinet temperature now is between 28 and 32 deg. C. and the fan runs never at high speed.
In a 15 minute test I have used a IC-756PROIII transmitting 100 W FM as load without the fan was switched to highest speed.
ICOM PS-125 PSU - a part of the revised diagram
ICOM PS-125 PSU - the surroundings of Q14
Updated: March 1. 2012