ICOM IC-756PRO3 modification

Since late 2006, I have been a very satisfied owner of an IC-756PRO3.
During the time, I have observed a smaller irritating thing - the surface temperature of the unit is too high after half an hour in receive mode.
For this reason, I will suggest a minor mod to the fan control circuit located at the PA board.
During receive mode the fan normally is switched off or is operational if the inside temperature is too high (fan control decision).
My suggestion is to supply the fan with a low voltage (app. 6 volt) during its off period.
OBS: The internal fan control will of course control the fan as before this modification.

Opening of the unit:

Remove 11 black screws from the top cover; 2 from the handle and 11 shining screws in the grey chassis cover; now you can locate the PA most left in the frame.

PA with fan control

Power amplifier with fan control circuit which is located at the bottom left for center.

Fan control

In the middle fan control

8V2/1,3W zener

The only component for the modification, an 8V2/1.3W zener diode.
OBS: To day, I will use a 7V5/1,3W zener diode instead!

Fan control with the zener

Fan control with the zener diode


Macro of the zener

Fan control schematic

Fan control schematic with my modification upper right.

Check circuit for possible short circuits and you are ready for testing the circuit. Switch on power and enjoy a silent fan and a mild breeze through the cabinet.

Switch off power to the unit!

1. Remount the grey chassis cover
2. Arrange the loudspeaker cable as earlier
3. Remount the top cover and handle.

How simple!

Good luck with the mod

mail link

Revised March 28, 2009