Mods & Repairs

Icom IC-756 Pro3 Over Voltage Protection Mod

This very simple and inexpensive mod requiring no soldering which may possibly save you frying an expensive radio should your power supply go bad and send too high voltage to the rig.

All that is required for this mod is a Littelfuse 1.5KE15A Transient Suppressor Diode. Basically this diode acts like a zener diode in that it will pass allow a reverse voltage to flow once it’s designed voltage has been reached. This very high current 15-volt zener diode can handle a peak current of over 70 amps, plenty to blow a suitably rated fuse.

The very simplest protection method is using the 1.5KE15A is to place it directly across the the supply voltage using the correct polarity, upstream of the fuse. You must connect the cathode, which is the end marked with the band, to the positive side, and the anode to the negative. This being the method I’ve used here.

To complete this quick mod, remove the top cover of the radio by undoing all the screws on the top and sides of the radio. You will also need to remove the carrying handle.

IC-756 Top internalOnce the top cover is off, you need to remove the screening plate by again undoing all the screws. Be careful when you lift it up as the thin speaker wire may be tucked under the fan bracket. Disconnect the speaker wire by unplugging from the rear of the front panel and place the screening plate aside. You can see the power socket in the middle section of the chassis. The main power feed is taken to screw terminals on the PCB near the rear of the radio. Note these screws will be very tight, so you will need to use a good screwdriver that fits correctly.

IC-756 protection diode installedTo fit the diode in place, simply cut the leads to length and secure using the screw terminals. Make absolutely certain you fit the diode in the correct orientation, and if you remove both power cables, make sure you replace them in their correct positions.

Re-fit the screening plate, not forgetting to reconnect the speaker, replace the top cover and enjoy your radio…

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