GRC-9 BIAS BATTERY REPLACEMENT

The AN/GRC-9 is a military transmitter and receiver from the 1950s. It is used for portable communication in the 2 to 12 mhz frequencies, and was used as a base station or a jeep radio. The receiver and the transmitter are separate units in a common case. They both use valves. The receiver has a 3Q4 audio output valve with 4.5 volts grid bias coming from an internal dry battery. The battery will need replacement, or the audio valve will draw excessive current, and produce distortion and shorten the life of the valve. The battery plugs in using a 2 pin battery plug, and is secured by a clamp. The bias current is very small, and the battery needs replacement, not because it goes flat, but because it reaches the end of its shelf life.

Photo of GRC-9 receiver (new battery fitted)

My radio had an incorrect dry battery fitted, with 2 pins soldered to an incorrect battery, covered in insulation tape and loosely pushed into the connector.

Photo of old and new battery.

I found a battery plug, and glued a plastic tube to it. I dismantled an AA cell type battery holder, and removed the positive terminal contact and the negative spring contact. I soldered 2 wires to these, and soldered them to the plug terminals inside the tube. The big pin is positive. I placed the positive contact at the bottom of the tube, suitabley insulated, then placed 3 hearing aid cells inside the tube, to give approximately 4.5 volts. I placed the spring as the top contact, then glued a fibre washer to the top, to hold it all together. It was clamped while the epoxy glue cured. The battery voltage was checked, then it was plugged into the radio and the clamp tightened. Simple, looks the part and works fine.

Photo of GRC-9 and new battery

This radio was written up by Mike Hoddy G0JXX in the VMARS Newsletter Issue 32 December 2003, and he describes the bias battery.

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