TESTING AND REPAIRING CPRC-26 RADIOS The CPRC-26 VHF radio was used by the Australian Army as a VHF backpack. AWA (Amalgamated Wireless Australia) at their Military Electronics Division in Sydney, had the repair contract from the Army, including sets that were returned from Vietnam. I recently spoke to a technician who worked on these sets in the approximate period from 1964 to 1967. He was working on these sets by himself, and there were others working on other army sets. He would call in the boss or the others if a set proved troublesome. They came in batches, varying from 10 sets to 50 sets. Generally, they were complete with whips, handsets, bags and instruction plates. These were supplied if missing, and the sets left complete, and ready to use, with fresh batteries. He could average 5 repairs a day. TEST EQUIPMENT The test equipment they used was not very specialised. They used an AWA VHF signal generator and an AWA Noise and Distortion meter for testing the receiver. They used a power output meter and a dummy load for testing the transmitter. They had a set of meters on a panel that was used to plug into the radio test socket. There was a bench power supply which was used in place of the battery pack. They had an ordinary Scope soldering iron for heating the modules bases for opening them, but no special tips. They didn't have a CTS-3/PRC test set for testing the modules, or a spectrum analyser for looking the output spectrum and harmonics. FAULTS The most common faults were in IF cans for the receiver and the Modulator cans for the transmitter. Low transmitter output was usually the output valve. Some resistors under the base plate would go high, and sometimes a diode in the discriminator would go out of specification. Failures and problems were usually due to microphonic valves, so the common repair method was to connect up the test equipment and a pair of headphones, and tap or flick the suspected module with your finger, and listen for ringing. The module would be replaced if noisy. Sometimes the module would fail completely after a tap or flick. The isolation of a fault to a particular module was done by trying a suspected module in a working bench radio, and also by using a known good module in the faulty radio. When the radios came in for refurbishing or reconditioning, they were cleaned and checked, and given a new set of modules. Some sets had broken switches, and split cases. He recalls one set that a bullet had entered and was still inside. After repair, the sets were heated in a lamp box, then sealed with a new dessicant inside, and given a final test. A red seal was placed over the case/front panel join, so that any tampering could be detected. Ray Robinson vk2ilv