Thursday, 8 June 2017

PRC-350 Volume Control Mod

One of the problems with the Clansman FM radios, the PRC-350 and -351, is that they have just two audio settings - Whisper (W), and Loud (L). When in W mode, the received audio is attenuated by 20dB, in order that the enemy cant hear your radio. But, at the same time, the microphone gain is also increased by 20dB, so that you can whisper and the enemy cant hear you! All well and good under battlefield conditions, but not terribly good for amateur use! Ideally, what we amateurs want is the mic gain, and hence the Tx deviation, to remain the same, and to be able to change the received volume.

So, being on a short shift today, and having done my good deed for the day dealing with a medical emergency, I decided to use my bit of extra free time this evening to sort this issue out.

The PRC-350 opens up by unfastening four captive hex bolts, and then folds out, taking care with the now very old flexi-connections! The bit I wanted to work on is Assembly 12, which is on the section folded out. Five captive screws hold this board into the case, so had to be undone to lift the board out to access the solder side.

From the service manual, I had found that when W mode is selected, this connects pin 9 of Assembly 12, to ground. This is detailed as the 0VW line. Grounding pin 9 selects a bias capacitor that increased the mic gain by 20dB. A similar line works on the audio amp module for receive, but I didnt want to change that

I first tried cutting the track, but it turned out that the pad to the right is just that, probably for an automatic test jig during manufacture, and the flexi-board connection was made via through hole plating.

So, in order to effect the modification, I got the joint nice and hot and removed the solder with a combination of a solder sucker and desoldering braid. After testing the ensure that the pin was indeed disconnected (with W selected you can do a continuity test to ground, there should be a path from the pad but the pin should be open circuit) I then carefully slid a little sleeving over the pin and into the hole, to insulate it. It can just be seen in the above photo. Finally I wrote the mod and the date on the PCB with an indelible pen.

A bit of adjustment of the audio levels, and I now have a PRC-350 that has the same Tx deviation in both modes, but now the L and W controls are simply a volume control.

The same mod, but I expect slightly different, probably in terms of the connections rather than technique, should also be possible with the PRC-351. That will have to wait for another day.

Ive also bought some superglue today, so could do a bit more work on the 4m transverter now I can glue the pads down! Not much more done this evening, just the Tx IF input attenuator, and a test of the changeover relay.

Next stage requires the positioning of an open air wound transformer - 7t secondary with a 1t primary. The problem I have is that im unsure where the primary should be positioned relative to the secondary!

Amendment - Ive had a few moments spare while my tea settles, to look at the service manual for the PRC-351. Exactly the same method is used in the -351 as the -350. In W mode, a connection to the Tx audio module is grounded. As it happens, it is again pin 9, but in the -351 on Assembly 13.

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