RCA PC41 audio problem

[Phil]

Ok, I'm embarrassed to admit to the cause I found, but I'm posting it in the hope it might help someone else from tearing their hair out in the future.

After going through the audio section, which in this set is split from the video after the tuner mixer, and aligning it with the VA62 (with chip to allow it to operate on the old IF frequencies) and not hearing this with the VA62 it seemed obvious, even to a dummy like me, that the problem was not in the set.

I had been using a bank of Ematic AT103 converter boxes feeding a bank of BT modulators to feed my sets for about 3 years, 24/7 with no problems. Because I am able to pick up more channels here than can be distributed in this way on VHF and thinking it would be good to minimize the wear and tear on 70 year old tuners, I and decided to do away with that setup and instead put a separate converter box on each channel. This also give remote channel change and volume control (as well as "mute"). I made this change during the time I was restoring the 648PV.

The problem in a nut shell turned out to be thus: the audio and video output of the AT103 are clean and works great, the RF output no so much. It is VERY microphonic. Touching the top of the unit, gently, results in quite a sound from the speakers. I tried several of these boxes and found them to all share the same problem. Taking the top off the box and tapping gently at different points showed the problem is apparently not what appears to be the RF modulator section but rather a square chip near the center of the board. Very strange. This box has an on board switching power supply and it seems the sound I was hearing from the set was coming from the mechanical vibration of an inductor/s in the power supply.

Replaced the box with another type and now the 648PV sounds fine with no junk in it at all.

I think the lesson here is: don't assume the source, etc., is ok because it worked in the past, especially if you have made any changes in the way you are using it!