RCA PC41 audio problem

[Phil]

I have been working on this 9PC41 off and on for some time now. The video isn't too bad but I have an audio problem which is very annoying. The audio has a squeal somewhere I would say around 3 kHz that is loud enough to make watching it for any length of time impossible. I've replaced all of the bypass caps, 1500pf with no change. It was my first guess the IF was oscillating but this doesn't seem to be the case. Tuning the IF doesn't effect the pitch although it does change the amplitude. The audio from the TV goes through the radio chassis for tone control and shares the same amplifier chassis as the radio. The radio sounds great.
Anyone have any ideas? Thanks

[Electronic M]

There are a number of possibilities, but it would be easier to isolate the source by trouble shooting.

Try shorting grids of each stage from the first sound IF through to the last AF to ground 1 at a time. If killing signal at the 1st IF grid leaves the squeal the problem is in the 2nd IF or later, but if the short kills the squeal then it is in the signal feeding the 1st IF grid..and that same logic follows down the signal chain.If you can pin it down to a stage it should be easier to fix.

[Penthode]

Does the volume control affect the squeal? Argh are you randomly playing with the audio IF alignment?

3kHz sounds pretty low and if it was an rf/if oscillation or heterodyne, it would likely vary in pitch and be unstable.

Sounds like acoustical feedback through the tuner local oscillator. Make sure the lead damper tube shield is around the 6J6 oscillator tube.

[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!

[kvflyer]

Originally Posted by Phil

Ok, I'm embarrassed...
I think the lesson here is: don't assume the source, etc.,

Thanks for telling us the problem. What was told to me many years ago was:

"When you assume, you make an "Ass" out of "U" and "me". Been there all too many times...