Audio "buzz" on a 1948 Motorola VK-101

[Resonance1]

Originally Posted by old_tv_nut

Wild guess:

Does the buzz change when you adjust contrast? I wonder if one of the video IF tubes is weak, causing you to set the first video/audio bias (contrast control) to a strange level to get a picture. If you can get non-buzzy sound with the contrast turned down, then maybe tube replacement or swapping around some of the 6AG5's may have an effect.

No, turning down the contrast does not reduce the buzz, until it is nearly all the way down and the picture is basically gone. But at any normal level, the contrast does not seem to affect it.

[old_tv_nut]

Originally Posted by Resonance1

No, turning down the contrast does not reduce the buzz, until it is nearly all the way down and the picture is basically gone. But at any normal level, the contrast does not seem to affect it.

The question was, is the sound good at any contrast setting, even so low that the picture disappears. If both sound and picture fail at low setting, then that is just too low a signal. But if the sound is good (buzz gone) when the contrast is very low, that could be an indication of an intermodulation problem in the common stages that carry both.

Are you able to test the video IF tubes or substitute good ones? What I was guessing at is: maybe one of the IF tubes is bad, causing the contrast to be set to increase the gain in the others to the point where that first one that carries both sound and video is overloaded.

[Resonance1]

Originally Posted by old_tv_nut

The question was, is the sound good at any contrast setting, even so low that the picture disappears. If both sound and picture fail at low setting, then that is just too low a signal. But if the sound is good (buzz gone) when the contrast is very low, that could be an indication of an intermodulation problem in the common stages that carry both.

Are you able to test the video IF tubes or substitute good ones? What I was guessing at is: maybe one of the IF tubes is bad, causing the contrast to be set to increase the gain in the others to the point where that first one that carries both sound and video is overloaded.

I just tried it, and the buzz is steady as long as the picture is there, but fades away together with the picture as I continue to reduce the contrast.

I have a tube tester and I can certainly re-test the video IF tubes. I'll give that a try.

Thanks!