Audio "buzz" on a 1948 Motorola VK-101

[Resonance1]

So, it sounds like the VT-101 is NOT an "intermodulation" design, because it does have a separate chain of IF amplifier stages for the audio. So, what is the buzz that I'm hearing. Is it still "intermodulation" buzz. It has all the symptoms, including almost disappearing during dark scenes and being loudest when the screen is mostly white.

[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.

[old_coot88]

Might try reducing signal strength by loose-coupling to the antenna input, i.e., no direct electrical connection. Reduce to just above where snow appears in the pic and note if buzz remains.

[Resonance1]

Originally Posted by old_coot88

Kinda surprizing this has intercarrier sound, considering it had just barely been invented in 1948.

OK, so this set is clearly split sound, because it has a separate chain of amplifiers and filters for the audio. Given that, does it still make sense that the buzz is "intermodulation buzz" as described above? It has all the symptoms, including being much louder when the screen is mostly white and much softer when the screen is mostly dark.

[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.

[Resonance1]

Originally Posted by old_coot88

Might try reducing signal strength by loose-coupling to the antenna input, i.e., no direct electrical connection. Reduce to just above where snow appears in the pic and note if buzz remains.

That's great idea, but I haven't done it before. How exactly do you "loose couple" the antenna input?

[Tim]

Just means put the antenna wire near the connections but not actually connected.

[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 Tim

Just means put the antenna wire near the connections but not actually connected.

Hi Tim,

I'm using a DVD player through a modulator and a balun to feed the video signal into the set. I tried disconnecting the balun from the set and just holding the leads close to the antenna screws, but no picture. I tried connecting both the converter and the balun to two different rabbit ear antennae, and putting them close together, but still no signal. Do you have a suggestion as to how i can loose couple the video into the set?

By the way, disconnecting only one of the balun wires makes it buzz like crazy, even with the modulator disconnected. No buzz (without video signal) if both are connected or both are disconnected. Don't know if that means anything.

[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!

[Resonance1]

OK, tested all of the video IF tubes - they all test good! May have to just live with the buzz. Thanks everyone for all your help!

[Resonance1]

Quick post-script. In my basement I stumbled across a modulator that I didn't even know I had - an RCA model CRF907R. Simple and compact. Must have purchased it many years ago. So, I tried it with the TV, and the buzz is almost gone. The audio is a bit quieter - I find myself turning the volume almost all the way up - but the buzz is noticeably reduced, almost totally gone.

I have ordered a few more of these units on eBay, basically new/old stock. They seem to be readily available online. So, if anyone else faces this problem, I recommend trying the RCA CRF907R modulator.

[Electronic M]

The buzz is caused by the video going into the modulator not the audio so you probably could build or buy a small flat response audio preamp before the modulator to get louder audio.

[Dude111]

Originally Posted by Electronic M

Often it's the RF modulator that causes the buzz (especially if it's a consumer grade modulator).

Yup that was gonna be my guess seeing as when no source is connected the buzz isnt there....

[Resonance1]

Originally Posted by Electronic M

The buzz is caused by the video going into the modulator not the audio so you probably could build or buy a small flat response audio preamp before the modulator to get louder audio.

That's a great idea - thanks!