Audio "buzz" on a 1948 Motorola VK-101
 

I'm nearly finished refurbishing a Motorola VK-101. At this point, the picture is fantastic. However, I'm having a bit of trouble with the sound. When no video input is connected to the set, the sound is very quiet. No sign of 60 Hz or any other hum or "buzz."

However, when I connect a video source, there is a distinct buzz. It's not horrible, but it would be better if it were not there. To my ear, the pitch of the buzz sounds like 60 Hz.

The loudness of the buzz seems to depend on which video source I use, and even on the content that is displayed on the screen. For example, if a DVD player (via a modulator on channel 3) displays a menu on the screen, that will cause a much louder buzz than when an actual video is playing. I've been using videos of B/W TV programs and movies from the 1950's as test videos.

The buzz goes away if I turn the volume all the way down, or if I turn the contrast all the way down (virtually a black screen). Also, it tends to be drowned out when the sound level of the video is high - i.e. the loudness of the buzz does not appear to depend on the loudness of the sound track. Overall, it seems that the buzz is related to the video, not to the audio, of the input signal.

At this point I'm a bit baffled. I'm not sure if the problem is with the video source(s), or something wrong within the TV itself. As many of you know, the VK-101 has an electrodynamic speaker with a voice coil that is used as a filtering element in one of the two power supply circuits. I have no idea if that's related.

I've used the same video sources with other 1947-48 TVs that I've refurbished, including a Motorola "Goldenview," a Pilot TV-37, and a DuMont "doghouse," and I never noticed this problem before. But perhaps this Motorola design is simply more sensitive than the others to some aspect of the output of a (relatively) modern video source.

Any suggestions would be appreciated!

Thanks.

What effect does the fine tuning have? Have you attempted an alignment?

The sound on these early split carrier sets can be tricky. The owner's manual typically tells you to tune for the best sound rather than best picture.

that is covered here...
https://www.youtube.com/watch?v=mazu7c_nHAg

about 1:30 into the video, intercarrier buzz , common with sets like this, you can never get rid of it completely with modern video.

Try tuning the sound detector coil or transformer ( quad coil) just
a little. Be sure to put a flag on your tool so you can go back to start.
Also if it has an AGC control AKA fringe lock back off on that & see if it
helps. That will cause any set to buzz from too much IF gain.

73 Zeno:smoke:
LFOD !

Often it's the RF modulator that causes the buzz (especially if it's a consumer grade modulator). Many modulators have crude automatic level control, and the carrier gets cascade AM modulated (by video) and FM modulated (by sound). When that's the case the video AM modulator will modulate over 100%. When that happens the RF carrier will change phase 180 degrees(which is BAD). Phase modulation and frequency modulation always happen together and thus will both be demodulated by an FM detector. It creates a 60Hz buzz because video repeats due to frame rate at 60Hz.

A solution (my preferred solution) is to use a professional grade modulator like a Blonder Tongue AM or BAVMz series which has manual video level control that can be set to stop overmodulation of the video carrier.

Wow - that's all really great information and suggestions! Thanks to everyone.

I'll try slightly tweaking the three audio adjustments that can be accessed from the top of the chassis. Unfortunately, the most critical adjustment that sets the carrier to zero volts can only be accessed from underneath. I did do an alignment on both the audio and video before putting it all back together. I don't know why I didn't notice the buzz before, but I think that most of the time I was working on bigger problems. Since I've already aligned the audio once, and since most of you said that intermodulation buzz can be almost impossible to eliminate, I don't think it's worth pulling the set back out of the cabinet and putting it back on its side.

By the way, yes adjusting the fine tune does help. I can find a spot where the buzz is minimized, but not fully eliminated. It becomes imperceptible when the picture is mostly dark, and noticeable when the picture is bright - which fits exactly with everything you all said.

I've collected '40's TV's for many decades and have learned to accept their quirks. Audio buzz is one of the most common. Usually it occurs when there's a lot of bright white in the picture. I just live with it.

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

Motorola was among the first adopters of intercarrier sound. I don't think Motorola ever made a split sound set post WWII.
A good contrast between RCA and Motorola is to compare their 630 to The VT-71. RCA was built to perform and be stable cost and parts count be damned, Motorola was built to have the lowest practical parts count that could perform stably. A similar comparison could be drawn between a CTC17 and a Motorola with a SODPIL single tube chroma circuit.... Only the RCA was less reliable due to PCBs and the Motorola wasn't as stable or good performing from asking a heck of a lot from that one tube....

The VK101 is their first postwar console set and it is split carrier. It's a robust design with two power transformers. One for "high" B+ and one for "low" B+ rather than use huge voltage divider power resistors like RCA.
Technical info here: https://www.earlytelevision.org/pdf/...sams_51-14.pdf

Yes, I was fortunate enough to be able to purchase an original Sam's Photo Facts for the set on eBay. I scanned it at 600 dpi and forwarded a copy to the early television museum. That's the copy that you see when you click on the link you provided. And yes, I've been up to my elbows inside that set for the past couple of months, so I've become very familiar with the dual transformers, indeed dual power supplies. Now that you mention it, yes there are no power resistors, ballast, etc. What I found especially remarkable was the interaction between the TV and the radio. The only power supplies are in the TV, but the control is largely in the radio. Most of the power generated by the TV is routed to the radio.

When switched to radio mode (by a switch on the radio), the power stays there and powers the radio, while only one of the power tubes and the audio output tube are powered in the TV. The radio "plays" through the final stage of the TV audio channel. When in TV mode, of course, all of the power is routed from the radio back to the TV.

By the way, now I'm curious. I think all 5 of the TV's that I've worked on so far have been of the intermodulation type. I never heard of "split sound" before. How does that work? Is "split carrier" the same as "split sound"? In the VT-101, the sound is split off of the signal before the second IF amplifier. Is that what "split carrier" means?

It means the sound has it's own IF stages that have to be precisely aligned to the video IF stages. Otherwise, you don't get the best picture and sound at the same point in the fine tuning.

With intermodulation the sound and video share the same IF with the sound being picked off after the video detector. Best video and sound tuning are always in sync.

Might be well to clarify that only the sound IFs should be touched. There was a feller on here several years ago who was trying to align the video IFs to match the sound. Set was an early DuMont, IIRC.

Good point. Typically the sound IF has a narrow pass band around 21.25 MHz. While the video IF has a wide pass band around 22-26 MHz

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.

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.

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.

audio buzz on a split sound set
 

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.

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.

Loose coupling the set
 

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

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

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.

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.

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!

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!

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.

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.

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

That's a great idea - thanks!