Audio "buzz" on a 1948 Motorola VK-101

[Resonance1]

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.

[bandersen]

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.

[Yamamaya42]

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.

[zeno]

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
LFOD !

[Electronic M]

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.

[Resonance1]

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.

[decojoe67]

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.

[old_coot88]

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

[Electronic M]

Originally Posted by old_coot88

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....

[bandersen]

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

[Resonance1]

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.

[Resonance1]

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?

[bandersen]

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.

[old_coot88]

Originally Posted by bandersen

It means the sound has it's own IF stages that have to be precisely aligned to the video IF stages.

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.

[bandersen]

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