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.