[murf]

HI guy's,
Have not been on here for a while, so its about time I bugged some of you zenith fans.
Picked up a 1941 10 tube console that had been restored around the year 2000 by a pro.
He did a very nice clean looking job and did not miss anything.
He even put a cord on it so you can play your music from another source.
Question is, I noticed a low frequency humm that got louder as the volume increases.
Whether on radio or Ipod hookup.
First checked tubes and replaced a couple 6V6 tubes that showed marginal.
No difference.
Next, replaced the filter caps thinking maybe one is out of spec.
No difference.
I did notice that I could almost eliminate the humm by pushing in the lower right bass tone control.
Could there be something going on in the tone control circuit that could cause this?
Otherwise, it is in immaculate condition.
murf

[tbone]

If I recall correctly, at least one of the wires in the harness for the buttons is shielded. I believe there is a wire that solders to that shield and grounds to the button assembly frame under a mounting screw. Give it a look over. Could be a bad ground where the wire is grounded under the chassis.

If it worked fine when you first had it, that would in theory rule out the external audio connection.

Good of a place as any to start.

[Phlogiston]

If it gets louder as you increase the vol on the set (not changing with input, MP3, vol changes) then yeah, it is probably picking up the hum in the audio stages, could be those tone controls and yeah I think that at lest one is shielded - green wire, I think. Many of these issues can be found by pushing a few wires and such around with your hand, but be careful.

You could use the MP3 input (switched?) and even ground (short) the input so that you get no nasty ,real, input to distract from your noise. Then you can also turn the volume up to make hearing changes easier.

I just pulled the tone/on/off button assembly out of my 12-S-568 (1941) and see that I replaced the shield that goes over all but the 2 power/switch wires. The button assembly is grounded to the shield. You could also have one of the old, crusty wires shorting to the shield, or even a new, not so crusty wire doing so. These switch assemblies can be a dirty contact bonanza.

Other than an induced hum, the other possibility is increased loading of the power supply, beyond what would be normal for increased vol (minimal difference). But the most common - bad bias on the output tube, should be constant and unchanging with vol.

Using a signal sniffer might help but they can induce hum as well.
Russ