Hum/Noise - hoping for some suggestions

[dnm]

Some hum / noise occurs when the amp is switched from "Flat" to either of the scoop settings on the scoop control.    Quiet when the scoop control is switched to "flat."  Its not real loud, but it is there.  I reflowed the solder joints on the scoop switch and what looks to be the ground path (black wires) back to the brake control along the terminal strips trying to make sure grounding in the input area is all good,  but the noise when the amp is switched off of "flat" is still there.   

It seems to acquire the hum from the switch area, because when I move my hand near the switch (from the back of the front plate) the volume of the hum increases.  But, that could just be because that is the area where the overall input to the amp is.

If I touch the panel, the noise is suppressed considerably, but it is still perceptibly louder when the scoop is switched from "flat" to either of the two other settings.   Also, the hum is the same whether it is EFish or EMish.

Chopstick test - probing/touching the circuit anywhere with a dowel doesn't have any effect, except that the amp does produce some slight thump and a bit of crackling when feedback is at its maximum and the brake is wide open when I tap the shielded lead from 15/16 to the EF 86.   The lead seems almost microphonic, is that normal?.

Thoughts anyone?   Thanks!

[Doc B.]

I think it's a bad switch. We can send another.

[Paul Birkeland]

Did you spray any coatings on the panel?  If touching the panel changes the noise characteristics, that suggests that the plate grounding isn't so stellar.

[dnm]

Hello, PB, and thanks!    No, the plates were kept completely clean.   Plus, this happens when the scoop control  is switched to put the caps/resistors in the filters in line with the input.   When set to flat, the input is just seeing a straight shot into the input of the EF86, and the thing stays quiet.  Thinking about testing the switch by bridging it. Not sure that continutity tests will work with the switch wired in now.   Maybe the connection to ground in the switch at the "lower" part of the filter isn't closing in when the scoop is set to EM or EF-ish.  Thanks again!

[dnm]

Still working this.  

A few voltages deviate as follows:

Back Chassis:
Terminal 11:  345v vs. 325

Front Chassis
Terminal 2:   28v vs. 35
Terminal 5:   29v vs. 37
Terminal 10:  28v vs. 36
Terminal 11:  343 vs. 325
Terminal 12:  317 vs. 293
Terminal 14:  228 vs. 215
Terminal 22: 315 vs. 295
Terminal 24:  19 vs. 30


Mains voltage: 120 Vac

2,5,10, and 24 seem too far out of spec to be right.   Thoughts?  

In the meantime, I'll be trying to annotate the schematic with pin numbers and voltages to try to get a sense of where electrically things could be out of kilter.

Thanks!

[Doc B.]

I don't think those voltages indicate anything serious. The tubes need to cook in a while and voltages may shift a bit as they do. I still think that the switch itself or the lead dress around it is probably the where the noise issue when the mid scoop is switched in lies.