Ampeg VT-40 woes

[Mars Amp Repair]

Hello folks,
I am working on a very tired V-40 that has a weird standby switch issue. When the unit is in standby, the B+ from the bridge into the unit is still present & the amp buzzes badly. In this model, the standby switch switches the ground to the bridge rect instead of the usual positive output. I've worked on many of these amps & know this arrangement obviously works.

http://www.drtube.com/schematics/ampeg/vt40pwr-jp.gif

I've tried to isolate any possible other path to ground around the switch & I cannot find one. there is no low resistance from the bridge side of the stby switch to ground (chassis) that I can find. BTW, the entire ground side of the power supply terminates to the chassis at the input jacks.

The diode in line with the standby switch I'm guessing is for isolation to allow the neon stby light to function properly.

When the unit is in stby, the B+ voltage with respect to chassis ground remains about at what it is with the unit in operate mode. I wish I had another of these amps to do some A-B comparison.

BTW, this amp had some water damage on the edge of the main pcb, but I've cleaned all that off & am fairly confident that there are not paths to ground there...esp a high current one!

Anyone else ever have an issue like this? thanx, glen

[Robert M. Martinelli]

Hi Glen,
weird st-by circuit indeed!
I would try to identify where the "0 potential" manages to flow starting from the rectifier, try to disconnect the "0" at the two negs on the diodes, if the buzz' s still there I would try to disconnect one by one all the points around the power supply, including the common lug on the ground selector switch. Seems like some gnd potential manages to make it to the 7027s, maybe it' s spurious but enough to cause the buzz....
Oh, I was forgetting... don' t overlook caps, maybe you' ve got a leaky one around!

Hope this helps
Good luck
Bob

[Mars Amp Repair]

Update,
Always nice to get away from a troublesome repair & come back to it & put your finger right on the problem.
Turns out the weird buzzing & such was coming from the first 2 stages. upon further disassembly, it turns out the rocker switches had moisture residue inthem that was causing weird grounding of things that shouldn't be grounded.

cleaned all that crap off & voila...no more weird buzzing in standby. Thanx, glen

[Enzo]

Is there still excess background hum?

I found them wired with teh switch going to the pc board with a 4-5' piece of wire from the board down to the filter cap/CT ground. Moving the return wire from the switch over to the othe end of that wire at the CT killed that residual hum.

[Mars Amp Repair]

Cool,
thanx for the info. That's just the kind of stuff that can take forever to discover...glen