Help troubleshooting hum/buzz in Ampeg GV-15

[oldmacman]

Hi all,

A while back, I got an old Ampeg GV-15 chassis for cheap, not working. The power and output transformers had been replaced, and the amp was in pretty bad shape, with rust and nicotine stains all over. I cleaned it up and replaced the electrolytic caps and a couple of bad resistors, and it's now working, including the reverb/trem. However, there's a hum/buzz that I can't seem to get rid of. It gets louder when I turn up the volume control on each channel, even when nothing is plugged in. Looking at the schematic (https://ampeg.com/support/files/Schemat ... ematic.pdf), there's not much going on before the volume control -- only a single gain stage -- so I'm wondering if something else in the circuit is arcing/buzzing and it's being picked up by that circuitry.

Short of removing/measuring and replacing all the resistors and capacitors in the circuit, is there an easier way to isolate where this hum/buzz might be coming from?

Thanks!

[martin manning]

Short of removing/measuring and replacing all the resistors and capacitors in the circuit, is there an easier way to isolate where this hum/buzz might be coming from?

Are the input grounds all clean and solid?

[oldmacman]

It ended up being the input grounds, but in a different way than I had thought. They were all solid, with a star washer. But the grounding of the aluminum front panel to the steel chassis was not good -- it had oxidized and developed some resistance. So I took the front panel off, sanded the mating surfaces with 400-grit sandpaper, cleaned it with IPA, and reassembled. Success -- the buzz is much improved!

[brewdude]

I’m surprised that a face plate’s grounding would make difference. (?)
Many amps use plexiglass. Train Wrecks use wood.

Or, did I miss understand?

[pompeiisneaks]

In some builds the jacks and the pots get their ground from the compression between the body and the faceplate/front of chassis.

This corrosion is why I always just create a separate grounding buss bar that goes across all the front panel pots and directly connects near the input jacks so I have a physically hard wired connection to chassis and corrosion on the front panel can't interrupt the connections.

~Phil