Ampeg VT-120 Restoration / Repair - hum on channel C

[q9522678]

Hi g1,

No, all controls are at zero across the board.

Are you suggesting channel C, without a master, is equivalent to Channels A and B with master dimed? If so that would make the channels sound more “balanced” in terms of hum!

[g1]

When you are comparing, you have the master set full up?

[q9522678]

Hi All

Despite telling myself that my time is better spent playing Guitar rather than fixing amps after getting the Silvertone 1484 in working order (still going strong!) I managed to come by a USA built 1990 Ampeg VT-120 (inspired by Mesa Mk III) that needed some love. Unfortunately not a lot available online about these and people seems to trash them rather than try to repair them. Seems a pretty good amp in terms of specs although it weighs a tone.

The amp wasn’t working at all when I got it, but after replacing all electrolytics throughout, some burnt dropping resistors, bleeder resistor and screen resistors (with higher spec ones), resoldering every single joint in it and giving it a whole new tube complement it came back to life. The amp is a three channel amp with Channel C being clean, channel B slightly less clean and channel A being a full on gain channel. Appears to be not a lot of difference between channel B and C In terms of signal path.

What’s got me stumped is that the higher gain channels are whisper quiet with nothing connected and controls turned down. However channel C has a constant hum that is loud enough to be annoying.

Channel C and B are very similar and use much of the same circuitry. Channel B audio runs through a resistor and capacitor that Channel A does not. It also runs through a master volume and an additional optcoupler (same with channel A)- was thinking maybe this is what is making this channel quieter than channel C. Perhaps OC4 (and OC5 for channel A) are simply acting like noise gates and filtering the noise out? That being said, I can’t see these being this noisy from the factory. The hum is loud enough to suggest something is still amiss.

Also, the hum doesn’t change volume or sound with changing any of the controls and only affects channel C, which theoretically should narrow things down to those bits unique to channel C. I’ve also tried moving preamp Tubes around. No change (not was I expecting there to be). I thought if it were a grounding issue than it would surely carry through to Channel B and potentially channel A.

I have to say this is the first time I’ve experienced the clean channel being noisier than the dirty channels! Not something I’m going to die in a ditch over as can simply use channels A and B which are super quiet and functional, but open to any suggestions in terms of what this might be. Was hoping it was something quite simple (famous last words).

Schematic for the amp is here: https://ampeg.com/support/files/Sche...SCHEMATICS.pdf

Also found a very handy layout that had been marked up by hand in terms of components.

Was thinking coupling caps - but can’t see any DC where it shouldn’t be - and Channels B and C share the same caps and tubes. So would have thought any noise present on Channel C would be present on Channel B (and A).

I’m not familiar with Optocouplers but perhaps OC1 on channel C is not functioning appropriately (assuming these are installed to help filter out noise). Not sure how I would go about testing this. A quick poke with an audio probe shows the noise present through OC1, OC2 and OC3, which further confused me. At which point I decided post here.

Would appreciate any thought or insights on what might be the cause.

Thanks in advance!