high volume oscillation - Thomas Vox Cambridge Reverb

[MWaldorf]

I'm back. So, my V1032 Cambridge Reverb is sounding great, with one exception - it goes into oscillation when the volume is turned up. The point at which the oscillation starts varies depending on where I've got the treble control set; high treble settings result in earlier onset of the oscillation.

Troubleshooting so far I've removed the modifications I made (top boost, distortion booster) and the issue is still there.

It sounds like this:
https://www.youtube.com/edit?o=U&video_id=pvWo8CnoE9s

Any thoughts on where to start looking for trouble?

[MWaldorf]

Oops - this is the correct url for the squeal video
https://youtu.be/pvWo8CnoE9s

[MWaldorf]

OK, I found the issue. I got the amp into oscillation and then poked around with a chopstick. I re-flowed almost every solder joint on the pcb and trimmed the lengths of several wires from the board to the controls, all of which made the amp more stable, but it would still oscillate. Then I found the culprit - the reverb send wire. I had inadvertently changed the route of the reverb send wire and grounded it to a different post. Not sure if the issue was the ground location or interaction with any of the circuit it was crossing over, but running the wire along the edge of the board and putting the ground back to the stock location made the oscillation disappear.

So, lessons - make sure to take lots of pictures before and during disassembly. Then make sure to reference said photos when rebuilding.

[Justin Thomas]

I had a similar experience... someone had hacked a Bassman 50 into a Mosquito-Fuzz-Tone (IOW, broken) box and I tried to restore it. Due to semmat least two different schems and no layout and originals hard to find pics of, I inadvertently grounded the Master Volume in a different spot. Wicked attack crunch on every low note, not in a good way, and all kinds of dropouts on the Volume knobs. Finally I started experimenting with where certain things were grounded and found a good spot by mistake.

It's even more of a PITA when it's a new build!!! Thanks for posting the solution, too.

Justin