Champion 30 Reverb Problem

[benoit]

A friend of mine brought over his Fender Champion 30 solid state combo. Dirty/clean channels, spring reverb. The amp is fine (for what it is) except on 1 or 2 notes (the main culprit is D or G string, 12th fret, can't remember which) the reverb does this weird oscillation. When I took the chassis out of the cab and connected the speaker out to the 15" speaker on my bassman combo the problem went away. I'm guessing it has to be vibration at a certain freq. from the speaker, travelling through the cab, and screwing with the tank, which is mounted in the chassis. It appears to be "isolated" with a bit of foam, which is actually the only thing attaching it to the chassis. Anyone have bright ideas? Schem is here: http://www.fender.com/support/amp_schem ... c_-61F.pdf though I highly doubt it is relevant. I could pull the tank off and remount it, I could try moving it out of the chassis, or I could just build him a plywood head cab and put a speaker jack on the combo cab to make it just a seperate speaker cab. Thoughts?