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If you want to 100% verify the reverb circuit is all ok and the issue is the switching, remove Q2 (or de-solder it's leg that connects to R22).
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Originally posted by Enzo
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Yes. Reverb works with Q2 removed.
So, if I understand this correctly. When U3 pin goes negative, that “turns on?” Q2. Sending signal to ground? Like opening a gate? And then of course. What is keeping U3 pin 1 positive?Comment
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Q2 is an n-channel jfet. It is 'on' (low resistance drain to source) in un-powered state. It shorts the signal to ground (kills reverb).
It needs a negative voltage at the gate to turn it off.
According to the conditions at U3 inputs (post #14), the output pin should be negative, but it's not. That's why I wondered about U3 being installed correctly. Maybe there is a solder bridge somewhere between the +15 supply rail and the trace connected to U3 pin 1.Originally posted by Enzo
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No bridges I could find. And the problem existed before the board was ever removed for repairs. The customer says he never uses the switch and is fine with it being on all the time. As much as I’d like to get to the bottom of this, in case any of us ever run into it again. I have way too much time on this amp. And can’t seem to figure out what could be causing this. I thought maybe the foot switch jack possibly. As it looks pretty corroded. But jumping contacts etc on it doesn’t change anything.
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