I got a Peavey Classic 30 off eBay. It sounds good except for the reverb. When I turn the reverb up, there's a hum. If the reverb is up and I turn off the power, there's a pop in the speaker. The higher the reverb, the louder the pop on power-off.

Reverb does work. I do hear the effect, but I also hear hum. I took the reverb tank out and inspected it. There are no broken connections. Instead of the RCA connectors I expected to find, the coaxial wires are soldered to the connectors. Because of this, I can't easily bypass the reverb tank to test.

Anecdotal advice has suggested there may be a bad electrolytic capacitor on the circuit board. I pulled the circuit board and didn't find any obvious evidence of leaking. I don't have a capacitor tester, but unless I remove the capacitors, I would need an ESR meter to test them in-circuit. It would be easier to just replace all the caps and not look back, but I'm not sure the symptoms indicates a bad cap.

I understand hum can be a result of the AC filter caps going bad, but the amp is very quiet with the reverb off. When the gain is all the way down, turning only the reverb up makes it hum.

If I insert a 1/4" plug into the EFX Return, the hum goes away. Plugging into this socket opens the EFX circuit (when the circuit is not completed back to the EFX Send).

Searching turned up fixes to AC hum by replacing filter caps, but I don't have hum except on reverb. Reverb fix results were broken wires in the tank, and covering the bottom of the tank. I replaced the cardboard cover on the bottom of the tank with a thin piece of plywood and 1/8" foam.

It is an older Classic 30 without the oval logo and without a tube guard, and it has the tri-fold circuit board. I don't know how old but I'm guessing it's at least 13 years old, but it could be from the 90's.

I'm wondering if I should order a reverb tank or capacitors, and if there's any other diagnostics I can do to narrow it down.
Last edited by pinenut at Feb 12, 2017,
If it is just reverb, I would check only the caps/components in the reverb circuit. If you plan on doing electronics repair in the future, an ESR meter may be something you look to invest in. It may also be that something is not grounded properly.