Pro-Reverb scratchy vol & bass pots

[rpbale]

I have a BF Pro Reverb with a scratchy Volume and Bass pots. The pots are fine as new ones didn;t fix it. The treble pot doesn't have the scratchiness.

I'm assuming I must have DC on the pots. I'm thinking it is one of the tone caps since they are the decouplers on this AA165 circuit. In looking at the order of the three tone caps it seems like the 0.47uf tone cap is the only cap common to both the tone and bass. Seems logical that it could be the leaky one. Is 0.25v the max allowable DC leakage?

I only have this condition on the vibrato channel. Maybe I should switch the caps from the normal channel to see if it follows the caps, but that seems like cheating. I do notice that when I move either the bass or the volume knobs a hair that the AC pattern justs up or down off center on the scope. It only does this for a second and then it recenters. Not sure if that's the scope's fault or part of the DC malfunction. Is that another example of a leaky tone cap? I just don't recall seeing my scope do that on other amps.

Is there a way to measure voltage at the tone caps and deduce which cap is bad?

Thanks

[Enzo]

Sure, use your volt meter.

I don;t expect any leakage through the cap, certainly not a quarter of a volt. if it is positive voltage it is leakage, if it is negative, it is grid leak buildup in the tube.

And don't forget to check for conductive eyelet board.

[pdf64]

Try swapping V1 and V2. Sometime a tube has excessive grid leakage current which can cause voltage to appear at it's grid. Peter.

[rpbale]

Solved

The tone caps had up to +35V on them depending on the position of the pot. They also were way off on their capacitance reading and one of them was shorted. Since the normal channel didn't have this problem I checked those tone caps and they meansured no leakage, MFD right on, and they measured as open circuits. So I traded them with the vibrato channel and the scratch pots problem follwed them to the normal channel.

I love it when reasoning finds the problem...of course I won't mention that I replace the bass and vol pots on the vibrato side last week. Lesson learned: noisy pots may mean DC is on the circuit.

Thanks