Drawmer 1960 help with dc on pots

[angelothewolf]

hi! im doing a recap on this nice Drawmer 1960.
A lot of old 47/25 caps with very bad esr and very drifted value, some around 60uf and 2.5ohm. i replace all of those with new (Elna Silmic II) caps, and replace tubes, etc...

Now the only issue left is this problem of very horrible and loud DC scratching on the gain pots



already replace all that electrolitic caps. the potentiometer apears in good shape. i got constant 650mVdc on all the legs of the first half of the gain pot, so what's the deal here?

[Randall]

Is it built on fiberboard? If it is, it's probably become conductive. You can tell by probing various places on the board with your meter. If you can measure voltages, there is the problem.

[52 Bill]

Now the only issue left is this problem of very horrible and loud DC scratching on the gain pots

i got constant 650mVdc on all the legs of the first half of the gain pot, so what's the deal here?

From the schematic that you posted, there is going to be dc on the gain pot. The second section connected to the op amp will probably have even more voltage on it.

[Mick Bailey]

I can't recall any scratching on the 1960 - the last one I repaired was for a very critical studio, so it would have come straght back if there was any problem. Did you double-check the cap polarity?

[angelothewolf]

From the schematic that you posted, there is going to be dc on the gain pot. The second section connected to the op amp will probably have even more voltage on it.

actually i only have DC on the transistor section...

[angelothewolf]

Is it built on fiberboard? If it is, it's probably become conductive. You can tell by probing various places on the board with your meter. If you can measure voltages, there is the problem.

well, i can measure around 12mV on the fiberboard surface, i think thats not the issue...

[Mark Black]

It strikes me that if there is 650mV at all legs of a pot section then the net voltage drop across the pot would be zero volts and shouldn't be a noise contributor.

Is it possible the pot is just plain dirty and in a sensitive portion of the circuit?