Voltage on pot

[drewl]

Let me pick your brain about this pick up preamp.

generic bass preamp, no name, has a quad op amp driving a bass/treble pot and a dual mid pot.

It works but has popping/white noise and I noticed there is voltage on the bass/treble pot and it's case, when it contacts the shielded paint in the cavity it causes the noise, isolate it and it's dead quiet.

The input to the bass/treble pot is tied directly to pin 7 of the opamp, one of it's outputs.
Ground the pot and it kills the signal.

Nothing looks strange, none of the caps are shorted, this sound normal? Having voltage on the pot?

[Helmholtz]

Having voltage on the pot?

Does that mean you're seeing a DCV BETWEEN the ends of the pot?

If not, it won't cause problems and might be normal.

[Leo_Gnardo]

This is a preamp built into a bass? Probably powered by a 9V battery. Similar to 9V effects pedals, the circuitry and signal are "floated" at half the supply voltage, so it would be normal to see 4.5V give or take, between the pot terminals and circuit ground.

[Helmholtz]

This is a preamp built into a bass? Probably powered by a 9V battery. Similar to 9V effects pedals, the circuitry and signal are "floated" at half the supply voltage, so it would be normal to see 4.5V give or take, between the pot terminals and circuit ground.

In these cases 4.5V often is the actual circuit ground.
As said before, a pot will only produce (turning) noises if its terminals are at different DC potentials.

[drewl]

There are different voltages on the legs.

Like I said, if the pot shaft or base makes contact with the low resistance shielded paint it produces static and popping noises.

Maybe scrape it away?

There's got to be a reason why it's doing this.

I swapped the opamp before which made no difference.

Thanks.

[drewl]

Okay, so after taking some more measurements, the volume pot is causing the problem.

There is voltage on the lugs of all three, bass/treble, middle and volume pots, but the volume pot is the only one with voltage on the base and shaft, so somehow there is around 10k of resistance between the lugs and the base/shaft.

The other two pots are around 2meg.

I'll try swapping out.

[drewl]

Okay, so I took the pot apart, blew it out, put it back together and it's fine!

No more noise, no resistance between legs and body.

Must have been dirt or something conductive in there.

Sometimes it just helps to talk these things out and the brain catches up.

Thanks.