dC on presence Pot

[Stevem]

Hi all!

I have a Friedman Runt 20 on the bench with a big time scratchy presence pot due to 12 some odd volts of DC being on it.
Is this one of those circuits where that's the way it is?

Does anyone have a schematic?

Thanks in advance?

[pdf64]

No schematic sorry, but yes, that's the way the original 5F6A bassman, and the earlier Marshall's it 'inspired', was. So yes, it could just be the way it is.
But if it's a problem, it's simple to add a resistor and rewire it to eliminate dc on the pot, eg https://drtube.com/schematics/marshall/2204u.gif
The original arrangement has the smoothest and biggest range of control though, it's just that damn 'scratch'!

[martin manning]

It shouldn't be hard to trace out the presence control schematic from the amp, and add some voltages. Post it here and people can comment.

[Stevem]

Thanks for the link to the Marshall schematic, but how is a resistor going to block D.C.?

I traced the voltage back thru most of of the rest of the circuit from what I can see on the dam black masked circuit board, and so far no caps are leaking the D.C. Voltage, from what I can see it's passing in thru a resistor.

[sluckey]

Thanks for the link to the Marshall schematic, but how is a resistor going to block D.C.?

I traced the voltage back thru most of of the rest of the circuit from what I can see on the dam black masked circuit board, and so far no caps are leaking the D.C. Voltage, from what I can see it's passing in thru a resistor.

The extra resistor does not block dc. It provides a path for the dc. The pot and cap are rewired in series and placed in parallel with the extra resistor. Now no dc can flow through the pot. BTW the current we are talking about is the cathode current for the PI. Look at the schematic.

Although this is an easy mod on paper it may not be so easy to implement on a pcb. I'd leave it be and tell the customer that's just the way it is.

[JD0x0]

Old early style Marshall presence circuit does that. I'm told leave it alone. There's isnt a problem unless someone is adjusting presence mid song, constantly, where the scratching would be problematic.

[Gaz]

It’s normal for all Friedman amps - old school Fender/Marshall presence Control. It has a much better taper than the “new” type, and frequency breakpoint changes depending on setting (in a good way).

[Stevem]

Confirmed by Friedman that yes, it's normal!

Thanks everyone!