Can anyone explain this phenomenon and what to do about it?

 

Not without a schematic to look over. Most audio amps are AC coupled, so any DC potentials don't make it between stages. Also, DC voltages don't contain signal elements that might cause noise.

 

Resistors create noise. It's very much like the noise you get when you put a nozzle on a garden hose.

To quiet the noise of a garden hose nozzle, you can drop it in a bucket of water, and it suddenly becomes very quiet.

For electronics, the electrons are bumping around a "rough road" in the resistor.

To eliminate the electrical noise, add an electrical "bucket", a capacitor - from the wiper arm to ground.

There's another kind of noise; as pots are used and age, they get "dirty"; corrosion and crud builds up on the conductive surfaces inside of it. Sometimes you can reduce or eliminate the corrosion and crud by using a spray electronic parts cleaner, available at various electronic stores.

 

There's another kind of noise; as pots are used and age, they get "dirty"; corrosion and crud builds up on the conductive surfaces inside of it. Sometimes you can reduce or eliminate the corrosion and crud by using a spray electronic parts cleaner, available at various electronic stores.

I have this old clock radio/phone combo that was shelved years ago primarily because it had a noisy, intermittent volume control. I had saved it (for parts?), but decided I would try to resurrect it for use as a radio in my shop. The pot had one of those serrated wheel-like knobs on it, and I could see the wiper by peering in by the knob, without even having to open the case. Not what you would call a good mechanical design, with the guts of the pot being exposed to the outside world. Anyhow, I blew hard through the little space - one good puff - and the pot is no longer noisy.

 

Hi.
Why do you attribute the noise to DC? How many DC volts did you measure at the potentiometer ?
-A capacitor connected to the wiper would load the audio signal-

 

Clean the control with an electronic cleaner, NOT WD40! If you can find it 'blue stuff' an old mechanical tuner cleaner is good though it may be out of production.

Ah
http://www.elexp.com/che_1677.htm

 

Corrosion/oxidation of contacts etc inside the pot can cause the noise, and DC voltages will accelerate the process due to electrolysis.

 

Not without a schematic to look over. Most audio amps are AC coupled, so any DC potentials don't make it between stages. Also, DC voltages don't contain signal elements that might cause noise.

A resistor setting on your shelf creates noise across its terminals. If only we could survive at absolute zero....

 

Agreed, resistors are noisy animals in some ways, but not often significantly so in audio applications. Be hard to get a working audio amp if so...

 

Not without a schematic to look over. Most audio amps are AC coupled, so any DC potentials don't make it between stages. Also, DC voltages don't contain signal elements that might cause noise.

Thanks. That's helpful. I shouldn't have used the word "noise". I know what a noisy pot sounds like, and the cause. I used the wrong word. The problem is that the signal passing thru the pot seems to drop out or down to a lower level when the pot's knob is rotated thru one part of it's range. Could "DC on the pot" (an expression I have heard engineers use) be the cause? Even potentially? No pun intended.

 

It's possible that the pot is worn and the wiper is making a poor contact in one area, which would sharply attenuate the signal. The only cure may be to replace the control.

 

If cleaning is not successful and the replacement potentiometer is not available, dismantling the ill one without breaking anything and bending the wiper contacts to sweep just next to the worn track surface, cures the problem. Done it several times.
Miguel