I was finishing up my restoration of a Philco 70 last night, having replaced caps & out of spec resistors and had everything sounding nice. Before I put the chassis back in the cabinet, I rememebered that the volume control was a bit scratchy and my eyes landed on a spray can of DeOxit sitting on my bench. A couple of squirts on the wiper arms and I put the radio back together, only to discover that the sound was now muffled and broke into the clear only when I rotated the volume knob to certain spots.

I assume that I can open her back up and wipe down the pot to return it to normal operations, but I didn't have the heart to crack it open again last night. I've heard all the claims about the miracle of DeOxit, but this clearly wasn't the right stuff for this application.

So my question is this: What is the best way to clean up a scratchy potentiometer? I suppose we should distinguish wire wrapped and carbon types.

Also, any suggestions for the easiest way to clean the pot on the 70?

Thanks,

Doc

You should never spray items into a potentiometer. Doing so will cause a leak between the carbon and the metal case. Carbon will also work its way into the substance, and form its own leakage path. This will alter tone, and cause hum problems.

Open the control. Clean it in soap and water with a tooth brush. Dry well. Rub a thin film of di-electric grease onto the carbon. If part of the carbon track is worn, and the unit uses a slider that actually slides on the carbon, move the slider(s) to an unused portion of the carbon. Make that sure no sharp edges will scrape the carbon. If the potentiometer uses the Clarostat (or maybe it was Centralab) method, where a fibre slider rubs over a stainless steel band, there is no easy correction for a worn resistance path. If the carbon is badly eaten away, you must replace the control or have it rebuilt.

If you wish, remove the C clip around the shaft (this takes some effort with one or two screw drivers). Then lubricate the shaft with di-electric grease. Removing the shaft also allows easier work on the sliding contact (wiper, whatever you want to call it). To put the C clip back onto the shaft, compress it with a pliers.

Thomas

Doc,
If it is a graphite pot/rheostat, its probably impossible to clean. I recently cleaned a noisy standard deposited carbon part with denatured alcohol after opening it. I recommend a small brush as opposed to wiping, and blow dry. I managed to get a Q-tip between the mechanism and the element, but it was a tight fit, and some of the carbon was wiped away as shown on the Q-tip. A 250K part became a 350K part, but the noise went away.

marv

:I was finishing up my restoration of a Philco 70 last night, having replaced caps & out of spec resistors and had everything sounding nice. Before I put the chassis back in the cabinet, I rememebered that the volume control was a bit scratchy and my eyes landed on a spray can of DeOxit sitting on my bench. A couple of squirts on the wiper arms and I put the radio back together, only to discover that the sound was now muffled and broke into the clear only when I rotated the volume knob to certain spots.
:
:I assume that I can open her back up and wipe down the pot to return it to normal operations, but I didn't have the heart to crack it open again last night. I've heard all the claims about the miracle of DeOxit, but this clearly wasn't the right stuff for this application.
:
:So my question is this: What is the best way to clean up a scratchy potentiometer? I suppose we should distinguish wire wrapped and carbon types.
:
:Also, any suggestions for the easiest way to clean the pot on the 70?
:
:Thanks,
:
:Doc
:
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