The PHILCO Phorum

Trying to do a thorough job on the 1933 Philco 14LZX chairside radio I am currently restoring, I pulled the mildly noisy volume control. When I popped the dust cover I was surprised by a potentiometer design I had never seen before. [Formerly, I was an electronic tech for about 15 years, and have been restoring old radios for a few years now. But, this is my first 1930s set.]

This control is built like more common carbon pots; however it has a ribbon of spring metal sitting above the C or horseshoe shaped carbon resistor. The wiper presses down on the ribbon, and the only place the ribbon makes contact with the carbon is under the wiper. This means that there is no physical wiping action by the wiper on the carbon. The only physical contact with the carbon is a perpendicular pressure at the point of contact. The center terminal is not connected to the wiper, but to the ribbon.

See the photographs below.

It appears to me that the intent of the manufacturer of the pot, CTS, was to build a high quality control that would have long life. Philco bought the idea and the pot.

Philco's part number is 8054. Rider's schematic gives the resistance as 0.35 Megohms, but my DMM measures 1.02 Megohms.

My questions:
Has anyone had experience cleaning and lubricating these pots? What is the best way to reduce the noise?

There is some lubricant remaining on the wiper side of the spring metal ribbon. It seems to me that lubrication on the wiper is important to smooth operation and minimizing wear (long life). But, what lubricant will stay on the wiper and ribbon, and stay off the carbon? Or, is there a lubricant that will not interfere or cause problems if it leaks onto the carbon?

Thank you for your help,
Perry


Note from site admin: Sorry, but the photos which were attached to this post are no longer available.

I think I'd try deoxit5 and swab the carbon ring gently with a q tip to clear away any crud. If the shaft seems stiff I would use a sparing amount of WD40 to loosen it up, again, blotting everything up carefully. If the shaft is binding in it's bushing, it can produce erratic results as the wiper bounces around on the carbon track. You can check for "open" spots by slowly turning the pot connected to an analog ohmmeter, and looking for jumps in the readings. You'll have to do this twice, once from each outer leg to the center leg.

Most of the high quality control contact wipers I have seen have several litle "fingers" that simultaneously press against the carbon track. Theory was, if one digs into and wears away the carbon, another will keep continuity. Looks like this control has had a lot of wear, and eventually there will be spots where all the fingers are rubbing against grooves in the carbon. Solution? If you can move just a hare left or right, they will find new paths in the unworn part of the carbon track, and thereby fix the problem, at least for a while. That is, if you can get the D**n thing apart, adjust it, and get it back together again in a reasonable amount of time. Some of the real good controls had a c-ring in the front of the control that allowed you to do this. If so, a tiny amount of lithium grease or phonolube should be wiped on the shaft after thorough cleaning, and before re-assembly.

1 meg audio taper pots are real easy to get, and unless there is something utterly bizzare about the shaft, a sub should be pretty easy to fix up.

Now for the lunatic solution: Get a modern control with a smaller od shaft, drill out the original shaft, gut the rest of the old pot, slip in the new pot with a drop of krazy glue, connect the leads, snap on the dust cover, and don't tell anyone about it. No, I haven't done that yet, I'm still working.

Thanks for your good suggestions. In my experience, DeoxIT products are very good. If I wind up using a commercial cleaner, I can' t think of a better place to start.

I will use an analog ohmmeter as you suggest, and look for a keeper on the shaft.

I like your idea of drilling out the old pot shaft to take the shaft of a new replacement. However, it may be a bit beyond my metal working skills!

Thanks,
Perry

Caig Laboratories (<!-- m --><a class="postlink" href="http://www.caig.com">http://www.caig.com</a><!-- m -->), the makers of DeoxIT products, recommends that a cleaner without solvents such as their DeoxIT 100% spray (D100S-2) be used to clean vintage potentiometers. It seems that phenolic, and perhaps some other materials commonly used at the time, are very porus, and that conductive solvents can be come trapped for extended periods of time in porus materials. DeoxIT D100S-2 contains no solvents, only their deoxidizing solutions. See their recommendation at: http://store.caig.com/s.nl/ctype.KB/it.I...ategory=21

I now use only DeoxIT D100S-2 on tube radios. I use it where needed as a contact cleaner, and I use it on noisy pots. It seems to work very well for me.

Perry