10-30-2009, 01:45 PM
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.
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.
