I'm repairing a 1936 Delco 635 car radio for my old Chev but I've ran into a problem with an open circuit volume pot. The pot is 1 meg but it has 4 terminals, one of which is a centre tap. I've never come across a pot like this and I doubt very much that I will find another one. Anyone got any ideas on how to get around this little problem or come across any other radios with this design of pot?

Regards, Mark.

Hello Mark

Some Australian receivers used a tapped volume control, I think to improve bass response at low volume. If this is the case for your radio it the tap might have a cap to chassis attached to it.

It might be possible to dismantle and repair. If not it might be possible to make a tapped pot using conductive paint on the side of the track and wire through a hole in the body of the pot.

Graham

Hi Graham,

You're spot on there, the centre tap is connected to ground via a 40k resistor and 3nF cap with a tone switch that bypasses the cap. Improved bass at low volume makes sense of it all.
The carbon track is open circuit most of its length so I might try your idea of modifying another pot. I think that will be better than modifing the circuit as I would like the radio to be as original as possible.

Thanks for info, much appreciated, Mark.

There is an organisation in the US that can rebuild the old pots & make new ones.

Your timing may yet be good as I have an order that may not have been sent. My issue was not only getting one, but getting one with a switch.

Those pots were not only used on Australian sets.

http://www.oldradioparts.net/

Marc

I went down to Jaycar and bought a pot and a conductive silver paint pen and I have successfully managed to modify the pot to suit. After a quick alignment due to age and cap changes the set is now working and the tone switch works too. I will definitely keep the old pot now that I know its repairable, it might be worth sending off as I don't know how reliable my made up pot will be. The four legged pot must be one of those evolutionary dead ends!

Thanks for the help, Mark.

They also have a conductive glue for joining LV wires.

I put a wire spiral around a Philips Triode Hexode that has shed its metalisation and painted it with that stuff.

Also sprayed it with laquer as reccomended for the glue.
Will be interesing to see how it performs?

Marc

These tapped pots were interesting enough to cause a couple of articles to be written about them eg


Nov 1999 EA Pg 66, VR, Tapped volume controls & Bass boost.

HRSA Radio Waves Jul-94 p12 Tapping potentiometers (Pot with tapped connection)

Astor seemed to use them eg later Mickeys