I just started restoring an old RCA 9K3 Tombstone Radio. I gave the electrical chassis a good cleaning. I'm now checking out the potentiometers, transformers and coils with an Ohm meter. I like to get a heads up if any are bad as these take some time to track down.
Anyway, the Volume Control Potentiometer is bad. It flunked the Ohm meter testing before and after Deoxit. I took off the cover there were lots of broken pieces inside.
The potentiometer has 5 lugs plus a ground. The schematic shows the potentiometer as having 3 sections, all in series.
- 2.5 Meg Ohm
- 250K Ohm
- 250K Ohm
One "tap" goes to a resistor and Capacitor in series and then ground.
The other "tap" goes to a resistor and capacitor in series, then a Music - Speech Switch, then ground.
This reminds me of some more modern radios that use a tap as a "bass boost" when listening at low volume.
- Does the "wiper" only "wipe" the 2.5 Meg Ohm Section, or does it "wipe" all 3 sections.
- This part looks like an "Unobtainium".
I'm thinking of using a 2.5 Meg Audio Taper Potentiometer and two separate 250K resistors in series with the Potentiometer. I will then install the resistors and capacitors per the schematic. I might lose the ability to play the radio at low volumes. If so, I can always reduce the size of the 250k resistors. If I'm really lucky, I might find a 2.5 or 3 Meg Audio Taper Potentiometer with a tap. Then I would hook the tap up to the resistor and Capacitor going to the Music - Voice Switch.