Marshall Amplifier 4-pin Potentiometers

[GiovannyS10]

Hello guys!
I won an old Marshal VS-100 Valvestate amplifier. Its potentiometers was so bad, the volume control wasn't acting correctly so a tried to clean, not solved. And i tried to replace. The but the original pots has 4 pins exactly like the image below:



I ignored the 4th pin and soldered a common potentiometer in it. Now i have no sound. Anyone knows how those potentiometers works? I tried to find datasheets but no success. Found it to buy, but i can't buy it now.
https://www.tubeampdoctor.com/en/poti-set-for-marshall-valvestate-vs100
In schematic it seems to use common pots, and it makes me more puzzled.
https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-VS100R-100W-Schematic.pdf
Thank you all!

[idy]

It's either a center tap or a dummy pin for mechanical attachment. They make a series of pots in the same package and some have 3 pins, and some the 4th is hooked up to the center of the track.

If you have the bad one you can open it up and see what's inside... assuming it is too broken to measure with a meter. Then look and see how the pcb attached to the pot. Does one of the pads on the pcb go nowhere?

[iainpunk]

an educated guess would be a ground/shielding pin, possibly connected to the metal bits of the pot.

have you tried penetrating oil and spending 10 minutes wiping them leftmost to rightmost back and forth? IME that works great, but it takes some time to do, and most people don't have the patience to ''scrub'' the pots for 10 minutes.

cheers, Iain

[duck_arse]

see here is a mouser page link, it has a pot and a datasheet for that series.

https://www.mouser.com/ProductDetail/Bourns/PTV111-4420A-C503?qs=%2Fha2pyFadug%2FZTWHFOVb2f2sGLToSTBjf0Y63%252BCgDOfOvDQtSo9nPg==


read the datasheet, and it shows a dummy "pin 4". you shuld be able to work it out from there.