Cleaning 10 turn pots

[uoficowboy]

I have a 10 turn, 10K potentiometer from a 30+ year old piece of HP gear that has a dead spot in it. Are these devices generally cleanable? I haven't done a thorough inspection yet but there were no obvious entry points for the device, so I'm concerned.

Also - does anybody have any pictures of what these look like on the insides? I've seen plenty of single turn pot guts, but never a multi-turn.

Thanks!

[Rufus]

How hard is it to type "multiturn pot teardown" into google?

[david77]

I've cleaned a few multi turn pots. As allways it depends on what brand it is and how good the person taking it apart is with delicate mechanics and extremely (!) fine wires.

Construction wise there usually is a helical resistive element, a resistance wire wound around a helical form. The wiper on its assembly travels along this, the wiper contact is usually brought out to its terminal via a rotary wiper contact in the back of these pots. All quite fiddly stuff.
The resistive element can usually not be taken out of the housing. I've cleaned it and the wipers with contact cleaner (Kontakt Chemie K60 + WL) and then lubricated the electrical parts inside with K61 and the mechanical stuff with some bearing grease.

From the few multi turn pots I have taken apart so far I'd say they're pretty susceptible to dirt and corrosion. Most seem to be reasonably well protected, but you never know what happened to decades old stuff...uu

[SeanB]

Normally you can pop off the back cover easily, or else drill a small hole in it ( carefully and using suction so as not to get swarf inside) then spray a small amount of contact cleaner into it and run from end to end a few times, then place some tape over the hole. If it is really work ( you will find the track resistance is much higher than nominal) then you will need a new one, sorry, as then the wound resistive element is wearing through and will break up and turn into a ball of fine metallic fluff inside it sometime in the future. Same again if it is open circuit between the 2 ends.

[uoficowboy]

Thanks SeanB and david77 - that was helpful!

I turned the pot to the bad spot with the machine on, then turned it one more revolution. I then took it apart. Turned out to be super easy - the case was made up of three parts - the front that also had the shaft fairly permanently attached to it, the middle which was mostly just a cylinder, and the back. The front piece had two metal clips that kept the back pressed against the middle and the middle pressed against the front. A screwdriver was all that was needed to pop it the clips off of the back.

The insides looked pretty good except for the wires looking a bit corroded. I was very impressed with how uniformly the teeeeeny wires were wound. After I opened it I cleaned the area one turn above the wiper with deoxit and then with alcohol. This fixed the problem area, but then I realized there were a couple other more minor problem areas. I gave up and gave the whole thing a wipe down with deoxit, then cleaned with IPA, and then put a little deoxit gold on it. Closed it up and it is working beautifully now.

[SeanB]

If you ever take one apart the track is actually a piece of enamelled copper wire wound with the nichrome resistive element, both wires being enamelled for insulation. Then one side is sanded down lightly through the enamel on the nichrome wire and it is wound into a spiral that is stuck down into the case. It is then polished and the assembly is finished off and tested. That is why the tolerance is so high on these, you have a 10 000 turn element on the wire.