Potentiometer contact surface between carbon track and solder pin lost its conductivity

[tohtorizorro]

I have a 10k stereo slide potentiometer with the carbon tracks pretty much intact but one of the short 'traces' that connect the ends of tracks to the solder pins seems to have somehow deteriorated and lost its conductivity. (Area between pin terminal A1 and carbon track in photo)



The pot is used as the master volume fader of a Tascam 244 4-track recorder. I know that the cheapest and easiest way would probably be just to search for a good enough match and replace the whole thing, but I have a lot of stuff with all sorts of pots waiting to get repaired so finding a decent workaround might save me a lot of searching and waiting for orders to ship.

I've only found one matching case on this VIDEO, but that still encourages me to think there are smarter people than me who've had similar issues and have come up with a working solution.

I haven't tried adding graphite from pencil like in the video since I think the added resistance would be too much for the 10k pot and cause the left and right channels to get out of balance. I did try applying solder on the trace but it wouldn't stick no matter how much flux I used.

I've looked a bit into conductive paints and glues and found that some glues used for fixing car window heaters might do the trick. Unfortunately at least Bison (Bison Electro) and Loctite (Loctite 3863 Circuit+) seem to have discontinued manufacturing these products, although you can still find the latter on ebay etc. The ones I've found can be ordered for 20€ (~ 24USD) shipping included but contain only 2 grams of glue.

Anyone here have experience in fixing similar problems using glues/paints? Is that 2 grams enough for several pot jobs? How about the shelf life? Maybe some totally different approach with good results? I've been thinking about fixing a piece of foil on top of the trace but haven't really figured out how to do that without the conductive glue.

I'd also be intrigued to know what might have caused this. All I can think of is some corrosive liquid getting into the fader.

 

[Bluejets]

One can see the actual carbon trace starting and ending long before the termination point.
Possible one might be able to clean a small area close to the rivet point both on the trace holder and the rivet and drop on some solder.
I would tend to think fine 60/40 solder, appropriate size solder tip and use of extra flux pen after scraping with a sharp knife.
Delicate but might work.
Cause is obviously crook joint to start with.
If it were contamination then the other point would also be affected.
Glue or adding graphite is simply out of the question and is basically "crap".

 

[tohtorizorro]

>One can see the actual carbon trace starting and ending long before the termination point.
Yes, that's what I referred as the 'trace', it seems to have been be the same material as the brass colored strips that
connect the wiper to its terminal.

I did try adding solder, but the metal layer seems to have totally vanished since solder just rolls off it just as it does with the carbon track. This is the main problem - how to connect the pin terminal and the end of the carbon track - and leaves no options that I can't think of apart from using some kind of conducting adhesive or just replacing the whole pot. As you mentioned the carbon based stuffs seem pretty weak ie. have high resistivity. However, I'm going to give a silver based glue a shot and give an update afterwards. As the pot is only 10k, it is somewhat sensitive for deviations between the channels, especially in the quieter end but I'd think about 100 ohms would still be pretty ok and since we're talking about 5mm (0.2''), it should be feasible

While you're right that it is strange that only so tiny area is affected (although there are two tiny spots on the wiper pin brass track also) I'm pretty sure it has to have been exposed to some kind of chemical, heat or something. What it most definitely is not caused by is stress from a crooked joint. As I said the whole conducting strip has disappeared - not like there's just a crack some where since meter shows no continuity no matter where you place the leads there. Also the the pot is chassis mounted and only thing touching the pins/joints are the wires.

 

[Bluejets]

Perhaps try to re-rivet the connection.
Depending on the circuit, it may not need the third connection point and you might be able to end for end the connections.
Therefore the dodgy connection not required.

 

[tohtorizorro]

Unfortunately the fader is in voltage divider configuration and at least I can't think of a way to wire it without the broken connection. For now I replaced it with two pairs of fixed 5.1k resistors so that I can carry on tracking the various other problems with the recorder. Not sure if someone might have already given it a try since by looking at the connections the left channel of the headphone out is dead, yet the signal comes through to the right channel only when signal is panned left... Maybe something more severe has happened and caused damage in multiple places which could possibly explain the weird problem in the master fader. Maybe the conducting material has burned off the trace instead of being exposed to some chemical although that sounds a bit far fetched too.