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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)

slide potentiometer with connection broken between carbon trace and solder pin terminal

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.

EDIT:

I tried fixing a slice of tin foil with scotch tape, worked just as well as one could imagine: 10k -> real noisy 1M - 5M

I also tried the same thing with solder wick soldered to the pin but could not make the connection to the carbon trace.

..Kinda losing my faith here but I guess I'll give one of those silver glues a try. Will update when I have tried it.

Rintala
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  • Isn't there a third component? Like a metal bar that is running across the top of the two conductors? – Voltage Spike Jun 25 '21 at 18:39
  • I can empathize, I know all too well about Tascam pots. Replaced a few, challenging to find. Call a music shop and ask if they can order it (could have inside market connections.) Might have to substitute a different part. In general, damage is non-repairable in my experience. – rdtsc Jun 25 '21 at 19:17
  • https://www.amazon.com/Silver-Conductive-Electrically-Adhesive-Repair/dp/B06XBCLHT8 – Tony Stewart EE75 Jun 25 '21 at 19:26
  • @Voltage Yes, of course. Here is only the board with the carbon tracks and pin terminals. The photo is missing the metal enclosure and wiper with brushes that runs along traces. – Rintala Jun 25 '21 at 19:41
  • @rdtsc Yeah, fortunately this one is panel/chassis mounted and the travel length is at least somewhat standard 60mm. I couldn't find one with the exact 80mm between the mounting screw holes but that could be solved with simple adapter pieces in each end. Still, it would be so nice to have some magic paste that you could just apply to the few millimeters needed. – Rintala Jun 25 '21 at 19:46
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    can you put it in upside down with only minor modification? I would rather lose the top shoulder than the ankle... – dandavis Jun 25 '21 at 19:57
  • I use silver conductive ink pen FROM RADIO SHACK (mine still still works fine, just thinned w/acetone.) Many on ebay... https://www.ebay.com/sch/i.html?_from=R40&_nkw=%22silver%22+conductive+ink&_sacat=0 Usually it works for some years, but may crack again eventually. – wbeaty Jun 25 '21 at 20:01
  • Oh, also soak/scrub off the solder flux w/alcohol – wbeaty Jun 25 '21 at 20:02
  • @dandavis It is symmetrical aside from the logarithmic taper but I can't what see flipping it would accomplish, it is used in voltage divider configuration. – Rintala Jun 25 '21 at 20:32
  • @wbeaty have you used the ink pen with pots? The whole thing is cleaned with IPA, but now that you said it there might be some residue left in the pin in the photo, if that's what you are referring to. – Rintala Jun 25 '21 at 20:36
  • @Rintala I've just used with carbon layers (on carbon-coated pcb, on membrane keypads.) Also with ITO transparent electrodes, with unsolderable platinum wires and thermocouples. It doesn't work so great above 1/4 amp per cm^2, the heat at the joint tends to screw up the contact. Besides silver-ink PCB-repair, there's also silver-filled epoxy, but I haven't used it much. Search SILVER CONDUCTIVE PAINT, I see many cheap syringes, not $30. HINT: these old Circuitwriter pens clog and dry up easily, so just shake up, unscrew, and dip a wire in the ink-pool, to use as a micro-paintbrush. – wbeaty Jun 27 '21 at 20:24
  • @Rintala oh also, your original pot appears to use nickel paint rather than silver. Dark gray color. It's the famous shield-coating inside the Apple-II plastic computer case. Much cheaper for mass production! – wbeaty Jun 29 '21 at 04:40
  • The end connections on the carbon track are simply pressed-together "rivets" that become loose and intermittent. Squeeze the rivet with pliers to make the rivet tight with good contact for awhile. – Audioguru Sep 06 '22 at 15:05

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In case anyone else is still interested in fixing a break on any carbon-based potentiometer, the fix is ridiculously easy. The track is made from carbon graphite just like (that's right) a no. 2 pencil lead. Just draw over the break several times with a pencil. It works every time, especially the old rotary volume knobs used in old audio equipment. I couldn't believe it worked, either, but it does for me. I was blown away by both the method and the audio!

toolic
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Kehk in a MiG
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