Potentiometer Repair

[Andy M0FYA]

Has anyone got any advice on a suitable repair method for the wirewound potentiometer shown in these photos, please?

The 20kohm track is 4” in diameter and 2” deep, with independent wipers top and bottom, and is part of an H2S Switch Box Type 207 which is extremely hard to find, so coming across a replacement is most unlikely.

As can hopefully be seen in the photo, the wire on the track is protected and held in place by a layer of wax. There a number of knocks, but the serious one has resulted in six turns of the winding being sheared through on the corner between the top edge where the wiper runs and the outer face, with the resultant ‘tuft’ of wires – the track is open-circuit and the wiper has to run over six wires on the upper edge which move as it passes over as they are no longer joined to the ends in the ‘tuft’. The damage occupies maybe a sixteenth of an inch or a bit more on the top edge.

I think to try to remove the track and attempt to rewind this part could easily end up in total disaster, so I would want to attempt to do this!

The only ‘repair’ I can think of is to coat the six wires on the top edge, and down the back (only accessible for about a sixteenth of an inch as the track on its former sits tightly around a Bakelite drum), with silver-loaded conductive paint, extending to an intact turn on either side.

Clearly this will be less than perfect as there will now be a discontinuity in a plot of voltage versus rotation, also seen by the lower wiper on the bottom of the track.

Will this be mechanically robust – I haven’t used this paint before, will it ‘glue’ the ends of the wires together where the wiper passes over?

Can anyone suggest any other repair method, please?

Many thanks,
Andy

[Nick]

The wire should be nichrome resistance wire - stating the obvious, but don't use ordinary magnet wire...

If possible and you are up for a repair like that, I'd remove the wax coating in some organic solvent (meths/white spirit/propan2ol etc.) and remove the failed windings cleanly.

Measure diameter of removed wire and using an exact-ish replacement (sure that 10% or so variation shouldn't matter), splice the new wire in and rewind failed number of turns.

If the replacement wire is not insulated, coat it a couple of times with FEV (French Enamel Varnish - available from "craft shops"), do the wrap, then very lightly sand the top to allow a conductive surface for the wiper. re-dip in wax should you want that "authentic" look

I did this some years back on an old US-made tube tester - it worked well...

[Radio Fixer]

Um! think I would have a go with your method before attempting Nick's. Think the wire might spring apart and it would simply go from bad to worse.

I doubt it solders but you could try cleaning off and see if does and then bridge with that. Or get a piece of thin copper or brass and bend it into a U so it hugs the track, push it down hard and superglue or epoxy it in place on the sides only.

Gary

[Nick]

Think the wire might spring apart and it would simply go from bad to worse.

Didn't spring apart for me - it was simple & quick - I used a bit of masking tape to hold down the ends of the wire whilst cleaning out the broken and installing the new wire.

Soldering NiCr wire is not tricky if you do it right but ordinary SnPb solder and fluxes rarely make a good connection - you have to use acid flux or acid core lead-based solder and then tin the end of the NiCr with a HOT iron and lead-based solder.

Some folk etch/Cu-plate the end of the NiCr wire using a Cu tinning bath (easy enough to do) before soldering...

Personally, I've always wanted an excuse to build a small spot-welder (I have a bunch of suitable capacitors etc. waiting for a good project) and joining bits of NiCr would be an ideal excuse!

Once its tinned, you can use lead-free/Ag-loaded solder etc.

Repair took about 1 hour start to finish... like new...

[Andy M0FYA]

I can foresee a number of unknowns if I try to dismantle it to replace the damaged part of the winding. Apart from the solderability of the wire, the track is wound on a thin former maybe a sixteenth of an inch thick which is then somehow tightly wrapped around the bakelite 'drum'. The inside parts of the winding turns are trapped between the two. I don't know how it is fixed. Does the wax go between the two and glue the two together? Somehow, the replacement turns, and the joints between new wire and old have to be tight and simply fill the space available.
I do fear that it could all go horribly wrong with no way back!

Andy

[Nick]

Point taken.

Being pragmatic, the damage is limited to a small percentage of the total wiper range - maybe 3% or so,.

Unless exceptional accuracy is a requirement, I'd seek a way just to clean up the mess and to make a bridge between the wire before & after the gap under the wiper thus ensuring continuity and to leave it at that. Even a "compression" joint using a small ferrule would be OK and would obviate the need to solder the wire...

[pwdrive]

Have a go... If I was having a go and assuming the whole thing can't be dismantled I would try to "fuse" some copper wire between the ends or just on top of the ends, don't know if copper will fuse to the resistance wire? If not try fuse wire? It's a try it and see job. If bridging then you need a good contact anyway.

Lawrence.

[Andy M0FYA]

Have a go... If I was having a go and assuming the whole thing can't be dismantled I would try to "fuse" some copper wire between the ends or just on top of the ends, don't know if copper will fuse to the resistance wire? If not try fuse wire? It's a try it and see job. If bridging then you need a good contact anyway.

Lawrence.

Lawrence, don't forget (perhaps not obvious as I photographed the pot with the wiper round the other side out of the way) that the wiper has to travel over the 'short ends' which remain on the top edge of the former the track is wound on, so there can't be a step there. The adjacent undamaged turns are accessible for only about a sixteenth of an inch down the back, but are accessible on the outer face. I need to both restore continuity of the track, and stabilise the 'short ends' so the wiper can run over them without dragging them aside.

Looking on ebay for 'conductive paint' I see that there some which look more like 'conductive epoxy' - I wonder whether this might be mechanically stronger than paint.

The problem is that any repair is in all honesty going to be a bodge, but this pot is such a custom-made item that finding another is most unlikely. They no doubt all ended up in landfill after people bought these units from the surplus shops to recover the more useful switches, bulbholders, relays etc! So there will inevitably be a discontinuity in the behaviour of the pot after repair.

There's no rush for a hasty repair, this will be part of a long-term job, so keep thinking, please!

Andy

[Nick]

Looking on ebay for 'conductive paint' I see that there some which look more like 'conductive epoxy' - I wonder whether this might be mechanically stronger than paint.

Conductive epoxy (aka "Silver-loaded" epoxy) is good stuff and probably ideal for this repair as it will bond to NiCr, maintain the right height profile for the wiper and can be sanded/shaved to the right dimensions. As mentioned before we are talking a small percentage of the overall wiper travel, thus bridging the gap rather than rewinding is probably perfectly acceptable. I've used MG 8331 epoxy before with good results - not cheap stuff, though... eBay have some...

Of the non-rebuild options, its probably the best.