Repairing a ceramic wire wound pot - UK Vintage Radio Repair and Restoration Discussion Forum

radio1932 · 10th Jul 2008, 12:42 pm

I am in the process of restoring a PYE B18T.

The brightness pot was not funtioning correctly and measured erratically on a meter. It turned out to be a break in the winding.

The wire break was not where I expected it to be, on the wiper contact where the all the wear and tear is, but on the opposite side !! Odd one.

Anyway, I thought, I had nothing to lose in trying to restore the pot to some sort of operational order. In any case, ( if the resistance is all to pot, pardon the pun ! ), I could always ballast it with a parallel resistor combination for the time being.

So out with the pot former ! A very easy job, since it just pops out of the wiper mechanism retaining clips. A nice touch from yesterdays designers !!!!

The wire is thinner than a human hair and I could only see the stray end by placing a white sheet of paper on the table top in good light. I had to look very hard to spot its' counterpart end using a magnifying glass. There it was still clinging to the bobbin !!

The stray end was long enough, so using tweezers, I carefully lifted the other end and unwound one turn. Enough length to work with at both ends !!!

One problem to over come is that a loose windings on a pot is not a good idea, especially on the wiper contact side, so I carefully retracked the wire, ( one end at a time), into the void where it came from and tacked it with a minute spot of superglue either side of the wiped section. It sets in a couple of minutes and the wiper track looked as good as new, ( albeit missing one turn ), but who cares at this stage of the game ?

I was careful to ensure that the two ends met on the other side of the pot former, away from anything that could snag on it in use.

Resistance wire is generally not easy to work with, since it is made from more exotic materials than plain tinned copper wire. It is usually oxidised to provide inter-winding resistance, which can not be soldered unless this coating is removed. ( this also depends on the material type of the wire being used.)

Using a blunt penknife, I sat the pot former such that one stray wire end lay flat on the white paper and VERY gently rubbed the surface. Then gave the other stray end the same. treatment.

It was a very fiddly job twisting the two ends together, but it was done. I tested the resistance - should be 10k, measured 10.42k. So this was looking good. Would it solder ? Well I applied some and the ends were held together. I do not know if the solder has actually taken to the wire, but it is holding it together reliably enough anyway.

Clipping the former back into the wiper recepticle, I tested it and it now works across its' range !! ( Yes, there is a minute flat spot at the wire break point, ( where a turn is missing ), but not really noticeable and the full dark to light path glides relatively well )..

So another job done !!!

Keeping you posted.

Paul

af024 · 10th Jul 2008, 9:20 pm

I know what you mean about the fine wire Paul - I tried to do a 50k pot not so long ago, but alas, mine had two breaks mid way so I couldn't do it. Well done for fixing yours. Carry on like this and you'll be soldering individual legs on surface mount ICs with a Weller soldering gun!

Regards,

Andy

10th Jul 2008, 12:42 pm	#1
radio1932 Retired Dormant Member Join Date: May 2006 Location: S.Wales Posts: 19	Repairing a ceramic wire wound pot I am in the process of restoring a PYE B18T. The brightness pot was not funtioning correctly and measured erratically on a meter. It turned out to be a break in the winding. The wire break was not where I expected it to be, on the wiper contact where the all the wear and tear is, but on the opposite side !! Odd one. Anyway, I thought, I had nothing to lose in trying to restore the pot to some sort of operational order. In any case, ( if the resistance is all to pot, pardon the pun ! ), I could always ballast it with a parallel resistor combination for the time being. So out with the pot former ! A very easy job, since it just pops out of the wiper mechanism retaining clips. A nice touch from yesterdays designers !!!! The wire is thinner than a human hair and I could only see the stray end by placing a white sheet of paper on the table top in good light. I had to look very hard to spot its' counterpart end using a magnifying glass. There it was still clinging to the bobbin !! The stray end was long enough, so using tweezers, I carefully lifted the other end and unwound one turn. Enough length to work with at both ends !!! One problem to over come is that a loose windings on a pot is not a good idea, especially on the wiper contact side, so I carefully retracked the wire, ( one end at a time), into the void where it came from and tacked it with a minute spot of superglue either side of the wiped section. It sets in a couple of minutes and the wiper track looked as good as new, ( albeit missing one turn ), but who cares at this stage of the game ? I was careful to ensure that the two ends met on the other side of the pot former, away from anything that could snag on it in use. Resistance wire is generally not easy to work with, since it is made from more exotic materials than plain tinned copper wire. It is usually oxidised to provide inter-winding resistance, which can not be soldered unless this coating is removed. ( this also depends on the material type of the wire being used.) Using a blunt penknife, I sat the pot former such that one stray wire end lay flat on the white paper and VERY gently rubbed the surface. Then gave the other stray end the same. treatment. It was a very fiddly job twisting the two ends together, but it was done. I tested the resistance - should be 10k, measured 10.42k. So this was looking good. Would it solder ? Well I applied some and the ends were held together. I do not know if the solder has actually taken to the wire, but it is holding it together reliably enough anyway. Clipping the former back into the wiper recepticle, I tested it and it now works across its' range !! ( Yes, there is a minute flat spot at the wire break point, ( where a turn is missing ), but not really noticeable and the full dark to light path glides relatively well ).. So another job done !!! Keeping you posted. Paul

10th Jul 2008, 9:20 pm	#2
af024 Octode Join Date: Jan 2003 Location: Ashby-de-la-Zouch (it's not by the sea) Posts: 1,255	Re: Repairing a ceramic wire wound pot I know what you mean about the fine wire Paul - I tried to do a 50k pot not so long ago, but alas, mine had two breaks mid way so I couldn't do it. Well done for fixing yours. Carry on like this and you'll be soldering individual legs on surface mount ICs with a Weller soldering gun! Regards, Andy __________________ Now where on earth did I remove that from?