Rebuilding Wirewound Pots - UK Vintage Radio Repair and Restoration Discussion Forum

Colinaps · 3rd Mar 2014, 3:45 am

To get my recently acquired Marconi 2207C working, I've just bodged up an inverse log wire-wound pot, one of a ganged pair - one reverse, the other not, by bridging a break with solder. However the wipers have both been worn through the plating and will certainly cause accelerated damage with further use. Also the resistance wire has worn very thin in places.

I guess I could maybe re-face the wipers with something or cannibalize similar units if I find any but I was wondering, is it feasible to remove the windings from both, then fit each one inverted in the other pot, cleaning the unused edges to give a wiping surface?

If not, I suspect I may as well set off on a lifelong quest for a pair of ganged forward/reverse log wirewound 5k pots!

Cheers,

Colin mm1aps

M0FYA Andy · 3rd Mar 2014, 9:34 am

A bit off-topic, but a general question, how is the track of a log wirewound pot made? Linear is obviously the same wire, evenly wound over the length of the track, but I'm puzzled by how a non-linear track is made.

Andy

Radio Wrangler · 3rd Mar 2014, 10:01 am

The wire is wound onto a rectangular card, then the card is curved to fit into the body of the pot, secured and connected--up. This makes a linear pot with the wiper running on the wire as it goes round the edge of the card.

Now make the card shaped, so that each turn of wire becomes progressively longer. Make sure the card is basically rectangular with only one long edge shaped. Curve the card so the 'straight' long edge is what the wiper runs on, and position it to get the wiper edge level, letting the other edge not go fully into the pot body except at one end. Secure and connect, and voila one log pot, or most other laws you might want.

David

Diabolical Artificer · 3rd Mar 2014, 10:05 am

I've tried to re build Colvyn wire wound pots,its a nightmare. the windings are made up of very thin wire which frays and breaks easily. also the bake a lite (or similar ) formers are brittle.

I have two Colvyn pots you can have the wipers out of if any use to you Colin. Check out Ebay under vintage pots there is usually a good selection on there Colin,you never know you may fine some.

Good question Andy. Maybe its done by putting more windings hence more R in certain spots of the track. If you look inside a wire wound pots the windings are made up of multiple turns of very fine high resistance wire not one wire as I originally thought.

Andy.

M0FYA Andy · 3rd Mar 2014, 10:08 am

Cunning!

Many thanks,

Andy

Phil G4SPZ · 3rd Mar 2014, 10:22 am

Avo used to make non-linear wirewound pots by using a former shaped like a sawtooth, as previously described, and by changing the type of resistance wire at each step along the winding. These are a nightmare to repair, but I did manage to fix one, the "set gm" pot on a twin-panel valve tester.

woodchips · 3rd Mar 2014, 10:54 am

Ok, another silly question. How is the resistance wire secured on the curved side of the former? A log pot ends up with a very rapid change in resistance to rotation, logarithmic in fact! So why doesn't the wire slide down the former? Is the wire coated so insulated turn to turn so each turn supports the one above?

Was going to try and simulate a stepped pot as in the Avo valve testers with a 24 way rotary switch and resistors, but not making much progress so far.

Diabolical Artificer · 3rd Mar 2014, 11:29 am

Here's a picture. Saves a long explanation. Andy.

Phil G4SPZ · 3rd Mar 2014, 11:52 am

Quote:

Originally Posted by woodchips

Was going to try and simulate a stepped pot as in the Avo valve testers with a 24 way rotary switch and resistors, but not making much progress so far.

This was a technique used by R.A.P for the volume control in some of their receivers, using a 12-way switch with the click stops removed. It works very well.

As far as I could see, in the Avo pot the winding is held on to the profiled former purely by being anchored at each end, the wire is tightly wound with the turns closely butted up against each other.

Colinaps · 3rd Mar 2014, 2:57 pm

So if the card is not rectangular before it's formed into a circle, there's no way they will work inverted. I think I'll get it apart again and have another look at the wipers - the smoother they are, the longer the rest of the assembly will last. I'm not sure if they are the same as Colvyns - the label is hand-written and they are very well made.

I guess it must be possible to use a pair of pots back to back with a tailor-made spindle if things get desperate. Access to a lathe and 1/4" brass stock isn't an insurmountable problem, though adequate skill might be!

Colinaps · 3rd Mar 2014, 4:52 pm

just opened an old Colvern - completely different wiper.

David G4EBT · 3rd Mar 2014, 8:42 pm

Wirewound log/inverse log pots were something of an abomination, and very prone to failure for a number of reasons, not least that to simulate a log law, the resistance wire is progressively finer with each segment, and apart from the fact that the welding of the joints being of dissimilar gauges for each segment is a weakness, because the gauges of the higher resistance segments is so fine, it's current carrying capacity is lower than the earlier segments. Then of course there is the wear of the wiper around the track, again problematical on the finer gauge of wire.

Apart from these shortcomings, sourcing small quantities of resistance wire of the necessary gauges is next to impossible. Back in 2010, Colin Wood - forum name 'Retired' (no longer into vintage radio and electronics, but still a forum member), set about attempting to repair such a pot and went to very great lengths - making a new Tufnol sawtooth former, making an oven to coat and dry the resistance wire with special varnish sourced from America. In the end, it all came to nothing.

Anyone who might wish to read of his efforts will find the thread here:

https://www.vintage-radio.net/forum/...t=60050&page=2

I've attached a rather crude sketch which outlines the basic principle. A sawtooth former enables each turn or wire to be progressively longer and hence higher resistance than the turn before it, then each of the following segments is of progressively higher (and thus finer), resistance wire.

I think there are rotary switches available with some 30 click positions. I would have thought it a much better idea to use such a switch with the desired wattage of fixed wirewound resistors (5 or 10 Watts for example), with values chosen to replicate a long law pot. Even a 12-way Lorlin rotary switch might suffice.

I've read quite enough about AVO valve tester flimsy GM pots, tired thumbwheel switches and fragile meters to remind me to never attempt to acquire one - not even as a gift!

Colinaps · 4th Mar 2014, 12:45 am

I wonder if carbon pots and a pair of transistors would do the turn?

Colinaps · 8th Mar 2014, 1:01 am

http://www.maximintegrated.com/app-n...dex.mvp/id/838 has a useful spreadsheet attached. Not sure what changes I might have to make to the existing components to suit but will cross that bridge when I come to it.

In the meantime, I'll be the bloke trawling through boxes of old pots at radio rallies..

Thanks for all your wisdom.

3rd Mar 2014, 3:45 am	#1
Colinaps Pentode Join Date: Jul 2009 Location: Roxburghshire, UK. Posts: 196	Rebuilding Wirewound Pots To get my recently acquired Marconi 2207C working, I've just bodged up an inverse log wire-wound pot, one of a ganged pair - one reverse, the other not, by bridging a break with solder. However the wipers have both been worn through the plating and will certainly cause accelerated damage with further use. Also the resistance wire has worn very thin in places. I guess I could maybe re-face the wipers with something or cannibalize similar units if I find any but I was wondering, is it feasible to remove the windings from both, then fit each one inverted in the other pot, cleaning the unused edges to give a wiping surface? If not, I suspect I may as well set off on a lifelong quest for a pair of ganged forward/reverse log wirewound 5k pots! Cheers, Colin mm1aps

3rd Mar 2014, 9:34 am	#2
M0FYA Andy Nonode Join Date: May 2007 Location: Preston, Lancashire, UK. Posts: 2,511	Re: Rebuilding Wirewound Pots A bit off-topic, but a general question, how is the track of a log wirewound pot made? Linear is obviously the same wire, evenly wound over the length of the track, but I'm puzzled by how a non-linear track is made. Andy

3rd Mar 2014, 10:01 am	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: Rebuilding Wirewound Pots The wire is wound onto a rectangular card, then the card is curved to fit into the body of the pot, secured and connected--up. This makes a linear pot with the wiper running on the wire as it goes round the edge of the card. Now make the card shaped, so that each turn of wire becomes progressively longer. Make sure the card is basically rectangular with only one long edge shaped. Curve the card so the 'straight' long edge is what the wiper runs on, and position it to get the wiper edge level, letting the other edge not go fully into the pot body except at one end. Secure and connect, and voila one log pot, or most other laws you might want. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

3rd Mar 2014, 10:05 am	#4
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,667	Re: Rebuilding Wirewound Pots I've tried to re build Colvyn wire wound pots,its a nightmare. the windings are made up of very thin wire which frays and breaks easily. also the bake a lite (or similar ) formers are brittle. I have two Colvyn pots you can have the wipers out of if any use to you Colin. Check out Ebay under vintage pots there is usually a good selection on there Colin,you never know you may fine some. Good question Andy. Maybe its done by putting more windings hence more R in certain spots of the track. If you look inside a wire wound pots the windings are made up of multiple turns of very fine high resistance wire not one wire as I originally thought. Andy. __________________ Curiosity hasn't killed this cat...so far.

3rd Mar 2014, 10:08 am	#5
M0FYA Andy Nonode Join Date: May 2007 Location: Preston, Lancashire, UK. Posts: 2,511	Re: Rebuilding Wirewound Pots Cunning! Many thanks, Andy

3rd Mar 2014, 10:22 am	#6
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: Rebuilding Wirewound Pots Avo used to make non-linear wirewound pots by using a former shaped like a sawtooth, as previously described, and by changing the type of resistance wire at each step along the winding. These are a nightmare to repair, but I did manage to fix one, the "set gm" pot on a twin-panel valve tester. __________________ Phil Optimist [n]: One who is not in possession of the full facts

3rd Mar 2014, 10:54 am	#7
woodchips Octode Join Date: May 2010 Location: Grantham, Lincolnshire, UK. Posts: 1,177	Re: Rebuilding Wirewound Pots Ok, another silly question. How is the resistance wire secured on the curved side of the former? A log pot ends up with a very rapid change in resistance to rotation, logarithmic in fact! So why doesn't the wire slide down the former? Is the wire coated so insulated turn to turn so each turn supports the one above? Was going to try and simulate a stepped pot as in the Avo valve testers with a 24 way rotary switch and resistors, but not making much progress so far.

3rd Mar 2014, 11:29 am	#8
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,667	Re: Rebuilding Wirewound Pots Here's a picture. Saves a long explanation. Andy. Attached Thumbnails __________________ Curiosity hasn't killed this cat...so far.

3rd Mar 2014, 2:57 pm	#10
Colinaps Pentode Join Date: Jul 2009 Location: Roxburghshire, UK. Posts: 196	Re: Rebuilding Wirewound Pots So if the card is not rectangular before it's formed into a circle, there's no way they will work inverted. I think I'll get it apart again and have another look at the wipers - the smoother they are, the longer the rest of the assembly will last. I'm not sure if they are the same as Colvyns - the label is hand-written and they are very well made. I guess it must be possible to use a pair of pots back to back with a tailor-made spindle if things get desperate. Access to a lathe and 1/4" brass stock isn't an insurmountable problem, though adequate skill might be!

3rd Mar 2014, 4:52 pm	#11
Colinaps Pentode Join Date: Jul 2009 Location: Roxburghshire, UK. Posts: 196	Re: Rebuilding Wirewound Pots just opened an old Colvern - completely different wiper.

3rd Mar 2014, 8:42 pm	#12
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,764	Re: Rebuilding Wirewound Pots Wirewound log/inverse log pots were something of an abomination, and very prone to failure for a number of reasons, not least that to simulate a log law, the resistance wire is progressively finer with each segment, and apart from the fact that the welding of the joints being of dissimilar gauges for each segment is a weakness, because the gauges of the higher resistance segments is so fine, it's current carrying capacity is lower than the earlier segments. Then of course there is the wear of the wiper around the track, again problematical on the finer gauge of wire. Apart from these shortcomings, sourcing small quantities of resistance wire of the necessary gauges is next to impossible. Back in 2010, Colin Wood - forum name 'Retired' (no longer into vintage radio and electronics, but still a forum member), set about attempting to repair such a pot and went to very great lengths - making a new Tufnol sawtooth former, making an oven to coat and dry the resistance wire with special varnish sourced from America. In the end, it all came to nothing. Anyone who might wish to read of his efforts will find the thread here: https://www.vintage-radio.net/forum/...t=60050&page=2 I've attached a rather crude sketch which outlines the basic principle. A sawtooth former enables each turn or wire to be progressively longer and hence higher resistance than the turn before it, then each of the following segments is of progressively higher (and thus finer), resistance wire. I think there are rotary switches available with some 30 click positions. I would have thought it a much better idea to use such a switch with the desired wattage of fixed wirewound resistors (5 or 10 Watts for example), with values chosen to replicate a long law pot. Even a 12-way Lorlin rotary switch might suffice. I've read quite enough about AVO valve tester flimsy GM pots, tired thumbwheel switches and fragile meters to remind me to never attempt to acquire one - not even as a gift! Attached Thumbnails __________________ David. BVWS Member. G-QRP Club member 1339.

4th Mar 2014, 12:45 am	#13
Colinaps Pentode Join Date: Jul 2009 Location: Roxburghshire, UK. Posts: 196	Re: Rebuilding Wirewound Pots I wonder if carbon pots and a pair of transistors would do the turn?

8th Mar 2014, 1:01 am	#14
Colinaps Pentode Join Date: Jul 2009 Location: Roxburghshire, UK. Posts: 196	Re: Rebuilding Wirewound Pots http://www.maximintegrated.com/app-n...dex.mvp/id/838 has a useful spreadsheet attached. Not sure what changes I might have to make to the existing components to suit but will cross that bridge when I come to it. In the meantime, I'll be the bloke trawling through boxes of old pots at radio rallies.. Thanks for all your wisdom.