Home made potentiometer tracks.

[Retired]

Hi,

I would like once again to beg the moderator’s indulgence by allowing me to add the following story in full.

I hope the following story will be of interest as it covers a project I’ve never seen attempted before. Wire wound potentiometers are used in many vintage radios and also in test equipment. This particular pot is from a petrol tank sender unit and as such added many extra complications involving the complete redesigning and making of a brand new track. I’ve retained the extra information regarding how I obtained the wiper sweep as this could be of use to experimenters wishing to design a pot for a specific application but have omitted all the mechanical modifications I made to the wiper drive due to the original drive being broken. Please sit back and enjoy the story of how a simple task grew into an epic spanning six weeks of my life.

A few years ago I bought my AVO VCM (Valve tester) and after a short while the meter went open circuit. I replaced the meter but also found one of the wire wound pots had failed. This pot had a very special track with a large saw tooth profile and a second hand unit cost me over £40. I remember looking at this track and thinking how hard can it be just to wrap a strip of Paxolin/Tufnol with a bit of resistance wire; surely a child could do this?

My friend Gerry Horrox (Crowthornetubes) emailed me asking if I could possibly wind him a coil for his petrol tank sender unit out of his vintage 2ltr Lagonda Low Chassis Tourer; I replied saying please send it after all a coil is only a length of wire with two ends.

I received a large carton containing the complete sender unit and to my initial horror found the coil was actually a wire wound pot track. Now I was in deep trouble because the problem had now been adopted by me and I had never done anything like this before but it rekindled the thoughts about my VCM. I sent Gerry a message saying please don’t hold your breath but if it’s OK I would like to treat this as a project and will do my best.

I checked the winding to find it still gave continuity at 96 Ohms but the Tufnol track had a profile resembling a graph as it was breaking up and had jammed the wiper. As I looked at the unit I noticed the main steel support bar had been cut and welded and thought the Lagonda factory would not have done this so I sent another message to Gerry asking for more information. Gerry’s reply was a classic and really complicated this project taking it from difficult to virtually impossible. The unit was not original equipment but one Gerry wished to fit after modification to his car and now Gerry asked if could make the track at 50 Ohms?

Although this pot belongs to a car I would like to be as comprehensive as possible because it threw up many problems and these problems not only covered designing and making a track but making a bespoke precision track and I now would like to explain how I went about it.

I surfed the Internet and could not find any information whatsoever about how to wind a pot track; I also checked two radio forums but again no one appears to have attempted this before so I started with a blank page and an open mind.

The original Tufnol track could not be used so I would have to make a new one. The track measured 0.040” thick and I had a piece of 0.060” Whale Tufnol to experiment with. I also had a small bobbin of 4 Ohms/ft resistance wire. Everything suddenly became very complicated. I hadn’t a clue at first how to work out the track size and already I was waking up in the middle of the night thinking about the problems. The original track was not fitted with wiper stops nor was the float fitted with limit stops. I asked Gerry for the tank sizes and worked from these. I drew the tank dimensions onto a piece of lining paper and now by placing the unit as it would be fitted I could ascertain the maximum float travel between tank empty and tank full. I replaced the track with a strip of thick card and now I could also see the wiper travel between the two points; I allowed ¾” clearance to prevent the float coming into contact with the tank and marked the card at both points. With the card removed and straightened out I could take the linear measurement which proved to be 1 3/4” (1.750”).

I now had some data to work with. The resistance wire measured 0.0135”, as this would be multiplied by the turns therefore multiplying any error I took the measurement very accurately. I divided the wiper sweep (1.750”) by the wire gauge (0.0135”) giving 130 Turns. 50 Ohms was the target so now I divided 50 by 4 (4 Ohms/ft) and would need 12.5 feet of wire.

Now I could work out the new track width; 12.5 ft is 150 inches divide this by 125 turns gives 1.154” of wire for each turn. The track has two edges and two sides so taking the edges first gives 0.060” x 2 = 0.120”. Now for the sides; 1.154” minus 0.120” gives 1.034” divide by 2 and the new track is 0.517” wide. The track length is 3.375”.

I cut a strip of Tufnol using my large bandsaw and carefully filed it to exact size using my digital vernier caliper for measuring. Tufnol is unforgiving material and breaks easily if forced into a curve so here was another major problem and another restless night. I reasoned that as Tufnol is formed with heat and pressure I would try heating it and to this end nipped a short piece of steel round bar stock in the vice also nipping one end of a wide strip of Tufnol. I protected my hands with heavy rigger gloves and applied heat using my Bosch 2,000W heat gun set at maximum. This scorched the test strip but proved my theory; Tufnol can be heat formed.

I then made a simple lever operated tool combining a lathe turned aluminium former to exact track inner diameter and a steel forming peg both on a common axis; this tool was mounted in the vice and one end of the track was secured and as heat was applied pressure was also applied to the lever which in turn formed the new track. The track was left in the machine to let everything fully cool and once removed I had formed my first Tufnol track much to my delight. This track was slightly scorched so I wondered if I could find a simpler way of heat forming tracks.

I then thought about using the same aluminium former but adding a metal strap which would offer full support during forming and the heat could be applied to the strap preventing scorching? My wife Bronwyn kindly gave me a biscuit tin and I cut this into strips; I cut a wedge shaped slot into the aluminium former using a hacksaw then tightly rolled one end of the tin strip to give a tight fit in the slot thereby securing it. The former was tightened securely to prevent it rotating. Again I set this up in the vice and as the tin strip was extra long I could safely grasp it in my gloved hand; with the Tufnol in place heat was applied but now at half setting and the track was slowly formed by pulling on the tin strip taking care not to let the strip slip and cut my hand through the glove. The tin strip was pulled very tightly around the new track forcing the track into tight contact with the former and the end secured with a clamp until everything cooled. I was over the moon at this success as I was now half way home. I visited Direct Plastics Ltd. In Sheffield taking along a number of newly heat formed Tufnol tracks and showed these to Gary. Direct Plastics website clearly states Tufnol cannot be heat formed so Gary was amazed as he inspected the tracks and said he would pass these on to the manufacturer. Gary was a huge help kindly giving me information about the various grades of Tufnol and other plastics and Nylons they stock. I bought a selection of sheet Tufnol and it was very cheap so I could experiment without becoming bankrupt.

I can now heat form pot tracks out of both Whale and Kite Tufnol; Whale is fabric based and Kite is paper based; Kite is very brittle and chips badly unless a no gap throat plate is added to the bandsaw when it then cuts cleanly. Both file easily but for anyone not used to filing to a thou (0.001”) then wrapping a short piece of planed timber with 100 grit Garnet abrasive paper quickly reduces the track to size. Carp Tufnol is best quality as it is made from very fine fabric; it is also expensive and only available in full sheets. Any shaping of the track is best done now whilst it is flat as is adding holes. The new Tufnol track needs to be accurate as any taper could cause trouble with the wiper contact once assembled

I thought making the track was difficult but I soon found what difficult really was with the next stage. The resistance wire I had was uncoated; I had wondered if uncoated wire would work and a friend suggested that as the pot would be working under low voltage and very low amps the wire would have a very thin oxide coating during manufacture which made sense at the time. I wound the track with bare wire and took a lot of care. The reading I obtained was 34 Ohms and as I had added a few extra turns I hoped to see a reading in excess of 50 Ohms. I removed the wire and stretched it out now it read 65 Ohms so now I had the answer; bare wire is no good for this application.

The nightmare then set in with a vengeance. I spent hours surfing the Internet for coated resistance wire; I sent many email enquiries but I was truly stuck half way through the project. By now I was heavily into this project and it dominated my life and thoughts keeping me awake many nights. I’m stubborn and quitting wasn’t an option so if I couldn’t obtain coated wire then I had no option but to coat my own wire. I went to bed and as I lay wide awake for hours by the time morning arrived I had a wire coating machine sorted out in my head. I have lots of junk and lumps of scrap metal kicking around and I cut all this and assembled it in my head so by the time I made a start in the garage I knew just what to do.

I wanted a method of dipping the wire; drying it and winding it from bobbin to bobbin. The only real problem now was what to use for the coating? Before making the machine I decided to try to coat a short length of wire by dipping; I tried auto lacquer; Zinsser; water based polyurethane and shellac. Things were bad enough but now they got so much worse. I’ve never tried to coat wire before and soon found that coating beads on the wire during dipping refusing to go on as a smooth even layer. I tried thick and thin coating and eventually filled a bucket with water only to find water also beaded. Spraying the wire with lacquer worked very well as excess lacquer was blown away but because the spray was directional it only coated part of the wire and using spray cans would be very expensive. But spraying gave me a clue because the air pressure from the can removed excess coating. Another sleepless night resulted in the weird idea of using a cyclone to remove excess coating. I’m experienced in woodworking machines and am familiar with dust extractors; cyclones are very efficient at separating dust from the air and I reasoned that if I could make a miniature cyclone to provide a vortex for the wire to be drawn through it should work? The following morning I quickly mounted a square section of hardwood in the woodturning lathe; deep bored a large hole axially using a Forstner bit and added a taper at the bottom of the bore; I also bored a small hole for wire entrance; I made an airtight lid out of plywood again adding a small hole for the wire exit. I bored a hole for the air inlet and added a stub piece of pipe for air hose connection. It is imperative the wire does not physically touch the cyclone.

I worked many hours resulting in a wire coating machine with precise wire tracking; a dipping unit; a cyclone and heat chamber; a hand cranked “V” pulley was to be used to collect the newly coated wire and wire tension was gained by adding an adjustable brake onto the bare wire bobbin. A rough opening was cut near the base of the heat chamber and a support was rigged up for the Bosch heat gun. The compressor was hooked up and the dipping tank filled with water based polyurethane varnish. Everything was double checked and then the heat and air was switched on; as I looked down the cyclone was much too efficient as the air not only removed the beading it was rapidly emptying the dipping unit of polyurethane!!

Adjusting the air pressure and now I could slowly crank the newly coated wire through the machine. This gave me an immense feeling of satisfaction. I only coated sufficient wire for this track allowing extra before switching everything off. At first I thought it had been a total failure until I realized that I couldn’t see the coating because it was clear but I could feel it. A very thin even coating had been applied and if I wanted extra thickness I could run the wire through again or experiment with the air pressure. I now had the track and coated wire.

My initial experiments showed shellac to be a clear winner as it dried so quickly but whilst surfing the Internet I came across a site saying never use shellac as it cracks? This threw me because I wanted to go with shellac. I then spent a full week surfing the Internet for wire coating information; eventually I found an excellent site giving details of various wire coatings and I also found the information that wire manufacturers use a product called “Polyurethane 180”. The 180 refers to centigrade and this is used because it enables copper wire to be soldered without having firstly to remove the coating. I sent many email enquiries out whilst trying to find a supplier of Poly 180 all without luck. Poly 180 is rarer than gold.

I then sent an enquiry to Rustin’s who manufacture a range of coatings and promptly received a reply saying their polyurethane would be fine and could withstand short periods at this temperature (180) but the downside was that it took a week for the Poly to fully harden. I then sent another enquiry to Rustin’s for further information but this time for data on shellac. Peter Robinson kindly replied with information that lifted my spirits considerably; shellac is used in large quantities by electric motor manufacturers and has very high dielectric properties and only starts to soften at 180 degrees taking a great deal more heat before it finally breaks down and it is also highly resistant to electrical tracking. I couldn’t thank Peter enough for this so my first experiment was bang on after all. Shellac also dries rapidly. I wanted to run the machine using shellac but noticed the wire I had was running out; I thought there were hundreds of feet of wire on the bobbin so this was a disappointment but also a blessing in disguise.

My friend Tony Thompson kindly suggested powder dipping as Tony has experience of this and I found it a very interesting process; PVC powder is used extensively and there are many websites offering a great deal of information; once I have more time I’ll look into this method in more detail.

By now I was becoming very familiar with the whole process of making tracks and felt entirely at ease; all the problems had been successfully resolved and I could now make a brand new track for Gerry’s unit to my own specification. I ordered bare resistance wire from Wires UK. I re-did all the maths and now I could order specific wire for this track so bought 50g of 0.28mm Nickel Chrome Wire (Nichrome) at 5.33 Ohms/ft. The bobbin contained 97 metres. £10.28 inc. P&P. (UK).

I made another Tufnol track to the new dimensions and once the wire arrived rather than spending more time using the coating machine I merely stretched the required length between two fixed points in the garage ensuring it was tight and soaked a piece of washing up sponge with its rough backing removed in shellac then simply walked holding the soaked sponge to the wire; I applied five coats but between each coat I dried the shellac using the heat gun set at half power. I ensured I did not stop or go over the previously applied coat whilst it was still wet as the wet sponge would remove it. Giving five coats was a quick process and possibly overkill as the coating added another 0.001” to the wire diameter. In coating terms this is thick but better safe than sorry. This now gave me the choice of either coating in bulk using the coating machine or by hand to coat a short length for a one off.

I then left the shellac a further two hours to dry. I left one end of the newly coated wire secured for something to pull against for tension during winding and wound the Tufnol track after first marking the start and finish points. To wind I started at the right hand end of the track and using a turning motion whilst keeping tension on the wire roughly wound five turns on as the wire approached the marker I added ten extra very neat turns bringing up the marker and I continued winding adding a further ten extra turns as the second marker was reached; I could always remove excess wire but it would be difficult to add more. Winding by hand like this felt very strange and extremely fiddly at first especially with the first few turns but exercising patience and taking my time I made a very neat job of the track. I found this method of winding produced a neat track whereas I had previously tried winding directly from the bobbin but found I could not control the wire whilst handling both the track and bobbin at the same time; the wire twisted and bent out of shape also maintaining wire tension was a problem preventing neatness.

I once saw a pot track with metal shims fitted to each end and at the time did not understand their purpose but now I knew them to be used for making a precision track. With the wire trimmed at 50 Ohms I left a short tail at each end and from each tail removed the coating by scraping. I intended to make a shim for each end out of strips cut from the biscuit tin. The shim would provide a direct short circuit between the resistance wire end and tag. This was most frustrating because it was easy to bend the tin strip precisely around a strip of Tufnol but I found it impossible to create the curved profile and rather than spend another six weeks experimenting with sheet metal work looked for an alternative method. I did not want to add any more resistance so finally settled on the idea of adding more turns of plain steel wire? I searched through my wire stock and found a short length of rubber covered multi strand wire each strand measuring 0.010” this would be perfect. I removed the rubber insulation yielding many strands. This track is a force fit into a housing slot; it is not secured mechanically so could not add extra thickness to the sides nor could I add a wire joint to the wiper edge leaving just the bottom edge for connecting between the two wires. I very neatly bent the tail of resistance wire to run around the bottom edge of the track then I also bent the steel wire to run parallel; this served two purposes; it would entrap the loose tails and enable winding to continue giving uninterrupted travel for the wiper. As a precaution I added a small amount of solder to secure the end turns and also ensure a good connection at the track base. Care was needed whilst soldering as excessive heat would cause the track to straighten out.

I forced the track into the housing and by using the steel wire soldering the connections was easy. It was with some apprehension that I took a resistance reading as I was worried the new winding could have been damaged during forcing the track home but all was well; the reading was exactly 50 Ohms. I re-designed and made a new wiper drive because the original was damaged; once the unit was fully assembled I adjusted the float to the track and added limit stops to the float travel. At last I had fully achieved my aim; the full wiper sweep gave 50 Ohms and the track ends gave 50 Ohms at the tags. The cost of this track works out at $0.60 wire and $0.10 Tufnol.

Whilst work was in progress my friend Gary Tempest very kindly offered to test the new shellac coating for resistance if I would send a bit of wire to him; fortunately I could test this myself. I cut two 6” lengths of wire twisting them together and hooked up my industrial Evershed & Vignoles megger testing at 1,000V; the reading was infinity. Gary also kindly gave me a tip regarding soldering resistance wire which is very useful; tin the wire first using silver solder then it will solder easily with standard solder.

Rather than omit the details of how I obtained the linear winding length by operating the float and marking the wiper sweep I thought this information too hard earned to be lost and is a good reference should anyone wish to produce a special track as it demonstrates how rotary movement of the wiper can be converted to linear measurement.

I enjoyed this project immensely and believe Gerry couldn’t have given me a better challenge because it has taught me how to not only make a new wire wound pot track from scratch but to actually design one to fit a specific application. This project is very relevant to radio work as many wire wound pots are found on vintage radios. Most radio pots are merely open circuit so the original track can be re-used and if the new resistance wire is coated using the sponge method then repair should be straightforward and quick. I wasted a great deal of time looking for information and sending out email enquiries but couldn’t find anyone at all who could give advice or who had attempted such a project so it gives me great pleasure to fill the gap by submitting this article. I now have the confidence to tackle the ultimate challenge in making the special AVO VCM pot tracks and hopefully I can submit a follow on article in the near future.

Being an electronics novice I emailed Gerry to enquire if he needed to balance the new track to the petrol gauge? Gerry replied saying a previous owner of the car had fitted a modern sender unit of 1,500 Ohms and this would never work as the FSD of the gauge was 50 Ohms hence the request to make a 50 Ohm track.

Gerry is well pleased to finally have a working petrol gauge and I wonder what he will be sending my way in future? Thanks for this project Gerry.

http://www.directplasticsonline.co.uk

http://www.omega-wire.com/motorwaymay.html

http://www.rustins.eu/web/default.asp

http://wires.co.uk/

Kind regards, Col.

[G4XWDJim]

Well done Col. Once again your ingenuity and determination set a standard for the rest of us to try to emulate.

Jim

[Aerodyne]

What a great and encouraging story, Col. Once again you've shown how your determination and ability - both very considerable - have solved another seemingly insolvable problem. Is there nothing you cannot do!
Thanks for this enlightening thread.
-Tony

[G4XWDJim]

About three months ago I dismantled two vibration analyser units which contained large and precision multisection pots. They were American in origin and all I wanted from them were the rescuable bits . The pots were fully dismantleable having tracks of about 2" diameter wound on what looked like black cloth bonded Tufnol. I kept the metalwork of them and toyed with the idea of offering them on here but I'm ashamed to say that I thought that nobody would want them so I binned them, going against my lifelong principle of never throwing anything away I'm sorry to say.
The point of me making this admission is to say that several instruments like bridges and so on that rely for their operation on a major 'pot' could well have similarly constructed devices inside.

Jim

[Retired]

Hi,

Many thanks Jim and Tony for your very kind and most welcome comments.

You are so right about throwing things away Jim; an item can be kept for many years just in case it might be needed then as soon as it is binned the next day it is wanted. Thanks for adding this and it demonstrates the many applications these wire wound pots are used in.

Peter Robinson of Rustin’s kindly informs me standard shellac requires modifying to enable it to be used at the higher temperatures but for use in this project I found the standard shellac suitable.

After a great deal of wasted time searching for professional wire coating material I am pleased to shout from the roof tops that at last I can give details of where to obtain special electrical insulation varnish.

http://www.aev.co.uk/

I contacted this company with thanks to a link from Peter Robinson from Rustin’s. I sent details of my project to AEV and was deeply moved by their kind generosity as they supplied me a 1 litre tin as a sample to experiment with.

This made up for all the companies I had contacted and not received a reply restoring my faith in human nature.

This is an industrial varnish and is harmful if the proper safety procedures are not adhered to.

The product I now have is Ultimeg 2000/372 Golden. XYLENE. It is available in 1 litre tins as shown in the pictures and also spray cans and can be bought worldwide. Distributors can be found by visiting the website.

Ultimeg 2000/372 is for use by manufacturers and repairers of coils, motors, transformers and PCB's.

The pictures show the original track and the Ultimeg.

Kind regards, Col.

[AlanBeckett]

Col,
Talking of not throwing things away I hope you've saved the wire from the original. You never know when you might need just that type
Alan

[Retired]

Hi Alan,

Good idea and usually I do save things for reuse whenever possible but I've found that old resistance wire once removed from one of these pots tends to break easily where the wiper has rubbed it especially on the thinner gauges.

The wire also has memory retaining the four right angled bends from each turn that never straighten out perfectly throwing out accurate resistance readings.

Attempting to measure resistance wire by stretching a length out on the bench whilst also trying to take a meter reading is a highly frustrating exercise testing ones patience to the limit. I finally gave up and knocked a pair of panel pins exactly one meter apart into a strip of wood leaving their ends protruding; gently securing the wire between the two pins taking care not to break or stretch it allows hassle free resistance measurement on the pins. I always try the hardest way first before coming up with such simple solutions.

I used this pot of Gerry's as a stepping stone as my ultimate aim is to succeed in making a brand new pot track for both my VCM MK1 and also for a CT160Vg; I see these as the ultimate challenge and work on both is progressing well. Hopefully I will be able to add another thread covering these because they are making Gerry's pot look like child's play in comparison.

Kind regards, Col.

[Retired]

Hi,

Are we being forced out of our hobby by the cost of materials?

I’ve spent a great deal of time and effort and also enjoyed myself whilst working on this pot track project. I’d like if I may to bring the story up to date so far.

With the success so far I was very keen to proceed with both the VCM (saw tooth profile pot) and also the CT160 pot tracks. I already had my original VCM Mk1 damaged track and “Pamphonica” very kindly sent me a damaged CT160 track to experiment with. I carefully removed the wire from each track allowing me to take resistance readings of each section. Each track has three sections of different resistance wire. To simplify the explanation I’ll concentrate on my own VCM track as this is the most complicated to copy.

One section of the Tufnol tack had completely burned through breaking the track into three pieces. I was able to unwind sufficient wire to obtain Ohms/m and gauge for all three sections so as long as I copied the original track perfectly I didn’t need to know the length of the wires. I carefully wound the wire onto a stick and put it away safely just in case I should need it at a later date. I now had three bits of Tufnol (Paxolin) track to play with. I tried to tape the bits together but the tape would not adhere to the charred surfaces; Glue would then obviously not help so I was faced with the problem of how to accurately align these bits of track to enable me to obtain exact measurements. I needed to know the total track length; the length of each of the three sections; the angle of each section; and also the inner diameter as I would need to turn a former to heat form the new Tufnol track around. Things were rapidly getting complicated and out of control.

As usual I thought about all kinds of complicated solutions first before settling on the most obvious and easiest. I could use the pot housing; I cut a strip of printer paper to accurately fit inside the housing; I found then with patience I was able to align each piece of the broken track allowing me to draw around the track onto the paper with a very sharp pencil. Removing the paper and taking care to cut to the lines I now had an accurate paper template of the track. I used the template and scribed around it onto sheet Tufnol; I cut the rough blank out on my huge bandsaw then hand filed the track profile to exact size.

I turned an aluminium former using my engineering lathe and set the former up in my vice allowing me to heat form the Tufnol as previously described. The new track looked wonderful but a pity it didn’t fit into the pot housing; it appeared to be 1/8” too long!! This was most unexpected as I had gone to great trouble to ensure I had reproduced the sizes to a thou. It took ages before I realized what had happened. I had used thin printer paper abutting this exactly to the pot housing; what I hadn’t taken into account was the thickness of the Tufnol. The thicker the Tufnol the smaller the inner diameter also the thickness of the wire needed to be taken into account. Trimming the length of the track would appear to be the simple solution but as these tracks are so special and made to fine tolerances it would invalidate the original specification?
I’ve made a bit of progress using CAD so scanned the paper template and using TurboCad resized it to the new dimensions I had worked out allowing for track and wire thickness. This was printed out and once again I made a new Tufnol track but this time it fitted OK. I then used the paper template to produce a master brass template for future use. I now had half the job done and was highly delighted with the look of the new Tufnol track; using my digital vernier calliper the new track was within a couple of thou all over; (0.002”)

I now turned my attention to the resistance wire and ran up against a solid wall which stopped me dead. I had bought 50g of bare Nichrome resistance wire for use on Gerry’s pot but now as I looked at the wire; the thinner gauges I needed cost considerably more; the thinnest gauge for the CT160 pot was over 1,000 ohms/m; here’s an example; 50g of 0.081mm 214.9 Ohms/m bare Nichrome resistance wire costs £54.75 plus VAT. For the two tracks I wanted to make I would require six bobbins of different wires. This assuming everything worked out first time but I would need to experiment as this wire is bare and adding insulation increases the gauge. At over £1 per gram I would not pay this for resistance wire even if I could afford to. The wire is also sold in 10m lengths at over £10 per length.

I thought making the new Tufnol track would be the most demanding part of this project so I’m deeply saddened to be prevented from experimenting due to nothing more than cost of wire; I expect to pay for materials but refuse to be ripped off knowing these wires were used in vast quantities years ago. Where I worked before retirement we used to buy enamelled copper winding wire at about £1 per kilo but I’ve seen this wire advertised at up to £35 per kilo; there is no easy answer to this as manufacturers need to make money but at these prices I’m afraid I don’t want to play any more and have taken my bat and ball home. In contrast I only paid about £23 for a 4’x4’ sheet of correct thickness Tufnol for these pot tracks and the Tufnol had to be ordered especially so top marks to Gary at Direct Plastics, Sheffield.

The pictures show the new Tufnol track; the middle picture shows the paper template; master brass template and two unformed tracks each in Whale & Kite Tufnol; so near but so far; I’m confident that if I had the resistance wire I would be writing up a success story. Ah well; I have my Bush SUG43 chassis on the bench so no time to sulk.

Kind regards, Col.

[Aerodyne]

Unfortunately this is an all too common experience, Col. Try to find reasonably priced output transformers, for example. It seems that as items slip into obsolescence they rapidly become expensive to the point at which only a very dedicated (or bottomless pocketed) person would purchase them. Valves - some valves, at any rate - are a case in point. A scan through Ebay reveals some truly incredible prices being asked for valves suitable for hi-fi audio use. Goodness knows who pays these rates, but sell they do... not to me, however.
-Tony

[AlanBeckett]

Col,
Could you Post in the Wanted Section?
You never know what people have stashed. I'm afraid I can't help, but I'll keep an eye out.
Alan

[dseymo1]

If it's any help, Greenweld do a pack of nine 1m lengths of Nichrome wire from 18 to 34 SWG for £9.00.
Still not cheap, but perhaps more affordable than full reels. I suspect that if you need longer lengths, they'd make up a similar pack for you.

Edit: I see that you need thinner wire - still worth contacting them, as some of these surplus dealers have all sorts of stuff lying around, and you never know...

There also appears to be a Facebook group dedicated to Nichrome wire! Perhaps someone there might be able to help.

[Retired]

Hi Tony,

I couldn't agree more. Fortunately I'm pretty well covered for winding transformers as I have four coil winders and have bought a decent selection of enamelled copper wire; I bought a pair of German Aumann electric coil winders over a year ago but just have not had the time to play with them; I also bought 10kg of suitable winding wire (primary) for DAC 90 output transformers as I hoped to produce a small batch just for the fun of it but keep getting bogged down with other projects. Sorry everyone I just can't take on any transformer work even though I'd like to.

Thanks Alan; I did actually post on ARF a while ago hoping a country the size of America would throw up some wire but it appears every time I want anything either the supplier is no longer in business or the price is sky high.

Also thanks dseymo1; if I remember correctly the CT160 track requires about 30' of wire just for one section and it is this that is causing the problem; the problem is further compounded that with projects such as this there is usually quite a bit of experimentation involved as it would be virtually impossible to buy exactly the correct coated wire in fact I had great difficulty trying to find the correct Ohms/m in bare wire. I'll certainly have a look at Greenweld thanks for the tip.

I've never been one to walk away from a challenge so I too will be keeping my eyes and options open; who knows perhaps there might even be some wire at the Golborne meeting on Sunday. I know there is demand for these pots; I'm not interested in making money but did hope to find a cheap way of making these pots to original specification; with winter upon us I can spend more time on the computer. Thanks everyone for your help.

Kind regards, Col.

[Retired]

Hi,

As the data for both pot tracks was so time consuming to obtain I would like to add the information for future reference.

The picture below shows my first attempts at using TurboCad and gives data covering my AVO VCM Mk1 pot track; all measurements are actual and taken from the original track using a digital vernier calliper. I broke each of the three track sections into two parts; the top part being the slope and as the slope was a straight line I was able treat the section as a right angled triangle taking the centre as the average. Doubling the average figure gave the wire length for the two sides of each triangle. The bottom rectangles were simply doubled giving the wire length for the two sides of the rectangle by adding these two results with the addition of two times the thickness of the track I could ascertain the length of each turn. This is so easy now looking at the solution but when I was initially presented with working out the total wire length for each section whilst simply inspecting the track the task was far from easy. The width of each section between the track bottom and the bottom of the “saw tooth” were all different but looked equal until measured.

The CT160Vg track is a simple parallel strip of Tufnol 0.0437” (7/16” wide). At the wire joints there is a shallow saw cut across each edge and I could not make out how the wires were joined as they appeared to be intertwined. With the track the correct way up (wiper edge uppermost) and the open side away from me starting at the left there is a 5/16” section of wire that is coated with some kind of conduction material and the tag strip covers and is connected to this.

Next to this is a section of 1,250 Ohms comprising of 0.003” thick resistance wire at 280 Ohms/m.

The middle section comprises of 2,500 Ohms of 0.0025” thick resistance wire at 460 Ohms/m.

The right hand section comprises of 6,250 Ohms of 0.0015” thick resistance wire at 1,250 Ohms/m.

The pot is marked; 10K Ohm special =/- 5% ABM.

The length of the track and inner diameter remains unknown and this data is based on my own measurements using a digital vernier calliper for the Tufnol track but I used my very accurate 0/1” micrometer for the wire gauge measurement as this measures to 0.0001”. (A tenth of a thou).

This track caused me a number of restless nights as I could not get the thickness of the wire multiplied by the number of turns to match the length of the track section; I was extremely baffled by this after all I had worked out Gerry’s track to perfection so knew my theory to be proved; I checked and double checked but it still would not come out correctly. As the resistance wire is so thin I concluded that each turn was wound with a gap of the wires thickness and suddenly the problem was solved. To the naked eye the turns looked to abut. For my purpose this is excellent as it could be possible to use thicker resistance wire abutting the turns but this is where experimentation comes in.

I’m trying to copy on a shoestring what a factory with greater finances and facilities made many years ago; I’ve put considerable time and effort into the project so far and have proved to myself that I can make brand new pot tracks as shown by the one I made for Gerry.

My friend Alan Beckett kindly emailed me suggesting “wire drawing” in order to reduce the thickness of thicker cheaper resistance wire. I had already considered this but wondered about obtaining uniformity of gauge so with a lot more thinking I wondered if it would be possible to make up a simple machine consisting of two steel rollers with a groove in each to suit the required wire gauge; each roller could be multi-grooved to suit a selection of wire gauges. The top roller could be hand cranked whilst the bottom roller could be allowed to rotate freely. I have a suitable engineering lathe but turning grooves to such small diameters is something I’ve never before attempted but an option certainly open to me.

Looking at the Tufnol invoice; I bought SRBF 1220 x 1220 x 0.8mm with an invoice total of £25.85 this was to special order and of correct thickness for the CT160 track.

Kind regards, Col.

[Pamphonica]

Colin,
just picked up on this thread. You are really making progress! How frustrating to be stymied by wire availability. Have you tried Gerry Wells at the Vintage Wireless Museum for wire? he has some amazing stocks of bits and pieces, and you would be a worthy recipient.
Anyway, more power to your experimental elbow!
all the best
Jeremy

[amp_mangler]

what a superb story Col,

I mentioned in an earlier post about dedicated masters, I bow to you as such a man.

It may not be of much help, but as a kid I had slot cars ( who didnt) and I used to make my own "speed controls" they are much easier than a curved pot as stright winding is all thats necessary. Anyway I always had the problem of how to terminate the nichrome wire ends, so dutifully asked dad ( another master whom I sadly miss) he looked at me like I was dumb and said "you weld it" how else "you cannot solder nichrome".

I asked how and he proceeded as follows take a scope soldering iron transformer ( 3 volts at about 30 amps intermittent I seem to remember) he broke open two D dry cells and removed the carbon rods. BUT was very careful to leave the (tin plated ) brass "caps" on the end. he sharpened these rods in an ordinary desk type, hand cranked, pencil sharpener, until the carbon rod end was about 1mm diameter.
a piece of copper sheet was used as a "base" to lay the wound up track on. The carbon rod had a heavy gauge wire soldered to the aforementioned top button and connected to one screw terminal of the scope transformer, the other obviously going to the copper base pad. the wire is then aligned to where you wish to terminate it ( in my case they were 4BA brass screws) and held in position with the sharp end of the carbon rod. power was switched on and applied for a few seconds ( I forget exactly, it was almost 50 years ago) and you have a near perfect almost zero ohms connection.

When you join two pieces of nichrome you use both carbon rods, i.e. one on the "bottom side" under the bottom piece of wire, the second rod is used to hold the 2nd piece of wire on top of the first. Again power is applied from the "scope" transformer and you end up with a near perfect wire join. the two "dag" ends of the wire are then VERY carefully ground off untill a near perfect round wire is obtained. ( this requires a VERY steady hand)

keep up the EXCELLENT craftsmanship

Best regards

Joe

[valvekits]

Hi Col
I have been enjoying your careful analysis of this problem.
Just like Alan’s suggestion, I was thinking that a suitable dangling weight could be used to stretch the thicker wire. But I was also thinking that maybe you could experiment with stretching the wire without worrying too much about the uniformity just to see what you can do. Maybe a few stretching exercises could lead to you being able to fabricate all three sections from one continuous length of wire.

Eddie

[Retired]

Hi,

Many thanks Jeremy. I’m sure Gerry Wells would be only too pleased to help if I contacted him but I don’t wish to pester Gerry as I’m not even sure what to ask for as I’m faced with too many variables at the moment. Gerry is an excellent option though once I can be specific as to my requirements.

What can I say Joe; I’m pleased you like the story so far and am extremely flattered by your kindness; thank you. Also thank you for adding the very interesting story of how to weld Nichrome wire; it’s certainly given me something to think about and demonstrates there are unusual solutions to problems if only we seek them out. My friend Gary Tempest passed on a tip he had been given by a friend on how to solder resistance wire and this is to tin the wire ends first with silver solder then the wire should solder as normal. I think my friend Alan Beckett sums me up rather well when he says I must the only one lunatic enough to tackle the projects that I do; its good to have such honest friends and I’ve even been described as “the mad professor”. In reality I’m still very much a novice but a guy who tends to get deeply involved once a project is started and I certainly don’t have enough common sense to quit. I like to select projects that are a bit different and hope I don’t bore anyone with my ramblings. I spend many hours researching and making all the mistakes possible but I am happy and am always learning.

A forum member sent me a most welcome PM a few nights ago with an excellent suggestion and it is worth sharing and I responded with a rather long reply (as usual).

The suggestion was to obtain suitable wire wound Colvern Potentiometers as a source of resistance wire as these pots are available in a great range of resistances. On face value this would be such a cheap and simple solution to the wire problem and one that Gerry Horrox suggested whilst I was doing his tank sender pot; Gerry also kindly offered to supply the Colvern pots.

I too have a selection of unused Colvern pots but I wonder how many pots I would destroy before obtaining the correct wire? The tracks I’m attempting to make have three different sections of wire and as the tracks are so special they are designed for a particular wire gauge; too thick the wire won’t fit into the track section and too thin it won’t completely fill the track section leaving gaps. If I wanted to make one of these sections 2,000 Ohms then a 2,000 ohms Colvern would be unsuitable; a 6,000 ohms Colvern might be nearer but the Colvern track will most likely be of a different thickness and also the width will most likely be different so this effects the wire gauge; so to strip a 6,000 Ohms Colvern would possibly yield nothing but a destroyed pot.

Another problem is that resistance wire has memory; I’ve mentioned previously that removing wire from a pot track gives a length of wire full of right angled bends there being four such bends to each wire turn also on each turn would be a small bare piece of wire where the wiper made contact. Assuming I had found the correct wire then with patience I believe the wire could be straightened out sufficiently but would the wire break easily due to being stressed? Wire subjected to wiper travel breaks easily so is totally useless.

All this might appear much too complicated when only a bit of wire and a strip of Tufnol are involved and I confess I thought I would have resolved all the problems by now but please consider just how difficult this project really is. Making a simple pot track such as Gerry’s involves one length of resistance wire and a Tufnol track parallel in section. There is some degree of tolerance allowing a selection of wire to be used as by experimenting I found I could use different thickness of Tufnol and vary the track width. By trial and error I became very proficient at producing these plain tracks to the extent that I made a bespoke precision track to suit Gerry’s application. I spent a great amount of time and not only found out how to actually heat form the tracks to my specification I eventually tracked down the correct insulating varnish so I thought I had solved many of the problems.

These special tracks though are something else and quickly burst my bubble. In order for the instruments to work correctly these tracks have to be very precise indeed. I’ve proved I can make the Tufnol track exactly to specification which was difficult enough and this completes half the job. Taking the saw tooth profile track of the VCM Mk1 pot; this is an incredible piece of imagination and a credit to whoever devised it and at the time suitable resistance wire was available. The wiper sweep between each of the threes sections has to be highly accurate therefore fixing the length of each section; into each section must be wound the correct resistance; the number of wire turns being controlled by the track length of the section divided by the gauge of the wire. These tracks where produced in large quantities and using the correctly coated wire could easily be guaranteed to be within specification.

Now I come along some 50 years later and try to copy one of these tracks. I’m a total novice with a blank sheet of paper and an even blanker gap between my ears but I do possess determination and patience; a great deal of patience in fact. I was even unsure as to what the tracks were made of; Tufnol or Paxolin is there a difference? After a lot of research I found Tufnol and Paxolin to be the same material but I also found out there are many grades; Carp is the best electrical grade but also the most costly; Whale is cloth based and a good general purpose material ideal for low voltage applications such as these tracks and is also used a great deal for it’s mechanical strength such as the new headstock bearings I turned for my lathe; Kite is an excellent electrical grade being paper based; it is very hard and brittle. I wasn’t aware this material could be heat formed so experimented and eventually I could produce Tufnol tacks to any size from cheap sheet material.

I thought most of the problems were surely behind me until the resistance wire kicked in; I had not even considered the wire to be a problem after all it has been used for so many years it must be still available? Yes it is but in limited gauges and also it is bare without insulation although some gauges are available coated but at extremely high cost. Now the problem bit deeply; the gauges I need are not listed by any of the many wire manufacturers I checked. Assuming the gauges were available would it help? No; because the wire is bare and adding insulation increases the thickness; the insulating varnish needs taking into account; at first thought it would appear this could be disregarded after all if it only adds a thou (0.001”) to the wire thickness what difference does it really make. Multiply this by say 145 this being the number of turns in a section then it suddenly grows to over 1/8”, this is a massive amount when I’ve produced the new track to a thou lengthwise. Obtaining thinner resistance wire to compensate for the insulation sounds easy but as resistance wire becomes thinner the resistance increases dramatically and it is this that is causing me so many problems because of the cost of the wire to experiment with. I still need to do a great deal of thinking about this but believe I have found a bit of adjustment as I can possibly re-design the track profile slightly to accommodate a slight change in wire gauge or resistance by adjusting the width of each track section. This project has really got a grip of me and I’m like a dog with a bone; I might never solve all the problems but I’m enjoying having my head messed with!!

My friend Tony (Aerodyne) has very kindly given me information regarding “wire drawing”, Tony informs me he used to do wire drawing whilst teaching and it was a very impressive trick to show his students how to reduce thick brass rod producing very long thin wire and I need to study this information in more detail. I came up with the steel roller idea previously mentioned by watching the late Fred Dibnah at a steel rolling mill in Sheffield and was thinking aloud if it would be possible to do the same but in miniature so I was delighted to hear I was on the right track and who knows I might eventually find a way of reducing thick; cheaper Nichrome wire to my own gauges; now that would be something?

Many thanks for your suggestion Eddie; I've just seen it as I was about to add this post. Alan Beckett also suggested this as we exchange many emails. It could indeed be worth a try as I do have some thicker Nichrome wire to hand. The only problem I foresee is by stretching; the centre section being stretched could possibly be much thinner than the outer edges; this of course is un-proved by me but just a feeling I have. Also the thinnest wire I need only measures 0.0025" but as I'm game for anything I'll try stretching a six feet length of Nichrome and let you know the result. One end nipped into the engineering vise the other gripped with pliers will provide a high tech test.

Many thanks for your patience and also for your kindness; also thanks to the member who sent me the PM; I’m always willing to learn and appreciate any suggestions or ideas put to me.

Boy I ramble on a bit. Kind regards, Col.

[amp_mangler]

Hmmm,

Dad was a back room boy and had access to all sorts of goodies. I used to end with all sorts of "offcuts", always the correct length and guage ,

He brought me several gauges of Cupronol ( I think its called) its a copper and nickel alloy resistance wire it was already varnished with winding wire varnish, it has a higher resistance per length/gauge and after winding the track you remove the varnish with automotive " colour paper ( 2000 grit wet and dry paper ) where the wiper will run.
I hope maybe this can help you too


regards

Joe

[Retired]

http://wires.co.uk/acatalog/cn_bare.html

Hi Joe,

Many thanks again for your continued information; it is all most welcome. Cupronol? Could this actually be Constantan wire? Constantan/Nickel Copper/Eureka is still available and is stocked at the above site it is also available as insulated wire. As with bare Nichrome wire the prices are reasonable for the thicker gauges but quickly become much dearer as the wire thickness decreases and the cost of enameled and silk covered wire brings tears to my eyes; I’d be terrified of making a mistake after spending so much on wire. Also looking at this site it will become obvious that the gauges available are nowhere near to the thinnest wire I need at over 1,000 Ohms/m. Whilst experimenting with Gerry’s track I did enquire about a specific wire gauge not shown on the site and received a phone call from a very nice lady explaining that the wire was not stocked but that they could manufacture a 500g bobbin of the wire for something like £150.

I’ve spent hours surfing sites in China for resistance wire and all the wire manufacturers sites I visited had a minimum order of 500KG (I’m put off buying a 50g bobbin because of cost so I shudder to think what half a ton of this wire would cost) I’ve also surfed sites in America. I’m pleased to receive your information Joe and it helps to turn this thread into a reference as well as a story.

I was wide awake again in bed in the early hours of this morning with all kinds of weird ideas running around in my head. Before dropping off to sleep I had come up with simple solution. Why not make my own brand new Potentiometer from scratch after all I can make the tracks. Rather than getting bogged down by wire gauges and costs I started with a clean page working through what I wanted to achieve whilst taking into account my equipment/facilities/knowledge. I have already gained a great deal of knowledge by just working on the tracks. I’m familiar with Tufnol sheet and round stock and also am now aware of the many plastics and nylons available in a vast range of sizes all at reasonable prices and available by a short car ride to Direct Plastics in Sheffield.

I had become pre-occupied with the restrictions imposed whilst trying to keep to the original specifications; these specifications not only covering wire gauges; track size and overall potentiometer size. Now what would happen if I could design my own brand new pot from scratch; that was a thought worth pursuing? Suddenly things started to click into place like pieces of a jigsaw; I could change the size of the pot body and use a short length of plastic bar stock turning this to exact size in my engineering lathe. I could not only increase the diameter of the track but also the depth; this would enable the cheaper thicker resistance wire to be used and also give a substantially more robust track not prone to burning out. I‘ve never seen a CT160 tester so don’t know what space is available around these pots; if the diameter is restricted then perhaps there is space to increase depth; as a last resort the pot could be remotely mounted allowing any size to be made; now I’m gaining many more options hopefully giving me greater opportunity for experimenting.

Space permitting the actual switch control could be retained together with dial. The dial range would not change either because if I’m correct I could increase the diameter of the track allowing a longer wiper sweep for each of the three sections; even a small increase in track diameter makes a lot of difference to track length; the control rotation would remain the same for each section for any track diameter; each section doesn’t have to be the same width? I could use thicker wire by increasing the track width whilst still retaining the correct resistance for the wiper sweep at each section. Simple BA brass screws/washers/nuts could be used for tags and a new wiper could easily be made to any dimension.

I know I have the ability to make a new pot and have already made a precision track from scratch so making a new complete pot would be the obvious next step for me and must be the ultimate challenge. I would need to do the calculations to determine pot and track sizes to match the cheaper resistance wire and suddenly I feel all the restrictions have been lifted from me giving me total freedom of design. It usually takes me time to resolve problems because the natural approach is to try to copy original items especially something as complicated as one of these pot tracks but stepping back and looking at what I want to achieve I come up with ideas such as this. Yes of course I’m barking mad but by adopting this approach to problems I overcame the major problem of cutting cast iron gears as shown in my Wave Winder thread.

I don’t have all the answers as I’m an electronics novice and I don’t even know if the original wire sizes were selected for a reason other than to retain a standard pot size; from my point of view all I want to achieve is a track section giving the correct resistance at the wiper and by getting bogged down with wire gauges I lost sight of this simple fact.

Thank you for allowing me to share my thoughts. I do need to break away from this project for a short while as I’m currently trying to fully restore my Bush SUG43 chassis which is a rusty wreck. I will be able to concentrate fully on this pot project once the Bush is completed; in the meantime I can start planning the new pot and enquire about a correct grade of plastic to make the housing from. What a project this has turned out to be. I’ll also have a go at stretching some resistance wire as this won’t take long. I’ve already searched for wire drawing information and found the following which might be of interest.

http://en.wikipedia.org/wiki/Wire_drawing

Kind regards, Col.

[Pamphonica]

Col,
I just had a probe inside one of my CT160s. You could increase the pot depth from 0.7" to 1.2" OR increase the diameter from 1.7" to 2.5". You can't do both unfortunately. The safety cutout impedes any more depth, and a thick wiring loom impedes any more width. You could get a bit more width (maybe even 3" if a couple of wires on the loom are re-routed), but maybe it's better to go for a drop-in replacement. As you know, the mounting is absolutely standard. Perhaps an alternative would be to take a larger pot to pieces and re-wind that. Do you want me to find a donor?
Anyway, once you get a prototype I can drop it into a spare CT160 and give it a try!

I think you do need time off for good behaviour - leave pots for a bit and refresh your valve interests. Good luck with the SUG43!

all the best
Jeremy