Home made potentiometer tracks.

[ppppenguin]

This may be a very stupid suggestion but how much resolution does this pot really need? Could it be done with a multiway switch and a set of resistors? I think I've seen 36 way switches though I may be mistaken.

[Pamphonica]

Very fair point Jeffrey. However it would need steps of 0.1V from 0-10V or even 0-15V, as these values occur regularly in valve test data. That's too many for even a 36-way switch, unfortunately. I know pot settings are not very accurate, but a humble pot allows these values to be set easily. Nowadays it would be an R-2R ladder with three decades of thumbwheel switches! In the Sussex valve tester the Vg is set with a 10-turn pot and monitored with a cheap DMM module - nice solution.

[Retired]

Hi,

Many thanks for your suggestion Jeffrey; it is far from stupid and most welcome; whilst at work I was always told there was no such thing as a stupid question or idea and I sincerely wish this thread to be the same by encouraging any suggestions. I believe Jeremy has already replied to your suggestion better than I could have done due to Jeremy’s superior knowledge of these testers compared to mine. I would like to add though that the CT160 pot kindly supplied by Jeremy is marked + or – 5% for tolerance.

This is so kind of you Jeremy to offer to find yet another donor pot but please hang on a while until I can concentrate on this project and come up with some solutions as to what I really need also you were my top guy to test the new prototype should I ever progress so far. I don’t understand electronics and have only ever scratched the surface; I can find my way around a circuit diagram as I treat it as I would an engineering blue print; I struggle with chassis repairs but this pleases me because I should hate to be classed as an expert in anything; I enjoy learning new things and get deeply involved in my projects. I’m in it for the enjoyment and I love to make useful items out of junk.

I exchange many messages with my friend Alan Beckett and Alan has just asked what I’m up to as I don’t even own a CT160 tester and my AVO Mk1 is working? The easy answer is that I see this as a very interesting project and one never attempted elsewhere (I could be wrong). This is only one of my hobbies but I like to try something that bit different and as such my projects usually have very little information available so I’m totally alone in finding solutions to what at times are complex problems. I leave myself open to the world by adding such a thread as this one and can only write about my own knowledge gained during doing these projects; I like to write story fashion rather than merely add a list of data in the hope that I can encourage others to leave their own safety zone and have a go at something new; this applies especially to novices joining such forums as this one.

At the risk of rambling on I would like to add a story with the intention of helping novices to look at things a bit differently and to inspire confidence. As an apprentice mechanical engineer I had to attend a six month long training course at Crigglestone Colliery near Wakefield at the age of 16. I progressed through the hand tools learning to file to a “thou” and moved onto a lovely Colchester engineering lathe. My project was a large tap wrench made of BMS (bright mild steel). There must have been a dozen of us lads there and we reached the point where we had to use the shaper to machine the tap holder jaws. There being only one shaping machine a queue quickly formed with me being mid way in the queue; all the instructors in their white coats were watching us and as I looked at the walls and ceilings whilst waiting my thoughts turned to the job in hand; all I wanted to do was to produce accurate flats on bits of steel? I looked at my lathe and wondered what if? I left the queue and mounted the four jaw chucking the first bit of metal and quickly faced it perfectly flat; this was incredibly easy and soon I had both tap holder jaws completed; I was so engrossed whilst doing this that when I looked up there was only one lad using the shaper and the other lads were now busy on their lathes; more importantly I suddenly gained a great deal of respect from the instructors; I completed the course in record time and was allowed to join in with the apprentice electricians learning how to wire up three phase motors then after dinner I would be allowed home.

This was a true turning point in my life and has served me well because making that one bold decision taught me to start thinking for myself. I applied this to my last job before retirement; I started in the loading bays loading wagons and 24 years later when I retired I was in charge of three departments. I also apply this thinking to my projects and am no longer scared of leaving my comfort zone.

I’m not bright enough to add highly technical threads but the threads I have added have all been most warmly received and have been written from my experiences whilst working on a project.

I’d like to see this project through but I’ve already spent so much time on it that a rest is called for whilst I catch up with other things demanding my attention; once I’ve completed the Bush restoration I should be able to fully concentrate on this pot project; at the moment I’m trying too hard to do too much at one go.

Many thanks to everyone for your support and kindness.

Kind regards, Col.

[amp_mangler]

hhrrrmmm just my two bobs worth,

My Mk IV had the open circuit pot syndrome, although I managed to repair it. I remember when looking that the different wire guages wasnt so much as the taper of the pot, but for power dissipation !!!
The pot had failed just where the former was burnt ( not badly but MUCH darker than the rest) and this point on the track was where the thinner wire joined to the thicker wire.

Am I barking up the wrong tree

regards Joe

[bodge99]

Hello,

Just a thought regarding the welding of resistance wire. Have you considered using/modding a cheap arc welder? You can buy spot welding attachments for these which might go some way in helping here.

Just an idea..

Bodge99.

[Retired]

Hi Joe,

Does your Mk 1V have one of these pots with the saw tooth pattern track fitted to it? I'm just curious because if it has then these pots must have been used in all the VCM's from Mk 1 to Mk 1V. Failure of these pot tracks appears to be caused by overload imposing a heavy current on the fine resistance wire section of the track as mine too burnt completely through the track and the CT160 pot kindly supplied by Jeremy has also failed at the fine wire section. I hope I can design a new track using heavier gauge wire that will handle much heavier current. In fairness these VCM's have been in use for many years and the fault might be more down to metal fatigue in the fine resistance wire; your guess is as good as mine.

Thanks for the idea bodge99; I own a 140A arc welder and many years ago tried using a "carbon arc" attachment; this attachment consisted of two carbon rods about the size of pencils and one of the rods was fitted to a slide; the rods were angled and they met at a point where the welding "flame" was produced; as the rods wore away they could be constantly adjusted by the slide. Although this worked it was incredibly crude and very cumbersome as a welding mask was needed and heavy gloves to protect the hands; in use the flame used to dance around a lot making trying to use it a most frustrating job. It would be possible though to expand on your idea by experimenting with a home wound transformer as I can design and wind my own transformers but this would be a project in its own right but worthy of thought.

My large Startrite bandsaw has it's own built in blade welder; the blade ends are cleanly cut using the guillotine then clamped butted end to end then a heavy welding current is applied; after welding then the welder is set to "anneal" and finally the weld is dressed using the in-built grinder.

The thinnest wire on these tracks is extremely thin being only 0.0025" thick; this is thinner than a human hair so the welding current would be small. I don't think it is critical to butt weld resistance wire as the wire could be laid side by side to be joined then simply trimmed but every idea is worth investigating; As you say the spot welding technique could be employed but in miniature as the wires only need fusing together. A good idea though and certainly worth consideration.

Kind regards, Col.

[bodge99]

Welding Nichrome wire got me thinking... and looking.

From an online discussion about welding 0.8mm nichrome wire:

I've welded nichrome wire two ways: oxy/gas and pulsed electrical spot welding.

For gas welding, carbon embrittles nichrome, so use a lot of flux (fluoride based flux is what I use) and prefer hydrogen before methane (natural gas) before propane before acetylene. Use a somewhat oxidizing flame. It should hiss a little. There's a very nice little torch sold under the name "little torch" which is very useful for that sort of work.

For spot welding, I use a homebrew capacitor/IGBT setup which delivers about 500A peak and tungsten electrodes. For that, you need to have a lot of pressure and make sure the wires being joined have been cleaned until shiny. One zap and it's done.

How this applies to much smaller diameter wire, I'm not sure..?

I forsee **another** long term project to be added to my list!

Bodge99

[David G4EBT]

Apart from this being tricky with fine gauge wire, the welding together of disimilar thicknesses of wire must be a further complication. All that this will serve to do is to heighten Colin's determination - it isn't in his nature to be put off by daunting tasks!

What a cleverly conceived design this pot was.

Ordinarily, wirewound pots are linear, though it wouldn't be too difficult to make a 'clunky' log law pot in which three wiindings of progressively higher resistance per meter were used in series, each more steeply sloping on a graph than the one before. But to taper the former in three sections so that each section would progressively follow a log curvature on a graph was very imaginative.

How frustrating for Colin that he's managed to make the Tufnol former to the right dimensions and bend it to the required diamater, only to be stymied by the ludicrously high cost of the wire. Ironic that many metres of nichrome wire are used in toaster elements and electric fire elements which are cheap enough to become landfill.

I guess it's another example of firms not wanting the business of small orders - only bulk trade orders, so applying what might be termed 'deterrent pricing', rather like minimum order quantitites and minimum order values. Maximum profit - minimum goodwill.

I hope someone comes up with some wire Colin - you managed to track down the Tufnol, devise a method of bending it, tracked down the correct varnish and developed a system for coating and 'oven' drying the coated wire, so the show isn't over yet!

Best of luck with it.

David

[bodge99]

Just another thought if I may... I really will be quiet now and go away and feed the fish...

Regarding the vcm in question (sorry, I'm not familiar with these, they are beyond my budget at this time.). Would it be possible to home manufacture a physically larger pot to replace the damaged one? With the skills and equipment "Retired" has this would be relatively easy. Assuming that there is space inside the vcm, could a larger pot be fitted? This might allow the use of larger gauge nichrome wire of lower cost and higher power ratings?

Just another of Bobs "thought experiments"..

Bodge99.

[amp_mangler]

Col,
I dont remember the exact "style" of pot ( your name of sawtooth I dont quite understand) I do remember how I did it though !!. the break was just over the "lip" of the tufnol i.e. inside the pot body, not where the wiper runs. I have resistance wire (cant remember the guage) but its 13.5 ohms /mtr ( you can have some if you like ) so anyway I carefully lifted the wire away from the tufnol and welded about an inch onto the origional "end" near the wiper and folded the tail back down into the pot body.
I lifted the floating end of the origional wire away from the former and welded the tail to that ( yes with a big floopy loop) and tucked it away from the pot shaft. its worked since. I guess if you were unlucky and the wire broke where its hidden only a new pot would suffice. I do remember I st***ed about the whole day to repair one lousy pot

PM me Col if you want resistance wire

regards
Joe

[G4XWDJim]

Col,

I have a length of new Nichrome wire of .013" diameter which I think is about 29 swg. It's wound into the form of a heating element about 3/16" diameter and in its former life was supported by a quartz rod through the centre and fed with the mains across the ends. Imagine close winding wire around a 3/16" former until it occupies about 5 inches and you get the picture. There's probably about 6 feet on there. I've tried to photograph it but because it's in a plastic cover to prevent it flopping about the light reflects off it and the image is useless.
If it's any good to you for your experiments you are welcome to it with my compliments. I'm sure on your lathe you could devise a method to unwind it.

Pm me please if you'd like it and I'll whack it in the post.

Jim

[Retired]

Hi,

WOW; 500A peak welding current bodge99; I weld RSJ’s at 140A with my arc welder but then of course it will depend on voltage. I’m very interested to learn more about your homebrew “capacitor/IGBT” set up if it’s not too much trouble to enlighten me as I’m willing to learn from others and as I have no experience of spot welding bow to your greater knowledge. I can gas weld though as I was taught as an apprentice how to use oxy/acetylene both for welding and cutting; I’m familiar with an oxidising and carbonizing flame and it’s many years since I heard a “hiss” mentioned. Please feed your fish but by all means keep the ideas coming and if you could manage to put time in to perfect a wire welding instrument it would be beneficial to many people who wish to join resistance wire; I’m rather bogged down to take this on but what a splendid project it would make. Your suggestion about making a brand new complete pot from scratch is very viable indeed and please refer to my post #19 as I’ve already discussed this as an option and Jeremy’s post #20 was most welcome as Jeremy has offered to try to find a larger pot for me to experiment with. You are thinking on the same lines as me though and I’m pretty sure one way or another I’ll come up with a working pot.

I’m sorry for any confusion Joe regarding my description of the VCM track as a “saw tooth” but I used this to try to separate both the VCM and the CT160 tracks as the VCM has the saw tooth profile to its track bottom whilst the CT160 track is plain it being parallel in width; I’ve included a picture of my master template showing the VCM profile which I think you will agree resembles a large saw tooth pattern. I’m not at all surprised it took you a day to repair your pot and you can guess how much time I’ve already put in on this project with no end in sight yet to making these special tracks.

Many thanks Joe and Jim for your kind offers of resistance wire and like Jeremy’s kind offer of a larger pot please hold onto them all until I get my act together as I will indeed accept such offers with thanks if I find I can use the items. I don’t wish to mess anyone around and am truly grateful for such kindness and generosity but feel at the moment I would be taking advantage.

David; what can I say other than thank you for your continued support and encouragement both on and off forum it is much appreciated and taking into account how much interest this thread has caused I’ve decided to shelve my Bush SUG43 restoration for a while to allow me to concentrate on making pot tracks or even a complete pot; I’m starting to feel more focussed again on this project and as winter bites I can spend time trying to resolve the many problems.

Please keep the ideas coming as I consider every one seriously and don’t care even if they appear weird; lets have some fun!!

Kind regards, Col.

[bodge99]

Hello all,

Retired:

Sorry if I gave the impression that the capacitor setup was mine... I'd actually copied & pasted from a welding site. Sincere apologies to anybody that thought that I was implying that this was mine. I must be clearer in the future! I think that the 500A peak **might** be a tad optimistic!

I've located the original post and have sent an email with a request for any information available. If I hear anything then I'll post it here.

I intend to start experimenting with a similar project as your original one. I just wish that I had access to some of the equipment that "Retired" has!

Bodge99

[Retired]

Hi

Many thanks to everyone for your continued support and encouragement but I feel this thread to be drifting a long way off topic as I actually posted it under a “Success story” which obviously it was but now due to all the interest it has quickly turned into a “Work in progress story”.

Of course I’m delighted by all the interest shown but I’ve requested this original thread now to be closed and a new thread to be started in the correct section of the forum allowing continuation but keeping it on topic. I knew my pot project would be of interest but I’m surprised just how interesting it is turning out to be and once the new thread is started I will share my trials and tribulations as I attempt to make a brand new pot from scratch.

I’d like to thank the moderators for their indulgence in allowing the project to continue but stress the request to close the initial success story is entirely my own as I was awake for many hours last night planning how to proceed with the project. Please watch for the new thread.

Kind regards, Col.