Log/Lin Pots

[pwdrive]

http://www1.electusdistribution.com.au/i...tprimr.pdf

Lawrence.

[Yorkie]

That's an interesting and informative datasheet Lawrence. I've often wondered how that make log carbon track pots. As to wire-wound long pots, coincidentally there's a thread running 'over the wall' about them which brought back memories of Colin's ('Retired') monumental efforts back in 2010 trying to rebuild one.

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 set about attempting to repair such a pot and went to very great lengths - making a new Tufnol sawtooth former, making a 'hot air' oven to coat and dry the resistance wire with special varnish (sourced from America as I recall). In the end, it all came to nothing.

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

http://www.vintage-radio.net/forum/showt...hp?t=60050

I've attached a rather crude sketch which outlines how 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 wouldn't have thought the pot needed to be so precise.

I've read quite enough sagas about AVO valve tester flimsy GM wirewound pots, tired thumbwheel switches and fragile meters to remind me to never attempt to acquire one!

[pwdrive]

Yes David, it was the post on the other side that prompted me, somewhere in the Archives is one of the most comprehensive books on the subject of potentiometers which I posted some time ago from a major manufacturer (Bourne) Here's the link again for anyone interested.

http://www.bourns.com/data/global/pdfs/O...ndbook.pdf

I don't think the common log pots of yesteryear were true log, just a compromise due to costs?

Lawrence.

[Radio Fixer]

As to AVO valve testers they are not that bad David.

I bought a Mk 1 that was spares really and a Mk2 that was restorable a dozen years ago. Both in the end were restored to good working instruments and still are. I did take the pots apart and clean and adjust them, not one was duff.

There only weakness really is the meters and some of that is due to operators overloading them. Yes! there is a cut out mechanism but it is electro-mechanical and on both of mine was well out of adjustment. I have read of restorations where no mention is made of checking and adjusting this. It may not catch every overload but is better than nothing. The best, and both of mine have this, is an OP-AMP circuit that even when over driven is arranged to not supply enough current to damage the meter. One of mine has the original AVO meter, the wrong one and about 30 micro-amps FSD, so really benefits from having it as well as making the AVO think it has the right meter. The other has a 100 micro-amp Maplin meter and again the OP AMP cheats the AVO.

Gary

[Retired]

Hi,

Many thanks David for the mention in post #2. I did indeed go to considerable lengths in trying to make a brand new "saw tooth" type pot track from scratch.

I was very successful in making a standard type wire wound pot to very close resistance tolerance so proved it could be done in a home workshop by a novice but I failed to complete a saw tooth tracked pot on costs alone. The thread explains in detail just how much time and effort I put in and had I been able to obtain the correct resistance wires without taking out a mortgage I'm sure I would have also completed a saw tooth type pot track.

When I first started the project my thoughts were that sourcing the track material and actually forming the track to shape would be the hardest part but how wrong I was; I managed quite easily to form the tracks and even sorted out how to coat the resistance wire but no way was I going to be ripped off by resistance wire prices just for something to experiment with. This was a big disappointment to me to get so near then have to give up but I did prove to myself that I could make wire wound pots from scratch. I tried the link David but it didn't work so I'll try the link to my thread again;

http://www.vintage-radio.net/forum/showt...hp?t=60050

I did gain a lot of interest and personal satisfaction out of making the pot tracks though; I've since got rid of everything associated with the project. Another project I intended tackling around the same time was to make from scratch complete speaker cones with the speech coil but other more pressing jobs cropped up. There is so much regarding vintage radios that an experimenter could find to play with? I'm currently bogged down restoring machinery.

Kind regards, Col.

[Yorkie]

Sorry the link didn't work Colin - I've edited it and it works now. (Same link as you've posted of course.)

[Retired]

Hi,

Thanks David; no problem because it was good of you to remember the article and post the details.

Kind regards, Col.

[Refugee]

I am pretty sure that the Paxolin type material the track formers used to be made from was pretty soft when it was new and that they were wound flat and then bent round before it fully hardened. At a later date the material becomes very brittle and brakes up easily.
I am pretty sure that is how they used to be made and it also may help to explain why the material becomes so brittle with age.

[Retired]

Hi,

Thanks Refugee. I spent hours browsing the web during my pot track making project and never did find any clue as to how the tracks were made hence I started from scratch coming up with my own method by trial and error.

It is possible the tracks were made as a flat strip and then formed into a curve but what I found out was that the Tufnol (Paxolin) is actually impossible to bend into a curve and for it to retain the curve unless a lot of heat was applied and the Tufnol strip required bending around a former.

There are different types of Tufnol and I've used two of them these being "Whale" and "Carp". The Whale Tufnol is brilliant for use in making bearings and sliding guides: I turned two lathe head-stock bearings to a very high degree of accuracy from Whale Tufnol. The Carp Tufnol is for use in electrical work due to it's extremely high insulation properties but Carp is very hard and brittle even when new. I believe Whale is cloth based whilst Carp is paper based? Tufnol is expensive material to buy but is still readily available.

I would be very interested to learn how the manufacturers did actually make the pot tracks; it's possible the strips were formed first then a special winding machine employed to add the resistance wire?

What is strange though is that whilst these pots must have been made by the million and possibly still are no one member from two radio related forums could enlighten me as to how they were made?

For my part I found the whole project to be fascinating and at times highly frustrating but what a learning curve it turned out to be; as with many of my projects I learnt lots of new things keeping my head and hands active; repetition bores me so I'm always interested in looking for new adventures.

Kind regards, Col.

[pwdrive]

I haven't read through all of the Bournes Potentiometer Book in the link I posted but there might be something in there that might give an insight as to how the pots were made, it shows the mandrel shapes for different resistance/law curves etc.

Lawrence.