Potentiometers - Printable Version
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Potentiometers - EdHolland - 11-13-2018
I was reading around the web over the last few days about potentiometers. Searching a 2 Meg pot, with switch and tone/loudness tap, I started wondering about options to repair the damaged track on the existing pot for my radio.
This rather interesting thread came up: https://www.radiomuseum.org/forum/repairing_old_potentiometers.html
Looking further, I was intrigued as to how the audio taper or Logarithmic pots are produced. That yielded this web page http://www.resistorguide.com/potentiometer-taper/
This implies that, except for the most specialised components the "curve" is really an approximation, achieved with a two part track, the first part having a relatively low resistance change vs pot roitation, and the second part having much greater resistance.
OK, knowing this, how can we ever make a pot new again? Conductive materials are needed, which can replicate the necessary resistances when deposited on a surface.
A while back, I bought some conductive carbon paint. This certainly conducts, but far too low resistance for a 2 MOhm track, without reformulating it.
This weekend, I was tasked with restoring and cleaning up an old wood burning stove. Up to my eyes in stove blacking, I had a notion to test this, rubbed on to a piece of paper. Quite promising results could be achieved with this approach. Next is to test on a piece of Phenolic.
Of course, this is a crazy project.... but I would really like to experiment if this could put the original pot back in my radio, rather than the modern replacement it has. at the moment.
Any other experience among forum members?
Cheers
Ed
RE: Potentiometers - Phlogiston - 11-13-2018
Yeah, I have tried using stuff like nickel print. But I came up with this formula:
Time + cost / success rate = get a replacement out of the parts box
Just making a linear taper pot, without taps, will be a challenge. Wear from wiper contact is the foremost issue.
RE: Potentiometers - EdHolland - 11-13-2018
Thanks Russ,
Parts box here is not quite the option it might be at your shop
Your other concerns are certainly good ones.
The type of pot on which I was going to attempt repair is the '30s design with no sliding contact directly on the track - a strip of sprung steel matching the track is pressed from behind by the wiper. Presumably this was to avoid/reduce wear of the then fragile track print.
As to accuracy of the curve, perhaps this is not so important, especially for audio use. Mostly I am interested to see what can be done, purely out of curiosity. My "radio time" is purely recreational, so wasting time on this is fun. First thing I'll do is try a test setup - an easy 1/2 hour experiment on some scrap materials.
Cheers,
Ed
RE: Potentiometers - Chas - 11-13-2018
Contact Mark Oppat: http://www.oldradioparts.net/controls.html
If he has not got an OEM pot, it can be assembled from Centralab, Clarostat, IRC, Mallory
If Mark doesn't have it, you don't need it...
Quote:The type of pot on which I was going to attempt repair is the '30s design with no sliding contact directly on the track - a strip of sprung steel matching the track is pressed from behind by the wiper. Presumably this was to avoid/reduce wear of the then fragile track print.I'm familiar with this pot. It is intended for circuits in which a very small current flow, known as "noiseless". It cannot be cleaned as the contact disk gets in the way. When "sprayed", the carbon deposit washes away with any solvent.
This pot is OEM in the National SW-3 as the regenerative control...
Do not be put off by the cost... I have known Mark for too many years, back in the '70's he was buying up control stocks, he was not wrong. Cannot even begin to imagine how many controls he has in stock... Having to maintain the inventory in a logical manner.
Chas
RE: Potentiometers - Radioroslyn - 11-13-2018
I just got a pot as you described for my 116B 2M tapped @ 400K w/sw. Order it from Mark and it was $26 + 4.50 shipping. I check it and it was correct it terms of values. The chassis is apart so I haven't had a chance to give it a try.
GL
RE: Potentiometers - EdHolland - 11-14-2018
I have one of these already, Chas & Terry, form the same source. However, I'd really like to have an original type part in there.
Hey - we restuff caps, why not rework pots too
.Yes, I'm stubborn enough to try this!
RE: Potentiometers - Chas - 11-14-2018
On occasion I have "messed around" with potentiometers. Only successes were making a pot from salvaged pots. Often several, to get the desired electrical characteristics. They didn't look like any OEM...
If you can create an emulsion that can be varied in electrical characteristics by virtue of its proportions, that is one step, the next step is applying the coating uniformly then curing to a final step. Wouldn't it be great if black toner could be put into solution with a binder, applied, then cured in a toaster oven
India ink, colloidal graphite 
I might add, that many control manufactures protected their products with patents. Thus a manufacturer must work outside these patents or pay license to make a robust control or a new design. That said the roller/contact disc potentiometer was short lived. For unknown reasons it did not stand the test of time.
Draw your own conclusions from that. Oh, how does one know if the control was OEM?
Please report your successes or failures.
Kinda reminds me of the steps I took to rejuvenate an A-K-30 bypass cap in a can. Opened the vent and put the cap in the oven, then plunged into melted paraffin and pulled the vacuum. It worked, I reduced the leakage to almost zero.
I rewind solenoid oscillator coils, so exact, that no alignment was necessary.. Then dipped bees wax coating identical to the other coils. Took three days, I wasn't retired then, I had more time
Other crazy stuff like re-plating Radiola III control arm, Malone-Lemon hinges and lid support.
So I feel your pain...
GL
Chas
RE: Potentiometers - EdHolland - 11-15-2018
Thanks Chas,
This isn't pain - it's fun. I already have a working radio, so there is nothing to lose fiddling with an old control. Sometimes I delight in doing things "the hard way" in order to learn something, or do a better job.
My background is in science, technical process testing and development. That has included physics, electronics and materials processing. My head is full of ideas, and my garage shelves are full of things to try
Will this work? Who knows? Who cares!
/Ed
RE: Potentiometers - morzh - 11-15-2018
1. Take a rotating multi-point switch.
2. Solder resistors according to the values and the taper function,
3. Solder in and enjoy.
RE: Potentiometers - EdHolland - 11-15-2018
Interesting idea Mike!
RE: Potentiometers - EdHolland - 11-18-2018
I fiddled a little with this today. I have a bottle of aquadag resistive coating, and tried various applications of this painted on to a piece of circuit board material.
By itself, resistance is fairly low, say for a 1" x 1/4" track. 10's of kOhm
Blending with PVA glue certainly made the coatings more robust and more resistive, as the carbon content of the film is reduced. It did not take much to get in to the MOhm range.
The next trick will be film thickness control.