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vintage potentiometer repair

A project log for Circa 1910 Electrotherapy/home medical device

When does collecting things become an addiction.

Boris van GalvinBoris van Galvin 09/18/2018 at 21:360 Comments

The pot used to adjust the power output to the person is kind of unique, its essentially a wooden circle with a layer of graphite that a wiper runs around. I have previously tried to repair these with little to no success as most coatings have a tenancy to wear off quite quickly or just don't seem to provide the resistance required.

Precious attempts included using a graphite power along with an acrylic binder much like what i used to coat 3D printed items so they can be electroformed in copper or nickel plaited.  This however tends to wear off quite easily or tends to chip in addition the resistance is not that low as the binding agent still creates insulation between the particles of graphite.

I would like to stress that if you are trying to make something like this you use graphite powder that is intended to be used as a lubrication for locks, some graphite is abrasive and you really don't want something that will wear the slider. 

I also experimented  with a mix of ABS, Acetone and Graphite. this seemed to work quite well, it was a little more robust compared to acrylic but again the resistance was a lot higher and in order to reduce this you need to build up lots of layers.

When I received the vintage unit it looked like someone had previously started the restoration process as there was some new wiring inside the unit and the batteries had been changed to use D cell's instead of the Number 9 ignition batteries. 

Wrapped in a small cloth was a block of Shellac. Shellac was used extensively to french polish wood and the process consists of dipping the soft rag in to meths the rubbing the rag on the surface of the wood, the shellac wold melt and  thin layer would be deposited on the surface. you then leave that to dry then repeat the process over and over until a thick layer is built up. Once done you can sand the surface with a fine grit sandpaper and polish it. 

The end result is a near mirror finish and the surface is prety hard. 

That give me an idea, if i mixed the shellac with the graphite and painted it on that may give me a reasonably robust surface. 

I ground up some of the shellac and mixed it with acetone, mainly because that is what I had on hand, it also dries fast and unlike meths has no colorant. I then added graphite to the mix. this was a bit of trial and error and by varying the amount of shellac and the amount of graphite I was able to fine tune the resistance.

To my surprise then dried the concoction performed really well. the resistance was exceptionally low compared to any other process I had tried in the past. 

The original POT had a resistance of around 3.5K 

I started painting the solution on to the surface and after only a couple of layers the resistance was about 2K, this was way too low for what I wanted and 2 layers was not going to hold up, If i added more layers the resistance would decrease :(

It was back to the mixing pot where I added more shellac, cleaned off the layers I had put on to the POT then tried again.

BINGO, 8 layers and a resistance of 3.59k :) a bit higher than the original but for this project that was not going to have too much of an impact. I then sanded the surface to make it mice and smooth using a 600 grit wet and dry, tested again and the resistance had dropped slightly to 3.48K still within a reasonable range.

The original wiper had worn out so I replaced it with a thin piece of nickel, reassembled the pot and gave it a test. It performed perfectly and other than a few little score marks where the edges of the wiper were touching the surface there was no other wear at all. 

So in short if you are looking at a way to make your own POT it would seem that Shellac, Acetone and Graphite Powder work well, something else I had previously had issues with was that both Acrylic and ABS when wiped with a cloth or your finger the graphite comes off leaving a black mark. The Shellac does not do this to the same extent. 

Reading a few articles there are warnings about cleaning vintage pots with  chemicals such as acetone, alcohol etc, so i do wonder if this was the way these pots were produced. 

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