Pot-rodding CTS mini potentiometers

For my ESB-4 with Dano pickups, I needed hard to get values in mini pots. 1M and 100K. I did find them, but mismatched...some Bourns, some Alphas, which would have meant using two USA and two Asian knobs.

I decided to go a different route. CTS make some mini pots for guitars. One 250K and one 500k. You can get them linear or audio, and they are available everywhere. You can get them with long threaded bushings that you need for semi-hollow guitars. So, I got four 500K pots to sacrifice, and started looking for some pots with the right value wafers...

The odd thing is that CTS labels the guitar minis differently than all the other minis in the series. The 200K and 500K pots have a long number starting with "EP." All the other pots in the series start with "270"

I found some 270 pots in the 1 Meg and 100K audio values I wanted. Of course the caps and shafts were weird...

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I took both pots apart, and was left with a housing, cap, shaft/rotor and resistor-wafer from. These parts are all interchangeable on 500K or 250K pots.

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However, on the 270 series pots I found, there was a critical difference. The hole in the wafer was too small to fit on the shaft from my 500K pots.

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To make the new wafer fit I bored out the hole with a reamer. Took about 5 minutes of careful work to get it the right size.

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I used compressed air and a toothpic to clean up the edge and make sure there were no rough edges or palstic shavings on the modified wafer. Once it all looked good and clean I reassembled 270 housing and wafer with the 500K pots shaft/rotor and cap.

Tested with a multimeter and later with audio to make sure that I had not scratched the wafer or damaged the rotor in the process.

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I have done 5 of these now (one test and four for the bass) and all have worked well with no problems.

Wow excellent work, DIY pots, what next?

Quote:

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Originally Posted by stan

what next?

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Well, since you asked... I have been trying to think of a way of keeping the wires from being visible through the F-Holes... My current idea is to make a sort of rigid frame our of stiff copper wire. Not sure if this is the same in Australia, but what I am using is the ground (earth) that comes in household wiring. It's about 2mm thick copper.

Tried it on on pots and a jack that I am not going to use, and it seemed to work well in the experiment:

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I did a test run partly because the ground wire is pretty thick and I thought I might need to leave the soldering iron in place so long to get everything hot that it might damage the pots. Good news is they seem to have come through without problems.

Took a couple of tries to get the technique down, but the key is (a) tin everything, the wire, the pot housing and the iron, and then (b) get the iron really hot. I can set my soldering iron from 10-50w, and for this I had it on 50w. I touched the tinned soldering iron tip to the tinned wire, got it hot, then touched both to the tinned pot housing for as brief a time as possible, and removed heat as soon as I saw the pot housing solder begin to flow. Tested each pot housing and the jack ring with a multimeter to make sure all there was continuity everywhere.

The other trick, since this is for a semi-hollow and must fit through an F-hole, is to replicate the hole pattern from the guitar on a piece of cardboard or, preferably, thin wood. Since the frame is rigid, it needs to line up pretty exactly with the holes.

Still thinking about the next step. What I am tempted to do is to wire up the pots with ordinary wire, then wrap them to the copper wire framework with aluminum tape. Any place the tape touches the pot or copper wire it will be grounded, and that should shield the wire.

I also have a little wire mesh tube on order that I may use with some shrink wrap to do the same thing. Whatever way I go, I am thinking the copper wire framework will keep it out of sight around the F-hole.

One note, I am using a C&K pickup selector switch. On the pic with the cardboard you can see that the framework does not extend to it. The reason is that I could not, as we say "for love or money" get solder to stick to the metal side of the switch. I was using an extra switch in the experiments, and the good news is that all my cooking did not seem to damage the switch. But nothing made solder stick to it either...

Nice idea. I assume you've tested to make sure you can get it in the holes while connected?

If you use tape and it connects at multiple points you may end up with earth loops... ideally you would want to run some sort of screening and only connect it to the earth wire at one end, but I don't know how much of a problem this would actually cause in practice.

I did a rubbing to get the hole pattern. Lines up well so it seems to have worked.

Didn't even consider groundloops...

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The more I read about groundloops the more I wonder if my idea tends more toward "brilliant" or "brain-fart". Looks like I'd be better off to use either the shielding or frame, but not both... ?