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Thread: Tapping a pot?

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    Bent Member Chuck H's Avatar
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    Tapping a pot?

    Anyone know a clever way to tap the carbon film resistance strip on a pot that's somewhat durable and won't get in the way of the wiper?

    TIA

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    Old Timer Leo_Gnardo's Avatar
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    I've thought of that but never attempted it. If you're trying to say, replicate the tapped treble in brown Fender EQ stack, Hoffman has tapped 250K. IIRC they have non standard bushing and PC leads but they're the only tapped pot I've ever found for sale currently.

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    There are a lot of electric conductive paints, glue, gels, etc on the market. Think it worth to try with.

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    Bent Member Chuck H's Avatar
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    Thanks. Yepper, I know about those. I've found a bunch of other cool stuff in my searches too, but this is a pretty custom project I'm working on. I'll need a dual ganged 500k log pot with both tracks tapped at 50% resistance. Believe it or not I did find a linear taper like this! But I need an audio. Or rather, I need to make one it seems.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by catalin gramada View Post
    There are a lot of electric conductive paints, glue, gels, etc on the market. Think it worth to try with.
    Yes. And thank you. I read about using such products on R.G.'s GEO article:

    http://www.geofex.com/Article_Folder...s/potscret.htm

    But I'm not sure how to make such a connection durable/solderable/etc. so I was hoping for practical experience.

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    If are good enough to replace a soldering job ,I think will be good enough to make a bridge between carbon trace and a mechanical point. There are also markers , can draw a whole circuit with it, just do some investigation to see what fit You demands

    First page I opened: https://www.amazon.co.uk/Bare-Conduc.../dp/B00CSMDT8S
    I.m sure are thousand of products available on the market

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    Last edited by catalin gramada; 12-17-2018 at 04:41 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Chuck H View Post
    I'll need a dual ganged 500k log pot with both tracks tapped at 50% resistance.
    Then have a look at hi fi offerings. That kind of pot would be used in stereo preamps that have a "loudness" switch. Not sure about the 50% figure. But it's worth a search.

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    I think that the 1M master vol in pull boost TRs has a tap around there. The pull switches are starting to fall to bits now, but if you can find one, the track etc may be salvageable, and other parts could be canibalised from a new CTS pot.

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    Senior Member Pedro Vecino's Avatar
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    I see it possible to do it on a 24mm potentiometer. With a thin solid cable (I have used one with a diameter between 1/3 and 1/4 mm.) inserted under the track to obtain pressure and bent over it. Then fixed with a light touch of a suitable conductive glue. Just enough to adhere it and that the cursor finds a soft ramp when crossing it (you would need an ultrafine brush and make trials in others).
    Even without it I do not notice nothing strange in the cursor when turning it.
    After that it would only be necessary to make a small hole in the housing of the potentiometer to take it outside.


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    I'm just spitballin' here...
    but I'm thinking very thin copper or brass sheet like shim stock. .001" or .0015" or maybe thicker? .003"?

    Somehow slipped under the track, if you can get the track away from the wafer without damage + conductive adhesive or even super glue.
    Could then solder a wire to the shim stock to bring outside the pot.

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    Supporting Member The Dude's Avatar
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    If you can without giving away any trade secrets, what's the circuit? I'm thinkin' there must be an alternative way of achieving your goal. I don't like the idea of making parts. If they fail, you're screwed or you have to get them rebuilt/remade.

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    Bent Member Chuck H's Avatar
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    That's good! Most of the pots I've opened don't have a resistance strip you can get under. A thin piece of copper glued to the resistance strip with conductive adhesive is something I can solder a thin wire to. Probably right at the edge so it won't even meet the wiper. I'm thinking I'll tie an overhand knot in the wire to act as strain relief and run it through a hole drilled in the case. Now I just need to find a dual 500k log pot that will allow me access to both resistance strips.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by The Dude View Post
    If you can without giving away any trade secrets, what's the circuit? I'm thinkin' there must be an alternative way of achieving your goal. I don't like the idea of making parts. If they fail, you're screwed or you have to get them rebuilt/remade.
    Still a trade secret for the moment. I wouldn't be making these things to sell, but I do want to build some working prototypes before I seek a patent and put together a prospectus to shop around.

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    Senior Member nevetslab's Avatar
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    You might be able to slip in flattened 30AWG or smaller wire underneath the resistance track of OMEG pots. I've never tried this tapping approach, but here's some photos I have of a disassembled OMEG pot, I think from a Vox amp.

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    Getting a dual-gang pot apart and then re-assembled.....most I won't tackle....made for assembly only.

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    Bothered too much. Drill a hole at border of track, hook a wire through to ensure mechanical stability, put a drop of conductive smack and is done.

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    Old Timer J M Fahey's Avatar
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    Well, I have adtually manufactured pots and think you can not do that.

    Standard carbon track is made out of a 25mm or 16mm wide, 1 or 2 meter long pertinax (phenolic paper) strip, which is roller coated (for evenness and controlled thickness) with phenolic varnish containing various amounts of graphite dust which makes it conductive/resistive after drying and further baking in an oven.

    Then you punch/cut suitable "omega letter" shaped tracks, add contacts at ends, attached by hollow or solid rivets.

    Slider is an elastic piece of brass which **smoothly** slides over the delicate carbon track, without touching or even less going over the end rivets.

    Any jump/bump will quickly make slider lose proper contact with delicate carbon track and make it useless.

    1) you can not insert anything between carbon track and phenolic base the sameway you can insert nothing between, say, a car paint and stamped metal body, plus the "bump" will quickly destroy slider contact pressure.

    Same with anything placed over the carbon track.

    2) to improve contact at ends, pot manufacturer applies a tiny dot of "silver splash" , same phenolic varnish base as for carbon track but with silver dust in suspension, which also gets oven cured.

    Layer is same thickness and strength as main carbon one so wiper can slide over it with no problems, but it must be applid while manufacturing, not later.

    3) track taps are treated as "one extra end" so puncher uses a special die which leaves an "ear" where needed, which also gets its carbon surface and a little silver patch, to better contact an added riveted terminal.
    Wiper never ever touches that rivet, it always slides along the main track.

    4) mmmaaaayyyybbbeeee you can add an extra tiny contact (bent wire, brass strip, "cat´s whisker" somewhere on a carbon track edge and where wioern does not btouch it, but it´s a most unreliable contact, no real melding of carbon + silver paint but just a tiny mechanical contact, poorly held ... what coud go wrong?

    In a nutshell: it requires a factory custom made track, going through all the processes.

    If you eventually will use thousands, you may custom order them.

    For testing/prototypes use a many contact rotary switch (12/16/24/32) where you can build any custom "curve" and add as many taps as you wish, and only later commit to a cheaper regular pot.

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    Bent Member Chuck H's Avatar
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    Fully agree. But I still think a hack job tap is possible, though of course not ideal. And still easier than wiring a hundred tiny resistors to a switch with so many small, stacked contacts you need loupe to see what you're doing

    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Senior Member nevetslab's Avatar
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    I knew I had some other pot in my drawers who's carbon track wasn't fully bonded to the phenolic base. This came out of an Ampeg SVT-CL, think it might be CTS, maybe Clarostat. I was able to slip the blade of my X-Acto knife under / between the base and track, so should be easy to place a super-thin conductor under the track, and not have a 'bump' on the surface of the track.

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    Bent Member Chuck H's Avatar
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    Well I don't know if/when I might find a pot with a lift-able track. I sure appreciate the efforts and I'll keep a lookout for those brands in a dual A500k. The problem as I see it is that the dual pots may not be serviceable on the front pot assembly. I know this is the case with most brands. Finding a pot I can kludge is my problem though. I was just wondering how I might tap the resistance track.

    My plan right now is to use a strip of adhesive copper foil and just touch the track. wrapping the remaining strip around the wafer in a spot that doesn't contact the case. Then I'll apply some conductive glue (possibly epoxy) to the spot where the foil meets the resistance strip. The goal will be to avoid the wiper path by as much as possible. I've used the adhesive copper foil on a couple of shielding projects before and soldered it without it lifting. This idea is also minimally invasive in case I only have a small gap to work within. I think this could work.

    I researched an overseas manufacturer that will make the pots for about a buck each in lots as small as 500 pieces! That's a pretty fantastic deal that I may take advantage of if I decide to make some of these widgets myself.

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by nevetslab View Post
    I knew I had some other pot in my drawers who's carbon track wasn't fully bonded to the phenolic base. This came out of an Ampeg SVT-CL, think it might be CTS, maybe Clarostat. I was able to slip the blade of my X-Acto knife under / between the base and track, so should be easy to place a super-thin conductor under the track, and not have a 'bump' on the surface of the track.
    Maybe I misunderstood, but my impression of what Juan said about the 'pertinax' strip was that the underside would not have continuity to the resistive side. I guess a simple meter check would tell you right away.

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    Bent Member Chuck H's Avatar
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    Even though I don't have the pot I actually need on hand... I think I DO have a couple of old CTS pulls. I'll go look and see if they allow access to the bottom of the resistance strip. Then report back on whether it's suitably conductive. Just a sec...

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  22. #22
    Bent Member Chuck H's Avatar
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    Ok... Life got a little sideways for a bit. I opened up the pots I had and only one of them had a lift-able track. And that track was backed by a thin phenolic sheet that matches the track profile. I expect most are like this.?. How else would you stabilize the resistance strip? So, not conductive on the bottom for the one I have.

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    Drill a hole and hook a wire in the edge of the track. Run the wire out through a gap in the case.Make it rigid with epoxy

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  24. #24
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by catalin gramada View Post
    Drill a hole and hook a wire in the edge of the track. Run the wire out through a gap in the case.Make it rigid with epoxy
    Definitely part of the plan.

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    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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