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  • #16
    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, "catīs whisker" somewhere on a carbon track edge and where wioern does not btouch it, but itīs 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.
    Juan Manuel Fahey

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    • #17
      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

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      "Take two placebos, works twice as well." Enzo

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      "If you're not interested in opinions and the experience of others, why even start a thread?
      You can't just expect consent." Helmholtz

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      • #18
        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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        Logic is an organized way of going wrong with confidence

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        • #19
          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.
          "Take two placebos, works twice as well." Enzo

          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

          "If you're not interested in opinions and the experience of others, why even start a thread?
          You can't just expect consent." Helmholtz

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          • #20
            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.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


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            • #21
              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...
              "Take two placebos, works twice as well." Enzo

              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

              "If you're not interested in opinions and the experience of others, why even start a thread?
              You can't just expect consent." Helmholtz

              Comment


              • #22
                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.
                "Take two placebos, works twice as well." Enzo

                "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                "If you're not interested in opinions and the experience of others, why even start a thread?
                You can't just expect consent." Helmholtz

                Comment


                • #23
                  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
                  "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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                  • #24
                    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.
                    "Take two placebos, works twice as well." Enzo

                    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                    "If you're not interested in opinions and the experience of others, why even start a thread?
                    You can't just expect consent." Helmholtz

                    Comment

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