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Re-manufacturing Carbon Resistance Strips in Slide Potentionmeters - any idea what binder

Discussion in 'Electronic Repair' started by David Forsyth, Feb 9, 2007.

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  1. I have a need to rebuild some slide pots from 1970's musical equipment that
    have worn-through resistance strips. The pots are long since unobtainable
    (made by CTS), and I have yet to find any suitable replacements. The
    resistance strips inside the pots are removable, and consist of a die-cut
    non-conductive substrate onto which a carbon resistance compound was bound
    and on which the slide contact of the pot slides. Each end of the strip has
    a metal connnector crimped to it to serve as an end terminal for the pot.

    I think if I knew what kind of binder and process is commonly used to adhere
    the carbon to the substrate I could restore the resistance layer. It would
    most likely have to be baked onto the strip, which is not a problem. For
    the resistance material itself, I was thinking of using material from carbon
    comp resistors (it would be trial and error to find the right value for this
    application). So, I am trying to find information on how resistance
    material is bound to a substrate for use in wiper/pot applications (it would
    have to be fairly resistant to abrasion, solvents, etc.) Please email
    foda01 at epix dot net with any helpful info.


  2. I suspect our time is just as valuable as yours, so you can see this here
    like the rest of us, if you're willing to look..

    Anyway, what about a conductive polymer? If the removable strips are in
    channels, or have enough width available, you could support a plastic
    conductor. If it's flexible, you can anchor it with small clamps based on
    the end connectors, or glue it in place. I doubt the value is very
    critical, and could be adjusted for by changes in fixed resistors nearby
    without compromising the design. They'd last longer than re-made carbon
    strips, too. I don't know a source for such polymers, but I know they
    exist, and someone here might know some standard part that is cheap enough
    to justify cutting strips loose from.
  3. Can't you just adapt the element out of a more available slide pot?

    Best regards,
    Spehro Pefhany
  4. Arfa Daily

    Arfa Daily Guest

    As if it's not going to be hard enough to make reliable, wear-resistant. and
    above all *linear* tracks, how on earth are you proposing to handle log or
    other tapers, which some are bound to have if this is audio equipment ? Is
    there really no similar sized off the shelf pot range that you could adapt
    the mounting brackets or PCB mountings of ? Even if they were a bit longer
    or shorter than the originals, they could be made to work . With many many
    years experience in the electronics repair and refurb business, I really
    feel that you are setting yourself a next to impossible task here ...

  5. SNIP
    I agree....
  6. So do I, but I suspect the guy wants to restore an old synthesizer or
    something, as close as possible to original. I doubt the makers of those
    would have ordered special pots made, they'd have made them in limited
    runs, each instrument would have been expensive, but I doubt many pots
    would have been needed, not enough to justify a custom run, so maybe they
    are spares/surplus bought from another firm that did get specials, most
    likely for mixers if log pots were used. (I bet they're all linear, just a
    guess though).

    If I were trying to restore an old synth or specialised audio item I'd
    contact the museums that spring up to keep such things, and get to talk to
    whoever does the restoration work.

    To get a better idea, we'd probably need more info from the OP, but he
    doesn't suggest he's coming back, he seems to expect us to take the time to
    go to him...

    Maybe someone will, if they think there's money in it. There might be, if
    the item is so special that the OP is prepared to contemplate such extremes
    to restore it accurately.
  7. Hi everyone -

    first off sorry for the confusion - I didnt mean to imply I wouldnt be
    checking back into the newsgroup, I just never know when I can get online
    and sometimes the older messages dont show up in my listings.

    second - good idea about the conductive polymer - I hadnt thought along
    those lines yet.

    The travel on these pots is 2 inches and the closest thing I could find is
    an alpha part but the construction is completely different internally and
    they dont offer all of the various values I would need. I wonder if I could
    have a lot of them custom made but I dont have megabucks to invest. These
    are for synths and there is a demand since the NOS stock either dried up a
    long time ago or the repair people who will even touch these things nowadays
    are probably clinging tenaciously to the reserve stocks they do have (or
    want an arm and a leg for them).

    thanks again to everyone who replied

  8. Think reconstruction. Perhaps a chassis that fits over the existing controls
    and lets you use available parts.
  9. Eeyore

    Eeyore Guest

    Can you post a pic of one alongside a ruler in
    alt.binaries.schematics.electronic ?

  10. N Cook

    N Cook Guest

    on one of my 2 tips files, URL below, is a formulation ofphotocopier toner
    and graphite i have used in such circumstances. Otherwise if only thin
    wipers , dismantle and move the wipers to fresh bits of track.
  11. Wow this is exactly the info I was looking for - thank you so much! The
    toner is the trick. I was thinking of trying to bind it with epoxy
    but I like the toner idea better. Do you think a heat gun would work in
    fusing the film onto the backing or would that be too hot or too
    In any case I have a place to start now - thanks again!

  12. N Cook

    N Cook Guest

    toner and graphite are both fine dust

    Diverse Devices, Southampton, England
    electronic hints and repair briefs , schematics/manuals list on
  13. ahh ok - for whatever reason I was thinking toner was a fluid - thanks

  14. Okay, well, many years ago I did some work along these lines. It's not
    all that difficult. CTS used to make some custom pots for us (very
    high resistance)-- they're right nearby, but I think they are
    concentrating on high-volume commercial/automotive sensor applications
    these days.

    You need to make the substrate (typically a paper-based phenolic). It
    would have been stamped originally.. the die would cost a pretty
    penny, but maybe you could get it cut using soft tooling with water
    jet or something like that (I imagine laser cutting would release some
    very unpleasant fumes).

    The resistive ink is then silk(sic)-screened on. You create the
    artwork and have a screen made at a supplier. It probably ought to be
    a stainless mesh in a metal frame. You will need the proper ink and to
    follow the manufacturer's recommendations as to what thinner to use
    and what screen cleaning chemicals to use. Communicate that
    information to the screen maker in advance so they can use the proper
    'hardening' depending on how harsh the solvents are. (Use a proper
    industrial screen supply house, not a T-shirt type operation).

    You will need a proper manual screening setup to hold the substrate in
    place. The gap between screen and substrate is critical to getting the
    thickness (and thus cross-sectional area, and thus resistance) within
    tolerance. It will probably take some experimentation to get within
    the 20-30% tolerance range, but trimming should not be necessary. The
    connections at the ends, I think, are typically made with crimped in
    riveted connections with a silver-bearing conductive ink or epoxy
    gobbed on around the rivet. You can probably get this material from
    the same supplier as the resistive ink. I don't recall the suppliers
    for this stuff, you're probably going to have to research it.

    You have to bake the element to cure it (possibly before attaching the
    end connections) with a specified time-temperature profile, so you'd
    need a small oven for that. It won't be very high temperature, since
    the substrate can't take high temperatures. Maybe 400°F for an hour.

    If you can sell 100 of them @ $100 each it might even be worth doing.

    Best regards,
    Spehro Pefhany
  15. But toner is ground hotmelt glue with some iron oxide IIRC.
  16. N Cook

    N Cook Guest

    iron in the form of iron filings is used in the process but not consumed ,
    even used in laser printers, no better process would seem to have been found
    for the "developer" that transfers the toner to the OPC.

    some notes of mine on copiers
  17. Arfa Daily

    Arfa Daily Guest

    Re-manufacturing pots to work in a semi-static application, such as a tone
    control, is one thing. I still think that doing the same for a pot that is
    going to get 'used', as just about every pot on a synthesiser *is*, due to
    the very nature of the beast, is going to be beyond doing reliably by
    amateur kitchen-table chemistry ... Just my opinion ...

  18. JeffM

    JeffM Guest

    David Forsyth TOP-POSTED:
    Note 1:
    If something from previous posts
    is not worthy of being ABOVE your post,
    it should be eliminated entirely from your post.
    It's called **trim and bottom-post**.

    Note 2:
    Excellent capture and retention here:
  19. Chris Jones

    Chris Jones Guest

    If these instruments are to be sold to the public then I agree with the
    others that you probably should make an obtainable pot fit and adapt the
    mechanical parts and circuit as necessary, because home-made pots will
    quite likely wear out quickly and your customers may not feel like they had
    their money's worth.

    If you do feel like making your own pots anyway, I wonder whether you might
    find that black conductive ESD-safe bags are suitable, at least for
    short-term use. In the UK at least, Farnell sells chips packed in black
    bags which I think are made by Vermason and are marked PE-LD. In my
    experiments I have found that if you cut a strip from one side of the bag,
    the strip of plastic seems to be made of three layers stuck together. Each
    surface of the strip is conductive but both conductive layers are insulated
    from each other.
    If the resistivity is right then you might be able to glue a layer into your
    pots. It is probably not easy to glue it in a permanent way. So basically
    the material is conductive on both sides but if you wiper is on the top
    surface then the end contacts need to be on the top surface also.

    Making pots with a taper slightly different from linear should be easy
    enough provided you can fit a strip of plastic which has a width that is
    not uniform along the track.

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