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

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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.

thanks,

Dave

 

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.

 

Can't you just adapt the element out of a more available slide pot?



Best regards,
Spehro Pefhany

 

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 ...

Arfa

 

SNIP

I agree....

 

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.

 

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

Dave

 

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

 

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

Graham

 

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.

 

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
inconsistant?
In any case I have a place to start now - thanks again!

Dave

 

toner and graphite are both fine dust

Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

 

ahh ok - for whatever reason I was thinking toner was a fluid - thanks
again!

Dave

 

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

 

But toner is ground hotmelt glue with some iron oxide IIRC.

 

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
http://www.divdev.fsnet.co.uk/copiers.htm

 

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 ...

Arfa

 

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:
http://groups.google.com/group/sci...._frm/thread/9e5865a448fa2327/58fb854b042b223b

 

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.
http://www.vermason.co.uk/products/packaging/conductive_flexpack.asp
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.

Chris