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

"David Forsyth" wrote in news:_15zh.2590$ snipped-for-privacy@news1.epix.net:

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

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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....

"martin.shoebridge" wrote in news:V57zh.11842$ snipped-for-privacy@newsfe7-gui.ntli.net:

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

that

has

adhere

would

carbon

this

would

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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

toner and graphite are both fine dust

Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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

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:

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.

Chris