Keyboards & Conductive Rubber

From: Dave Dunfield <>
Date: Tue Sep 7 07:31:37 2004

>I've seen this happen. My TRS80 M4 keyboard (which uses individual
>keyswitches which use this technology) had this sort of problem. Of
>course there I could move them around to put the bad ones on the least
>used keys (like the numeric keypad).
>Two ideas :
>1) Rub a _soft_ pencil (I manged to get a 6B) over the rubber pads. This
>will put a new graphite layer down.
>2) Chemtronics sell (sold?) a repair kit for these pads. You mix the 2
>parts and then put a drop on each of the pads. The problem is that once
>you've misxed the 2 parts you have to use the kit in a day or so, and
>there's enough stuff for quite a few switches -- and it's not cheap. It's
>therefore probably not worth doing for one TV remote or something, but it
>might be just right for a keyboard.

>The pads either wear or "dry out" or something, never worked out which.
> I talked a mate of mine through repairing some vintage synthesizers
>which use conductive pads for the control panel (Sequential Six-Trak,
>Alesis drum machine and sequencer) by taking the panel apart and
>painting a blob of silver-loaded paint for repairing heated rear windows
>in cars. The bottle was a couple of pounds from Halfords, and probably
>cheaper in your local independant motor factors. A tiny bottle lasts
>for ages because you only need a little spot.

> I've tried several things on my Sony TV remote control. The various
>paints and other expensive fixes don't last very long. What finally worked
>for me was some heavy aluminum foil tape used for sealing air ducts.
> Cut some tiny squares of the tape just big enough to cover the two
>contact pads on the circuit board and stick it to the surface of the key
>plunger. Evidently the resistance value is not critical, as long as it's
>below some minimum value -- zero ohms is OK. My repair has been working for
>a couple of years now.
> You can find the tape at home centers (Lowe's/Home Depot), and appliance
>or plumbing parts suppliers.

Hi Guys,

THANKS for the good ideas...

I think I like the tape idea the best - I had thought about trying to glue
in a bit of tinfoil - I've done that for "round" pads, however these are
very thin/long pads, and the tape might be easier to work with.

Btw, they are NOT the kind where the PCB pads are interleaved "fingers" -
these are two separete pads which are perhaps as much as 1/8" apart - the
rubber bit is nearly 1/4" long and has a "bump" on each end to contact
the pads - this means that the rubber flexes as it pushes down, and I
think that graphite or paint would probably crack and flake off.

I don't think it would be easy to replace the rubber pads, as you would
need the right shape, and also the pad is bonded to a little strip of
material like this (cross sectional view):

     | | <= Key plunger
     | |
  | | <= open space
  +--------+ <= "Carrier strip"
    [====] <= Conductive Rubber Pad

Btw, this is for a Commodore PET - I just remembered that I have an
extra C64 keyboard, so I'm going to pull it apart and see if by any
miracle the key assembly is the same.


dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools:
com             Vintage computing equipment collector.
Received on Tue Sep 07 2004 - 07:31:37 BST

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