Keyboards & Conductive Rubber

[Dave Dunfield]

Tony:
>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.


Gordon:
>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.


Paul:
>    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.

Regards,

Dave
-- 
dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Vintage computing equipment collector.
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