Keyboards & Conductive Rubber

[Tony Duell]

> 
> Hi Guys,
> 
> I'm trying to revive a keyboard (integrated into a vintage computer so
> it's kinda important) - this is the kind where there's a PCB with lots of
> "pads" in the scanning matrix, and the keys push a little pad of "conductive
> rubber" down onto the PCB pads to make the connection.

[...]

> The good keys measure 200-300 ohms from one end of the conductive rubber
> pad to the other with only mild pressure - the bad keys measure 5k-10k
> unless you really squeeze them - under pressure they drop to 1k - 2k,
> which appears to be barely enough to trigger a detection.

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).

> 
> Anyone know what the failure mode is?  I though perhaps the rubber bits
> had cracked, however this does not appear to be the case.
> 
> Anyone have any tips/tricks to fix this problem?

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.

-tony