Keyboards & Conductive Rubber

[Dave Dunfield]

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.

Several of the keys on this keyboard require excessive pressure to make
contact - I've tried cleaning the pads, and (gently) the surface of the
rubber, however it appears that the rubber has increased in resistance.

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.

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?

Regards,
Dave
-- 
dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Vintage computing equipment collector.
                http://www.parse.com/~ddunfield/museum/index.html