6 Sep
2004
6 Sep
'04
11:50 p.m.
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