is and generates the appropriate ASCII code and a strobe. Fine, it sounds good
in theory, and given good close proximity of the pad on the key and the pads
on the pcb, it works.
How does muck affect the keyboard? Where does it go wrong? The lacquer on
the pcb is quite thick as far as pcb lacquers go, but is still only about
0.05mm. I haven’t worked out the capacitance involved, but the capacitance is
proportional to the area of the pads, and inversely proportional to the
distance between them. This is also multiplied by the permittivity (dielectric
constant) of the lacquer, which I will assume to be about 3 – 4 (air has a
dielectric constant of 1). Muck under the keys holds the keypad off the pad,
and the chunks of stuff I’ve found under the keypads is at least as thick as
the laquer. So the muck halves the coupling capacitance for a start, next,
because the keypad no longer makes contact with the lacquer (except for a
small area) the dielectric is now air and not the lacquer, let’s be generous
and assume that this only reduces the permittivity by half. The capacitive
coupling between the keys and the pads is now only a quarter of what it was,
so the capacitive change detected by the CPU is now between 0 and some rather
less than the 2pF (minus) which was present before.
Now this has puzzled Rotec, as the keyboards were designed with a certain
amount of muck in mind, after all that’s why the sense lines have 2pF in
series, to minimize the affects of the muck. After much poking around, they
have discovered that a whole batch of keyboards were made with a 10nF ceramic
capacitor which was marked as 10% tolerence component (to their spec) which
was in fact a wrongly marked –40/+100% component intended for use as a
decoupling capacitor. This of course degraded the sharpness of the scanning
pulses, with the predictable result that when the coupling capacitance was
reduced by muck, then the key failed to work.
Two further things compound the liability of key failure through muck
egress, the first batch of keyboards made no attempt to keep muck out, the
next and susbsequent batches have rubber strips round the edges between the
pcb and the keyframe to stop muck getting in, and between the keys to stop any
muck that does get in from moving around (and also to cut down noise). All
well and good, except there are 16 unfilled keyholes on the top of the
keyboard frame covered with sticky tape (not really tape, it’s quite thick).
Sad to say, this can peel off after a few months use leaving half inch square
holes on the top of the keyboard for the odd passing brick to drop through.
Secondly, the pads on the bottoms of the keys seem to be slightly sticky and
any muck that gets under a key promptly attaches itself to the keypad and
So onto the cure. Sling the thing back at your Gemini dealer, who in turn
will sling it back at Rotec, via Gemini, and shout at them. Actually, I
understand Rotec have been very good over this and all faulty keyboards
returned have been sorted out. That of course is the legal recourse and you
would be entitled to do just that, (provided that it is till under warranty)
unfortunately, it’s not a lot of help if you have just written a couple of
hours rubbish using PEN (not having backed up of course) and then discover a
passing brick has got under the CONTROL key, and all PEN does is print ZZZZZZ
instead of breaking out of the insert mode (^Z, for those who don’t know PEN).
That’s what happened to me once, which is the reason for this bit. The well
tried cure is to turn the keyboard on its end and ‘thumping it one’ (known in
the trade as a technical tap) in the hope that the muck will get dislodged and
the key will become functional. Needless to say this didn’t work. So in my
wisdom I decided to take the keyboard apart. When I’d fixed the keyboard, I
found that the action of removing or replacing the keyboard had crashed the
video card, and using RESET to clear it lost about half of what I had written.