The TRS-80 Journey Continues

Tony Duell ard.p850ug1 at
Wed May 25 08:20:38 CDT 2022

On Wed, May 25, 2022 at 2:07 PM Bill Gunshannon via cctalk
<cctalk at> wrote:
> Another question for the masters here.
> I just tried to revive my Model III.  More than half the
> keys don't work anymore.  What is the conventional wisdom
> on cleaning these old TRS-80 keyboards?  Is compressed air
> usually enough?  Can I spray the switches with something
> like DeOxit safely?  I expect when I go to revive my
> Model I's they are likely to be in the same state.

I think there were several keyboards used, the one I know (I have
versions in the Model 3 and Model 4) was made by ALPS

Anyway the switches in that have little conductive rubber pads on the
end of the key plunger, These are pressed against a pair of metal
contacts at the bottom of the switch to complete the circuit. Of
course the rubber wears. Cleaning,trying to re-surface the rubber,
etc, makes only a temporary repair, but it's better than nothing.

I started by pulling the keycaps off. Then I desolded all the switches
from the PCB (it sounds a worse job than it is!). Took the PCB off,
then unclipped the switches from the metal frame.

The switches do come apart by unclippng the 2 halves of the housing. I
cleaned the metal contacts with propan-2-ol, lightly rubbed the rubber
on fine wet-n-dry paper and rubbed a soft pencil -- the softest I
could get, a 6B I think -- on the face of the pad. Put the switch back
together and measured the resistance when it was pressed. Not that the
input circuit on the CPU board of these machines is quite high
resistance, so a switch that has a resistance of, say 1k ohms will
work fine.

In the end I got it down to having 3 or 4 that were a bit dubious,
needing harder-than-normal pressure or not working every time. Those I
put in the numeric keypad area of the keyboard, I could easily live
without that...

Incdentally old Model1 machines have the metal contact keyboards, same
contacts as the DEC VT52, VT100, the HP85, etc, the TI99/4A etc. Those
are a lote more reliable.


> bill

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