TRS 80 Model 1 Question

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu Mar 11 23:07:03 2004

>
> Hello.
> I am about to take a TRS 80 under the knife to repair a keyboard. The
> computer works fine, 16K Model 1/ 008 (never seen one with a part number
> ending with "1008". The S/N 0000738) ...Anyway the E key does not work on
> the main keyboard, and the 2 key does not work on the number pad. I
> believe it's a contact problem, not a chip problem, based on what I have

The latter _must_ be a contact problem (the 2 '2' keys, on the main part
and the number pad, are just wired in parallel), the former almost
certainly is too (it could be a very strange ROM error, but I think we
can ignore that for now).

The M1/3/4 keyboards are electrically a matrix of switches. They're
scanned by the CPU address lines (!), suitably buffered. The outputs go
to the data lines, also via buffers. A chip fault will take out an entire
row or column of the matrix.

I am not sure what sort of keyboard you have. Old model 1s have the leaf
spring switches, like on a VT100 or HP85. With those you can pull the
keycaps, plungers and springs and if necessary desolder individual
contacts. Often, though, just pulling the keycaps and cleaning things
cures problems.

M3s, M4s, and I believe later M1s have indidual keyswitches (and I think
there are at least 2 flavours). You can pull the keycaps, desolder the
switch pins from the PCB, and then unclip the switch from the frame. The
switch then comes apart for cleaning. Some of them use conductive rubber
contacts -- if you can't get this type to work, rub a very soft pencil (a
6B if yo can get one) over the rubber contact inisde the rubber dome.

I had to do this to my M4 keyboard. I then moved the dubious switches to
the number pad, since I never use that area. That way I had all good
swtichs on the main part of the keyboard...

-tony
Received on Thu Mar 11 2004 - 23:07:03 GMT