TRS-80 Question

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sun Mar 2 14:16:00 2003

> The thread has inspired me to pull my Model IV out of the closet and fire
> it up. Still works great. Some of the keys on the keyboard are a bit
> stubborn. Anyone have any tips for negotiating with the Model IV
> keyboard. I've never tried taking it apart.

It's a common problem, but the fix is long and tedious. You do have to
dismantle the keyboard...

First open the case -- one screw on the back, lots on the bottom, then with
the machine in the normal operating position, lift the top case up, turn
it counterclockwise and put it down alongside the machine. The monitor is
in the cover -- take great care not to break the CRT neck.

Then unplug the edge connector and ground wire (faston tab) from the
monitor PCB so you can put the top cover out of the way.

Remvove the keyboard bezel (half a dozen screws). 'Friendly' machines
have the keyboard ribbon cable plugged into the keyboard PCB -- if so,
unplug it there and remove the keyboard. Others have it soldered to the
keyboard PCB. With those you have to remove the shielding and a cable
clamp over the CPU board at the back, unplug the cable at that end, and
feed it under the disk drives.

OK, you've got the keyboard out. Pull off all the keycaps, including the
reset button (make a diagram of the positions -- graph paper is good for
this).

Flip the keyboard over, and desolder (!) all the keyswitches, but not the
reset button. Make sure all the pins are free. Undo the screws, and lift
off the PCB.

Unlcip all the keyswitches from the metal frame.

Nowtest the keysswitches. Measure the resistance between the pins when
the switch is prssed. A 'good' one is a few hundred ohms at most. Sort
those out first.

Take the 'bad' ones apart (the housing unclips, then there's a spring,
plunger, rubber dome with a conductive rubber block inside). Clean the
parts, particulalry the contacts inside the base, and the conductive
rubber contact. Put them back together and test again. Any that test OK
now are classed as 'possibly OK'.

If you still have 'bad' ones, take them apart again, and rub a 6B pencil
on the conductive rubber contact. That should get them working. But class
these as 'marginal'

Put the PCB back on the metal frame and screw it in place.

Take the 'good' switches and put them in the most-used places on the
keyboard. When I did this, I had enough 'good' ones to completely fill
the alphanumeric area, leaving 'marginal' ones for the number pad (which
is not essential). And then, as I jsut implied, put the 'possibly OK'
ones in the slightly less-used parts, and the 'marginal' ones in places
you can do without.

Solder them all in place (this doesn't take that long!), fit the keycaps,
reassemble the machine.

-tony
Received on Sun Mar 02 2003 - 14:16:00 GMT