My Experience with TRS-80 Mod III Keyboard Repair

[johndias]

Hi all, I wanted to share this because I found it interesting and I've gotten so much help from these forums I'm excited I have info to give back to the community!

 

Anyway, the keyboard on my model III worked fine with the exception of the "," key. So, I searched here, watched some videos and read some blogs and I was fully prepared to address with repair to the graphite pad on the plunger. To my surprise, that is not at all what I found!

 

First, I noticed that the PCB looked a lot less "busy" with fewer solder points...

 

 

Indeed, after removing the problem switch I understood I was dealing with something different than what I've seen in tutorials. The switch only has two conductors.

 

 

Here's the switch disassembled

:

 

The way that this works is the plunger (on the right) has a shim that pushes against a plastic tab on the contact mechanism (left). This makes the connection between the two contacts which are separated by a plastic insulator.

 

Some closeups of the contact mechanism... maybe hard to see, but on the side view there's a thin plastic layer between the two conductive plates.

 

 

The metal tab is springy and is pushed down by the plunger shim.

 

Here I've removed the metal spring to expose the plastic tab that forces contact between the two conductors.

 

Testing the contact mechanism by pushing down on the plastic tab with a screwdriver tip.

 

In the end, I found nothing at all wrong with the switch. It wasn't even dirty, although I suspect that maybe some debris got lodged in the mechanism and when I disassembled it the debris was knocked loose. Anyway, I soldered it back in place and my "," key works fine now!

 

Hope this helps someone who ends up with a keyboard similar to mine! In the end, this looks like a much better design than the graphite and rubber boot switches I've seen others have to deal with - hopefully less prone to problems.

[Bruce Tomlin]

Tandy used ALPS mechanical keyswitches for the III/IV and I think also the later Model I with the keypad. You can tell from the sculpted matte finish key caps, but also from the signature T stem on the key mechanism. The main downside is that ALPS stopped making them, and Cherry won the mechanical keyboard market.

 

The original Model I used a very hard to repair type of keyboard that was molded as one big piece of plastic, with metal finger contacts and a square frame plunger thing that the key cap slips into. Pulling out the plunger is likely to mess up the metal fingers. I have an old Lear Seigler ADM3A terminal that I'm restoring where those square frame plungers have split and the keys stick. TI-99/4 and Atari 800 also used those kind of keys. I've read where people used a hole drill to cut an entire key mechanism out of an Atari 800 keyboard to get repair parts.