Tandy TRS-80 Model 4

[Michael B. Brutman]

Tony Duell wrote:
> The keyaord is a simple matrix of switches (similar to the M3 keyoard in 
> your manual (I assume) but with extra keys in the unused spaces of the M3 
> matrix. The switches can, and do, fail. What I did to my M4 was pull all 
> the keycaps (make a diagram of where they go back), then desolder all the 
> swtiches from the PCB (!). Take the whole lot apart, and then test the 
> switches with an Ohmmerter. Don't expect a dead short (they're contuctive 
> ruber contaics inside), but you can seprata the good from bad ones this way.
> 
> The swtiches do come apart. Clean the conductive rubber part and the 
> contacts and try aggain. This might get some of them going. The remaining 
> duds come apart again, and I rubed a 6B (very soft) pencil on the 
> conductive rubber part. That really reduced the resistance!. I then 
> worked out the least-used keyoard locations (in my case the number pad) 
> and put the dodgy/repaired switches there. I had enough good ones to 
> fully populate the main area with a few dubious ones in the number pad.

So far so good - the disassembly instructions were perfect.  The printed 
circuit board on the back of the neck started to snag a bundle of wires 
but I was able to peer through and spot the problem before it got serious.

The keyboard seems to be a major problem though.  I'm not too keen on 
desoldering 65 keys (4 contacts per key), but I don't see any way to 
avoid this.  Soldering is not my strong point, and even if I can do it 
cleanly that is a lot of work.

I'm looking for other ideas on rejuvenating this keyboard.  I'm not 
above sending it to a pro who knows what they are doing.  (The Computer 
News 80 guys/gals in Wyoming are starting to look appealing.)

The rest of the machine looks pretty good.  I'm going to set the 
keyboard aside for now and work on cleaning the drives.


-Mike