Tandy TRS-80 Model 4

Tony Duell ard at p850ug1.demon.co.uk
Sat Dec 27 14:23:17 CST 2008

> Tony Duell wrote:
> > As I mentioned, it is possible to take the swtiches apart and clean the 
> > contacts and/or coat them in a bit of pencil graphite. But how long it'll 
> > last I don't know. I did it to mine, they're still working _but_ I put 
> > them in places where I'd not really use them (I rarely use numeric 
> > keypads, so I put the bodged switches there). You;'ve got far too many 
> > duds to do that, though.
> This is an old thread - March of this year.  It was about a TRS-80 Model 

Sure, but I'm still here (and I am still using that Model 4)

> 4 with a really bad keyboard.  Only a handful of keys had any sort of 
> life in them.
> I now have a replacement keyboard for the machine that is significantly 
> better.  I was able to cleanly desolder a dead keyswitch and disassemble 

I found it relatively easy to desolder the keyoard. I think it's just a 
single-sided PCB. From what I rememebr, it's easiest to pull all the 
keycaps, then desolder the 2 pins of a particular switch and unclip it 
from the frame. Dont' try to desolder the whole lot at once and take the 
PCB off before removing any swtiches.

> it to examine it.  The keyswitch mechanism is exactly as you described - 
> a rubber dome with a conductive pad in the middle.

_Many_ years ago, I needed a replacement keyswitch for my Model 3 
(similar keyboard, indentical switches, but with a few fewer keys). Back 
then you could order them from Tandy National Parts (at least in the UK), 
but I head to give the maker of the switch. I was told that there had 
been several keyswitches used, and they weren't all interchangeable. Mine 
(and the ones in the M4) were made by Alps, which might help you find a 
sourve of replacements. I've never mamanged to do that, but I suspect 
they were (at least 20 years ago) a stnadard part.

> The question now is about the conductive pad.  Cleaning with rubbing 
> alcohol has not helped anything.  Is there anything besides pencil

Id didn't help for me either. Nor did abrading the surface with wet-n-dry 

> graphite that will work to lower the resistance?  I'm looking for 
> something that will last a while, and your concerns about the pencil 
> graphite have me concerned too.  I'm not going to desolder and repair 

There are kits sold to repair(TV, etc)  remote controls which use a 
similar technology. The one I used (not on this keyoard) was made by 
Chemtronics IIRC, it was a 2-part thing. You mixed the contents to the 2 
pots and applied the result to the surface of the bad. You had about 30 
minutes to do that after mixing (so have all the swithces open at the 
start) and then you left it overnight to set.

The prolem is that the kit is not cheap, and once you've mixed it it only 
lasts 30 minutes. And you have to mix the lot in one go.

> every keyswitch so I won't be able to move dodgy ones to the keypad to 
> avoid using them.  I'd rather take a little more time and find a better 
> solution ..


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