Tandy TRS-80 Model 4

[Michael B. Brutman]

Tony Duell wrote:

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

Tony,

We expect you to be around for a while because rumor has it that you can 
be fixed with parts commonly found in a kitchen drawer.

> 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.

Desoldering the single switch was fairly easy.  I just wasn't looking 
forward to doing 70 of them.  My particular key switches have four 
contact pins.  The only inscription on the switches is 'SMK'.  The trick 
was figuring out how to pull the switch through the metal mounting plate 
without having ready access to the sides of the switch.

  > 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.

Bingo - somebody else informed me about these kits over at Erik Klein's 
web forum.  Except for the mixing and expiring part, the kit sounds like 
the perfect fix.

I paid a visit to Rat Shack (Radio Shack for you non-US citizens) and 
did not find the conductive ink pen.  The store manager is pretty good 
about his parts, and he knew it was discontinued.  I told him I was 
restoring a Model 4 and his face just lit up. :-)

I'm going to experiment with the conductive ink pen first because it 
looks easy to apply.  I only need to fix a few keys on this keyboard. 
If that fix doesn't hold up then I will go for the rubber keypad repair kit.



Mike