Tandy TRS-80 Model 4

Tony Duell ard at p850ug1.demon.co.uk
Sat Dec 27 15:37:52 CST 2008

> 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.

Well, in _our_ kitchen, that's probably true...

On the other hand, I'll only be around if I don't do something silly when 
repairing a SMPSU, a monitor, or whatever. But if I don;t carry on 
working on such things, I won't get new repair tricks to share with you 
all. Oh well...

> > 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

Thinking about it (and it's been about 10 years since I've been inside my 
M4), mine might well have had 4 pins too.
> was figuring out how to pull the switch through the metal mounting plate 
> without having ready access to the sides of the switch.

Is it easier to remove the ones near the edge of the array (can you reach 
between the PCB and the frame)? If so, that may be why I took the whole 
thing apart. On some keyboard -- again I can't rememebr if the M4 is like 
this -- you can get to the locking clips on top of the keyboard, I 
modified an IC extractor tool to squeeze those in and release the switch 
when I was repairing HP9826 (etc) keyboards.

>   > 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.

The miixnig is trivial (IIRC you pour the little pot into the bigger one 
and stir it :-)). But the fact yuou have to use the whole lot at one go 
is a pain.

> I paid a visit to Rat Shack (Radio Shack for you non-US citizens) and 

Oh, I've some across Radio Shack (traded as 'Tandy' over here, until 
about 10 years ago). The thing to watch for is that when UK people talk 
about 'RS' they almost certainly mean 'RS components' 
(http://www.rswww.com/) who are a well-known component distributor over 
here and nothing to do with Tandy/Radio Shack.

> 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.

How many? If it's less than 12, I really would move them onto the numeric 
keypard. On these machines the keypad is wired in parallel with the 
appropriate swiches in the main part of the keyboard (so shifting the 
keypad keys does giuve you the top-row punctuation characters). You cna 
manage without it. 


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