Tandy TRS-80 Model 4

Michael B. Brutman mbbrutman-cctalk at brutman.com
Fri Dec 26 17:33:17 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 
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 
it to examine it.  The keyswitch mechanism is exactly as you described - 
a rubber dome with a conductive pad in the middle.

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

If I press hard enough on the bad switch eventually the resistance will 
come down from infinity to around 6000 Ohms.  A good keyswitch shows 
under 100 Ohms of resistance.

A picture of the insides of the keyswitch (including the rubber dome 
contact I'm trying to rejuvenate) can be found here:



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