12 Sep
2006
12 Sep
'06
3:53 p.m.
So, I'm fooling around with my Tandy PT-210 printing terminal again.
Upon disassembly, these switches are revealed to have rubber domes inside
that electrically connect two isolated semicircles by pressing a circle of
These sound very similar to the switches used on the TRS-80 Model 3 and
Model 4 keyboards. They may even be the same parts -- the ones in the M3
and M4 were made by Alps IIRC. Any maker's name on these? If they are the
saem, a junk M3 or M4 keyboard would be a source of spares.
conductive rubber. Wiping with alcohol and blowing
them out gives a
closed resistance of around 300 ohms, which is good enough to trigger the
keyboard circuitry. I've come to one that refuses to work at all,
regardless of how I clean it.
So, I have two choices:
1) Glue tiny discs of aluminum to the bottoms of the rubber domes. A
preliminary test of this using water as "glue" suggests this won't work
for long.
2) Find some switches with the same footprint, height, and cross-shaped
acutator. The first two seem easy to get. Mouser lists absolutely no
switches with a cross top.
So, has anyone here any pointers?
A couple more suggestions. Chemtronics sell (sold?) a rubber keyboard
repair kit. It was a 2-pack thing that you mixed and then put a bit on
the conductive rubber pad. The problem was that it's expensive, and once
mixed you have to use the whole lot (enough for about 100 switches I
think).
When I did my M4, I had a few dodgy swtiches. I found rubbing the
conductive rubber pad with a soft (6B, if that means anything across the
Pond) pencil helped a lot.
And then I put said switches in little-used places, like the number pad.
I don't know if that's applicable to your terminal, though.
-tony