12 Sep
2006
12 Sep
'06
12:09 a.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
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?
--
David Griffith
dgriffi at cs.csubak.edu
A: Because it fouls the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?
12 Sep
12 Sep
6:31 a.m.
Upon disassembly, these switches are revealed to have
rubber domes inside
that electrically connect two isolated semicircles by pressing a circle of
conductive rubber. ... I've come to one that refuses to work at all,
regardless of how I clean it.
...
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.
I fixed one of my PET 8032 keyboards in a similar manner - the keyboard
uses plungers to push circles of conductive rubber against pairs of
interleaved "fingers" etched onto the board - this particular PET had been
sitting a LONG time and the conductive rubber had increased in resistance
so much that almost all the keys did not work at all - some of them would
work if you really pushed down, but all were much higher resistance
than I measured on other PET keyboards - cleaning etc. made no difference.
So what I ended up doing was cutting small squares of tinfoil and using bits
of tape, I fixed one over each set of "fingers" etched on the board - then
bent them up slightly so that they did not connect unless pushed down. I
fastened them to the board instead of the plungers for several reasons:
- Taping them to the board was easier than gluing them to plungers,
and less risk of contamination on the contact from glue.
- By fastening them to the board they would not "move around" with
the motion of the plungers - preventing undue wear from the metal
on metal (don't know if this would really be an issue or not).
- Completely reversable - no glue, no holes or other modifications.
The only thing I might have to do to remove it would be to clean
some tape residue from the board.
It worked very well, and the keyboard is going strong a couple of
years later. I probably would look for a better solution if this were
an "everyday" machine, however it's mostly used for the occational
demonstration etc.so the keyboard does not see a whole lot of use.
Dave
BTW: I have similar vintage PETs with identical keyboards in
which the conductive rubber is fine ... anyone know what
conditions accelerate the deterioration of conductive rubber?
--
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
http://www.classiccmp.org/dunfield/index.html
6:53 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
7:55 p.m.
On Tue, 12 Sep 2006, Tony Duell wrote:
I found out last night that the switches are the same. The question then
would become "are the switches on the junked keyboard still good?". See
http://home.online.no/~kr-lund/repair.htm
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. 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).
You can't mix up small batches? 100 is about how many switches I
have to fix (two terminals worth)
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.
This makes sense as conductive rubber is graphite-impregnated to begin
with. I think that repair kit mentioned above might be powdered graphite
with some sort of binder.
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.
See http://userwww.sfsu.edu/~hl/c.TR210.terminal.html for pictures of one.
No number pad.
--
David Griffith
dgriffi at cs.csubak.edu
A: Because it fouls the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?
13 Sep
13 Sep
5:54 p.m.
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.
I found out last night that the switches are the same. The question then would become "are the switches on the junked
keyboard still good?". See
http://home.online.no/~kr-lund/repair.htm
Maybe, maybe not. My experience is that you'll get _some_ good switches
from such a keyboard. How many do you need (no, I
don't have any spares...)?
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).
You can't mix up small batches? 100 is about how many switches I have to fix (two terminals worth)
Well, if you need to do all the switches, it may be worth buying said kit
(if it's still available) and giving it a go.
-tony
6:58 p.m.
On Wednesday 13 September 2006 05:54 pm, Tony Duell wrote:
I'd feel a whole lot better about taking some company's word that this is
necessary if they'd say _why_ it is.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
You can't
mix up small batches? 100 is about how many switches I
The instructions specifically tell you to mix the entire kit, and not to
measure out, say, half of it. Of course they want to sell more kits :-)
7:26 p.m.
On Wednesday 13 September 2006 05:54 pm, Tony Duell wrote:
I'd feel a whole lot better about taking some company's word that this is
necessary if they'd say _why_ it is.
I would assume it's because at least one part is not totally homogeneous
and may partially separate in storage. If you take, say, half of the
contents of the container you might not be getting half of everything.
-tony
You
can't mix up small batches? 100 is about how many switches I
The instructions specifically tell you to mix the entire kit, and not to
measure out, say, half of it. Of course they want to sell more kits :-)
14 Sep
14 Sep
2:12 p.m.
On Wednesday 13 September 2006 07:26 pm, Tony Duell wrote:
That makes sense as one possible reason. I'd prefer not to have to assume,
though.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
On Wednesday
13 September 2006 05:54 pm, Tony Duell wrote:
I'd feel a whole lot better about taking some company's word that this is
necessary if they'd say _why_ it is.
I would assume it's because at least one part is not totally homogeneous
and may partially separate in storage. If you take, say, half of the
contents of the container you might not be getting half of everything. You
can't mix up small batches? 100 is about how many switches I
The instructions specifically tell you to mix the entire kit, and not
to measure out, say, half of it. Of course they want to sell more kits
:-)
6:28 p.m.
[Mixing part kits of 2-pack stuff]
That makes sense as one possible reason. I'd prefer not to have to assume,
though.
Well, apart from that, and the 'business' reason (that they sell more
kits if you waste most of them), can you think of any other reasons?
-tony
I'd feel a whole lot better about taking some
company's word that this is
necessary if they'd say _why_ it is.
I would assume it's because at least one part is not totally homogeneous
and may partially separate in storage. If you take, say, half of the
contents of the container you might not be getting half of everything. 
15 Sep
15 Sep
10:05 a.m.
[Mixing part kits of 2-pack stuff]
That makes sense as one possible reason. I'd prefer not to have to
assume, though.
Well, apart from that, and the 'business' reason (that they sell more
kits if you waste most of them), can you think of any other reasons?
I've worked with a number of 2-part molding and casting supplies, mostly
urethanes and variations on RTV rubber, and they're often sensitive to
oxygen and/or atmospheric moisture: once you open the factory seal, they
start a slow chemical change unless you can stop the effect (say by
displacing the air: http://www.smooth-on.com/rubbaccess.asp#xtend for
example). I'd suspect the company's saying "mix the entire product all at
once" may be based on something like this. (The fact that you end up
using it up faster is doubtless a benefit in their eyes, too.)
But, yes, unless the manufacturer will _tell_ you what's going on, this
would only be a guess. :)
...ObClassicCmp: If you're looking at casting replicas of various small
plastic pieces for no-longer-manufactured items, a look through
smooth-on.com will likely give you useful ideas and a place to buy useful
materials. I _like_ toys which give you the capability of building more
toys. :) :)
-O.-
I'd feel a
whole lot better about taking some company's word that
this is necessary if they'd say _why_ it is.
I would assume it's because at least one part is not totally
homogeneous and may partially separate in storage. If you take, say,
half of the contents of the container you might not be getting half
of everything.
5:51 p.m.
I would assume
it's because at least one part is not totally
homogeneous and may partially separate in storage. If you take, say,
half of the contents of the container you might not be getting half
of everything.
That makes sense as one possible reason. I'd prefer not to have to
assume, though.
6:26 p.m.
Quoth Tony Duell:
Incidentally, do you have any experience of the 2-pack
'synthetic
rubbers' that Devcon (and others) make? I may well need to use such a
product to make a roller (for a classic computer -- an HP9820's internal
printer), and I do need to mix small amounts. The smallest Devxon pack I
can get over here is something like 500g, and costs \pounds 30.00. I am
going to go broke very quickly if I have to pay that out for every test
run (I know it won''t work first time).
I've worked with silicone RTV rubber. For molding/casting it has
excellent properties for holding small detail, releasing cleanly from
pretty much ANY material that isn't itself, and lasting for multiple
pours; it's also done a good job staying stable in the container for long
periods (read: 4-6 months) even after opening (so it can be used a little
at a time), and have never had a mold lose any stability at all once the
RTV had cured.
Having said that, I've never tried it for applications like printer
rollers, rubber capstans and such. Whether its hardness or other
properties would be suitable for that, I can't say; but I do think it
would be worth a try. If you do try it, let me know what happens.
My only advice: Whatever you cast it in, make sure that cast is completely
free of oils, grease, sulfur-based modeling clay (though "Sculpey" is
okay) or latex. It'll prevent the stuff from curing, leaving you with
either A) something a bit too unstable to use or B) one hell of a mess.
:/
Presumably the people at Devcon will have some documentation for their
stuff which can be downloaded (and if they don't, try www.smooth-on.com;
they do for their products).
-O.-
7:37 p.m.
I've worked with silicone RTV rubber. For
molding/casting it has
My only experience of RTV is as the 'gasket replacement' stuff. It works
well for that, but that grade would be far too soft to make a roller.
excellent properties for holding small detail,
releasing cleanly from
pretty much ANY material that isn't itself, and lasting for multiple
The Devcon stuff is a 2-pack -- adter mixing it lasts for 30 minutes
(according to the data sheet I've read) and cures fully in 24 hours. They
also sell a mould release agent to coat the mould with, and a primer to
coat things you do want it to stick to (in my application I do want it to
stick to the spindle). They specifically mention 'transport rollers' as a
possible application,
My main concern is that I know I'll have problems the first few times.
Maybe I'll misdesign the mould, or be unable to get the part out, or
whatever. The roller in question is about 3" lon and 1/2" in diameter, a
lot less than one kit's worth of stuff. And I simply can't afford a
complete kit for every attempt.
IO've read the book 'How to cast small metal and rubber parts' (or some
similar title). The authors say that Devcon used to recomend mixing
part-kits, but now recomend against it. But that it should still be OK.
-tony
12 Sep
12 Sep
7:58 p.m.
On Tue, 12 Sep 2006, Tony Duell wrote:
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.
6B pencils are easily obtained from art supply shops here. How long did
this treatment hold up?
--
David Griffith
dgriffi at cs.csubak.edu
A: Because it fouls the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?
13 Sep
13 Sep
5:55 p.m.
On Tue, 12 Sep 2006, Tony Duell wrote:
I didn't know if the designations for grades of pencils were the same in
the States or not.
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.
6B pencils are easily obtained from art supply shops here. How long did this treatment hold up?
It's till working (8 or so years later), but I don't use the M4 that much.
-tony
16 Sep
16 Sep
9:32 p.m.
Tony Duell wrote:
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).
I found, while repairing mid-80s synthesizers, that roughening the
rubber bit with sandpaper then painting on silver-loaded paint (from
Halfords, the stuff they have for fixing heated rear windscreens) worked
just great.
Gordon.
18 Sep
18 Sep
1:21 p.m.
On Sun, 17 Sep 2006, Gordon JC Pearce wrote:
Tony Duell wrote:
I'm thinking that the finer side of an emery board would be the roughest
one would use, right? This seems even better than rubbing the pads with a
soft pencil -- less likely to wear off.
--
David Griffith
dgriffi at cs.csubak.edu
A: Because it fouls the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?
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).
I found, while repairing mid-80s synthesizers, that roughening the
rubber bit with sandpaper then painting on silver-loaded paint (from
Halfords, the stuff they have for fixing heated rear windscreens) worked
just great.
19 Sep
19 Sep
3:14 p.m.
David Griffith wrote:
On Sun, 17 Sep 2006, Gordon JC Pearce wrote:
Oh, just enough to scratch up the rubber to key it so the paint sticks.
I used fine wet&dry, can't remember what grade though.
Gordon.
Tony Duell wrote:
I'm thinking that the finer side of an emery board would be the roughest
one would use, right? This seems even better than rubbing the pads with a
soft pencil -- less likely to wear off. 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).
I found, while repairing mid-80s synthesizers, that roughening the
rubber bit with sandpaper then painting on silver-loaded paint (from
Halfords, the stuff they have for fixing heated rear windscreens) worked
just great.
6497
days inactive
6504
days old
17 comments
6 participants
participants (6)
-
ard＠p850ug1.demon.co.uk -
dave06a＠dunfield.com -
dgriffi＠cs.csubak.edu -
gordon＠gjcp.net -
ohh＠drizzle.com -
rtellason＠verizon.net