11 Jul
1997
11 Jul
'97
12:09 p.m.
In message <51194C00BD39CF11839000805F385DB20578947C(a)RED-65-MSG.dns.microsoft.com>
classiccmp(a)u.washington.edu writes:
REPAIRING CAPACITIVE KEYBOARDS
v1.0 7/10/97 Kai Kaltenbach
---> What are capacitive keyboards?
The vast majority of capacitive keyboards were and are manufactured by
Keytronics of Spokane, Washington. The mechanism is really quite
simple. A keypress pushes a foil circle onto the surface of a printed
Note that the foil (actually a metal layer) is on the inside surface of a
plastic disk on the end of the key plunger. It doesn't actually make
contact with the PCB
circuit board, thus completing the circuit and
registering the keypress
More correctly it increases the capacitance between 2 tracks on the PCB.
to the computer. Capacitive keyboards are very
reliable; however, they
will fail over time even if unused.
Each foil contact circle is backed by a cylinder of PVC foam. The foam
is subject to environmental deterioration. Once the foam loses its
elasticity or dissolves entirely, the keys will stop working because the
foil circles will no longer make good contact with the PC board.
Various classic computer systems, such as the Processor Technology Sol,
exhibit this problem after their 20+ year lifespan.
I've once seen a fault where the metal layer vanished, leaving the foam
intact. No idea what happened, but it didn't work, of course.
---> How do I tell if I have a capacitive keyboard?
Just remove a key cap -- if there is a separate spring under the key
cap, around the plunger, it's extremely likely that it's a capacitive
keyboard. They won't always say Keytronic on the outside of the
keyboard housing.
These keyboards are _everywhere_. I've seen them 'made' by PERQ, Apricot,
Acorn, Tektronix, FTS, and probably more. The only problem is that some of
those are classics in themselves, and probably shouldn't be raided for
spares.
---> What do I need to start a repair job?
First you need a set of replacement foam pads. Each foam pad is a
cylinder about 1/4" in diameter and 3/16" long. There is a semi-rigid
clear plastic circle attached to one end of the pad, and a foil cylinder
attached to the other end.
The only toosl you'll need are a #1 Philips screwdriver and a very small
standard screwdriver such as those pen-clip types frequently included in
Every Keytronics keyboard I've dismantled has needed a smaller Phillips
screwdriver (size O?) to remove the PCB from the keyframe.
computer tool kits (plus whatever tools are necessary
to remove the
keyboard from its enclosure).
---> Where do I get replacement pads?
If you're a perfectionist, you can order replacement pads from Keytronic
Corporation (http://www.keytronic.com/). However, it's much easier to
Seriously? Do they do other spares as well. There's a couple of custom
chips (see below) in almost all of these keyboards, and a source of
spares would be handy.
simply find a capacitive keyboard in your local
computer junk/surplus
store and strip it for replacement pads (see above for info on
identifying such keyboards).
-----> So, What are the Custom Chips ?
There are, in general 2 off 20 pin custom chips in a keytronics keyboard (some
later ones have a single 40 pin chip, but I know nothing about that).
22-908-3B
8-channel sense amplifier
1 : Row 0
2 : Row 1
3 : Row 2
4 : Row 3
5 : Reset/sync output
6 : Row 4
7 : Row 5
8 : Row 6
9 : Row 7
10 : Ground
11 : Out 7
12 : Out 6
13 : Out 5
14 : Out 4
15 : Out 3
16 : Out 2
17 : Out 1
18 : Out 0
19 : Reset in
20 : +5V
Outputs are believed to be open-collector
22-950-3B
keyboard column decoder/driver.
1 : Col 8
2 : Col 9
3 : Col 10
4 : Col 11
5 : Latch Clock
6 : Select 0 (LSB Input)
7 : Select 1
8 : Select 2
9 : Select 3
10 : Ground
11 : Sync Input
12 : Col 0
13 : Col 1
14 : Col 2
15 : Col 3
16 : Col 4
17 : Col 5
18 : Col 6
19 : Col 7
20 : +5V
-tony