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
circuit board, thus completing the circuit and registering the keypress
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.
---> 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.
---> 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
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
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).
Note: If you're stranded on a desert island with only your computer and
a screwdriver, you can get the important keys working again by using
pads from the numeric keypad, which is usually in good shape because
it's not used very much.
---> OK, I've got all of the necessary items, what are the repair steps?
1. Remove the keyboard from its enclosure. Place on an open workspace.
Putting a pad of some type underneath may help avoid marring of the
keycaps, but in any case be careful to prevent static discharge if your
keyboard model has ICs on the PCB. A ground wrist strap is recommended.
Lock up the cats.
2. With the keyboard face-down, remove all of the screws on the back of
the PCB (there are a lot!) Don't worry, nothing is going to fall apart
when you remove the PCB.
3. Remove the PCB. Place to the side. If it shows any signs of dirt on
the contact side, you'll want to clean it with a non-residue,
non-lubricating contact cleaner.
4. Identify pads that need replacement. If you're replacing all of the
pads, you can skip this step. First, look for any foil circles that are
damaged. Then, while looking down the length of the keyboard on the
back side, press each row of keys and make sure that each foam pad
protrudes sufficiently and cleanly out the back of each key plunger.
The most-typed keys are the first to go (return, space, A, E, etc.)
When in doubt, replace a pad -- that's a lot of screws to remove to do
this over again! Replace pads one at a time.
5. Carefully remove the keycap and spring. There are actually keycap
removal tools available, but whatever you use, just make sure to pry
from both sides at once, not just one side. Don't bend the key
sideways.
6. Holding the keyboard perpendicular to the table, place the Philips
screwdriver into the back of the key plunger and push gently to pop the
plunger out the other side.
7. Pick up the plunger. Remove the foam cylinder by inserting a
fingernail under the foam cylinder's clear plastic circle. A tiny bit
of leverage and it'll pop out.
8. Put the replacement foam cylinder in place. Gingerly use the very
small standard screwdriver to pop the clear plastic circle into the
plunger by pressing around its outside edge. Be careful not to damage
the foam or the foil circle.
9. Replace the plunger in the keyboard chassis.
10. Replace the spring and keycap. Use the very small standard
screwdriver to press on the edge of the bottom of the plunger to firmly
seat the key cap.
11. Repeat as necessary.
Note: If you're skilled at delicate work and have various sized tools
available, you can probably replace the foam pads from the back without
removing the keycaps, but this is not recommended for beginners.
Received on Thu Jul 10 1997 - 17:06:53 BST