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[] HOW TO CLEAN THE IBM PC KEYBOARD []
[] By David R. Bivens, Richmond, Va. []
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In the process of owning and operating an IBM PC, you will, no doubt,
find a point in time that the keyboard causes strange and irritating things
to happen to your fond finger strokes. Nine times out of ten, this is
caused by DIRT!
It is amazing how much trash can be found under the keytops of any keyboard
that has been in use for more than a few weeks. Human hands carry a great
deal of trash, including dead skin cells. In addition, leaning over a
keyboard, as computer people are prone to do, can cause much "head garbage"
to fall into the mechanism. If you smoke (as I do), you'll find it next to
impossible to keep ashes in the ashtray.
The symptoms of a keyboard dirt problem are manifold, but the most common
are: mistyped characters, missing letters (Did I miss the key?), and having
the wrong character appear at the most obnoxious times.
The fix to most of these problems is to clean the bugger out. To do this
you must remove the keytops and blow out the mechanism with air. The
purpose of this message is to explain the operation of the PC keyboard and
to outline the steps necessary to clean it. I've included some notes on
some possible pitfalls that a few folks have run into.
The PC keyboard is a capacitive device, that is, its internal circuitry
detects each keystroke by a change in the capacitive potential of a wire
matrix the keys are mounted on. This matrix is contained in the form of a
circuit board that contains small "plates" that connect to a device that
monitors capacitive levels. Above each of these plates is a "toggle" made
of conductive plastic (like they ship IC's in). The toggles are connected
to a small spring that is held straight and slightly compressed in a small
cylinder. The thing that hold the spring compress is a small plunger
attached to the bottom of each key. When the toggle moves in its holder, it
changes the capacitive potential of the wire matrix and causes the keyboard
logic circuits to send a scan code to the 8088 bus.
When you press a key, the plunger moves down in the cylinder, compressing
the spring. This spring is already holding the toggle away from the matrix.
As the pressure increases, the spring becomes "unstable" and bends rapidly
to form a loose "U" shape. This action is felt (and heard) as the "break"
of the key. When the key breaks, the direction of the pressure on the
toggle is reversed, causing it to swing down about 1.5mm closer to the
matrix. This is detected by the keyboard logic circuitry as a change in
capacitive potential and the appropriate scan code is sent to the 8088 bus,
to be processed.
When you lift your finger from the key, the spring pops back to its
straight position and lifts the toggle away from the matrix.
As you can see, there is alot of mechanical interaction going on inside
of the lowly keyboard. What amazes me is that the devices are as reliable
as they are!
To clean the keyboard, turn off the PC and place the keyboard in you lap.
Remove each key by prying it STRAIGHT UP with a penknife of small
screwdriver until it pops off. Check each key for any gummy or sticky
matter, placing the dirty keys in a pile to be cleaned with soap and water.
DO NOT REMOVE THE SPACE BAR!!! If you do, youll find it is almost
impossible to replace properly. After removing the keys, take a low-
pressure air hose (or hair dryer) and blow a stream of air down into the key
tubes.
The air will remove quite a bit of loose dirt from the inside of the
keyboard. This dirt, if left in place, will alter the capacitive potential
of the matrix, thus "fooling" the keyboard circuitry into thinking a key has
been pressed when it has not. A WORD OF WARNING: Do NOT-NOT-NOT blow into
the key tubes with your mouth!!! You will blow moisture into the mechanism,
and possibly ruin the keyboard unit ($125.00 spare part). Nine times out of
ten, all that is necessary to correct keyboard errors is to clean the keys,
and the rest of this is not required.
If you did not heed my warning about the mouth air, you will have to
remove the matrix board from the key tube holder and clean it. THIS IS A
VERY DELICATE OPERATION! DO NOT-NOT-NOT ATTEMPT IT UNLESS YOU NEED TO! To
remove the key matrix, turn the keyboard over and remove the two screws on
the bottom. Remove the bottom panel from the keyboard and disconnect the
Berg connector that holds the keyboard cable to the circuit board inside the
unit. Place the keyboard in your lap and remove ALL the keys. Carefully
turn the unit upside-down, taking care not to bend the springs. With the
keyboard upside-down, and the PC (printed circuit) board on the right side,
you will see several metal "clips" holding the silver matrix board to the
black key tube holder. One of these clips (probably on the lower left) is
bent over the silver matrix holder. Bend this clip away from the silver
metal. Hold the two parts together FIRMLY and slide the silver matrix
holder to the right (on mine, at least). THIS TAKES ALOT OF PRESSURE! BE
SURE YOU DON'T ALLOW THE TWO PARTS TO SEPARATE SUDDENLY, OR YOUR FLOOR WILL
BE LITTERED WITH TOGGLES. Carefully remove the matrix holder from the tube
holder and place the tube holder in a safe place. Use a clean, slightly damp
rag to clean the surface of the matrix board. DO NOT ALLOW WATER TO GET
UNDER THE BOARD! Allow the unit to dry. You may perform an interesting
test by reconnecting the matrix board to the cable and powering-up the
computer. You may then "type" on the matrix board by touching your fingers
to the dark panels visible under the surface of the board. Cute, huh?
You will need to practice reassembling the space bar spring before you
try to put the unit back together. To do this, remove the spacebar tubes
from the tube holder. Remove the toggle and place it in a safe place. Note
the wire running the full length of the space bar tube assembly. This wire
is the spring that keeps the spacebar from tilting to one side during use.
When the spacebar is inserted into the tube assembly this wire MUST fit into
the slots in the "legs" on each end of the spacebar. Try it a few times
before you plan to reassemble the keyboard.
To reassemble the keyboard, replace the spacebar tube assembly in the
tube holder. Install the toggle in its slot. Insure all other toggles are
correctly positioned. Hold the tube holder upside down and GENTLY replace
the matrix board in its place. To complete reassembly, you will need to
press the matrix holder down onto the tube holder while sliding the "ears"
into their correct notches. THIS TAKE A LARGE AMOUNT OF FORCE! BE SURE THE
UNIT DOES NOT JUMP OUT OF YOU HAND!!! After replacing the matrix board,
bend the holding "ear" into place to secure the mechanism. Reinstall the
key unit into the keyboard case and reconnect the cable. Replace the bottom
cover. You should now check the unit for proper operation. This is tricky
without keys, but it can be done.
To test the unit, place the rear edge (nearest the number keys) on the
table with the front edge (nearest the spacebar) in the air. This will
allow the springs to fall backwards and insure against false input signals.
You may have to shake the keyboard a bit to get the springs to fall.
Bring the system up and test each key by "tweeking" the spring with your
finger. (The IBM diagnostics have a good test for this.) If all is well,
replace the keys, WHILE HOLDING THE KEYBOARD IN THIS POSITION! If you don't
hold the keyboard up like this, the springs will be "pre-bent" and won't
function properly. Replace the spacebar first, as this is the one that is
most likely to give you problems.
The object of the spacebar replacement game is to get the wire into the
slot on the bar legs. This MUST be done one side at a time, without
depressing the bar too much. Pressing the bar down causes the wire springs
to retract and makes their reinstallation impossible. You can tell the
spacebar is installed correctly by the fact that the ends are difficult to
lift up and the bar moves vertically as a unit.
After replacing the spacebar, replace all the other keys, WHILE HOLDING
THE KEYBOARD VERTICAL!
Run the diagnostics of the keyboard to insure everything is working
properly. If not, check that the small springs were in the correct (leaned-
back) position when the keys were installed. If they were and you still
have problems, then you reassembled the matrix board and key tube holder
incorrectly and will have to take it apart and start all over again...
A common problem is getting the toggles jammed up when installing the
matrix board.
Hope this was educational -- it sure was a pain to type! GOOD LUCK!
David R. Bivens
Richmond, Va.