COMMODORE KEYBOARDS 2-26-08
Troubleshooting and maintenance of C64 and C128 (including SX & C128DCR)
Sooner or later with use, computer keyboards show their age. The most
common "first sign" of a problem: some frequently used keys require more
force to make them work, or they don't work at all. The Shift keys,
Spacebar, and commonly used letters such as E and A are the first to show
symptoms. To anyone who is proficient at the keyboard, it quickly becomes
intolerable to have to stop and restrike a key that didn't work the first
time. The most common way to overcome the problem, at least temporarily,
is to strike the keys with more force. Very soon after, even that becomes
useless... the keys do not respond no matter how much force is used, and
the keyboard needs to be repaired. Other keyboard problems take a bit of
detective work to diagnose and solve.
The design of a Commodore keyboard is simple and elegant. It works
well and lasts a long time. When keys do become unresponsive, it can be
from several causes. If only one or two are not working (or require a lot
of force to make them work), it's usually because those internal keypads
are worn or dirty. Sometimes dirt can work it's way into the internals of
a keyboard. A bit of dirt, hair or a liquid spill can render the
key(s) inoperative if such debris gets between the conductive rubber
keypad and the underlying PC board. If many keys are not working (a whole
row, for example), the cause is normally either a failing CIA (keyboard
interface) chip in the computer, or a bad connection between the keyboard
and the motherboard. No problem with the keyboard itself will -prevent-
the computer from displaying the startup screen, by the way. The keyboard
can even be unplugged from the motherboard for diagnostic purposes, and the
computer will still display the opening screen.
Electronically, pressing a key connects a matrix "row" (one of eight)
with a "column" (one of eight) and provides "keyscan" data input to a chip
called a CIA (Complex Interface Array). The coded 8 bit data output of
that chip tells the microprocessor which key was pressed. Some of those
keyboard data lines are "shared" with the joystick inputs. One key that
doesn't go to the CIA is the RESTORE key. It simply grounds an input line
to a timer chip. Two more inputs in a C128: the 40/80 switch, and the Caps
Lock switch are likewise not part of the rows/columns inputs.
IF YOU MUST PRESS HARD TO MAKE KEYS WORK
Fortunately, most keyboard problems are of the "I have to step on
this key to make it work" variety. Commodore keyboards are easy to pull
apart and clean. That is sometimes all that's needed to make them work
again. At worst, the little rubber keypads deteriorate and must be
replaced, either with new ones or swapped with ones from seldom used keys.
The pads are attached to "plungers" under each keytop... normally the
plunger (with keypad attached) is swapped out as a unit, but you could
remove just the rubber pad... it comes off the plunger easily. Very worn
pads will look "shiny". If failing because they have been used a lot,
unresponsive key pads don't benefit much (if at all) from cleaning...
they must be replaced.
Cleaning the keypads and PC board will correct non-functioning keys
-if the problem is dirt or other debris-. After removing it from the
keyboard top, wipe the dust off the PC board with a clean cloth. Don't
rub or scratch the conductive spots on the board. They normally have a
coating to prevent corrosion and oxidation of the copper underneath. If
there is some corrosion that you must remove from a copper clad board,
rub the spots -gently- with a pencil eraser, then clean the board with
alcohol and wipe dry. If bare copper is exposed, it will eventually
oxidize, and the repair will not last. It's best to avoid rubbing those
conductive traces if they don't need it. Clean any fingerprints from the
board contact areas. Oil and moisture from your skin can cause problems.
Some boards have a black conductive paint over the contact areas. Unless
you encounter a liquid spill that needs to be cleaned off, it's best to
leave those areas alone. If the conductive coating is rubbed off, the
board may become unserviceable.
The rubber keypads can be cleaned with alcohol on a Q-tip. It's best
not to spray anything inside the keyboard. The spray goes everywhere and
just attracts dirt. To wipe off any residue from cleaners, lightly rub a
clean sheet of paper over each key pad while pressing the key. The rubber
is thin and flexible, so be carefull you don't tear or dislodge it. Clean
only the pads that need it... leave the others alone. Blow any dust out
with compressed air, and remove any hair or fibers left behind from Q-tip
or cloth. Don't touch the keypads with your fingers, but if you do
accidently, make sure you clean off the oil with alcohol or other solvent
before reassembly. Never use abrasives on the board or keypads in an
effort to clean them. It only makes matters worse and may render the
entire board unserviceable.
CLEANING VS REPLACEMENT
I did some experiments with good and bad keypads. I tried various
kinds of cleaners including solvents, and rubbing with paper and emory
cloth. After all this, I have come to the conclusion that for a key worn
out by normal use, cleaning is useless. The conductive rubber itself
deterioriates and cannot be "rejuvinated". A new key will measure less
than 1000 ohms (1K ohm) across the two ends of the rubber keypad. If used
a lot and starting to fail, it will measure more than 10K ohms. At that
point, the user will notice that it takes a lot of force to make the key
work. Nothing I tried would make a bad key better (unless the problem was
dirt or contamination of the PC board), and interestingly enough, nothing
I could do (including sandpaper and solvents) would increase the
electrical resistance (and therefore the pressure needed for proper key
response by the user) of a good key. The good ones stayed good and the bad
ones stayed bad, no matter what I did. So, with that in mind, if cleaning
doesn't help (or you don't want to have to open the box again), just
replace failing keypads or the entire plunger with pad attached.
REPLACEMENT OF KEYPADS
The quickest way to repair a keyboard if you have no spare parts on
hand is to swap heavily used keys for ones not often used. The Shift keys,
Spacebar, and letter keys like E and A for example, represent ones most
used by the average user. Such keys as the Up Arrow, Back Arrow and pound
key could be used as a "source of parts" for a quick keyswap. If you have
an old C64 or VIC20 laying around, it could be used as a parts source for
another similar computer. Many parts are interchangeable between computers
and all keys are the same inside. To make sure you have a good one to
install, either measure it with an ohmmeter across the two protrusions of
the pad, or just examine it closely. Worn pads will look shiny... good
ones will have a dull looking surface. New pads will measure between 100
and 1000 ohms (1K ohms). When they start to require more pressure to make
them work, the resistance of the pad will be up around 10K ohms or more. If
the resistance is too high, the computer does not sense a keypress.
STUCK KEY SYMPTOMS
A stuck key can produce some rather confusing symptoms, depending on
which key is stuck down. If the keyboard has ever suffered a liquid spill,
one or more keys can get "sticky" from days to months later. Of course a
broken key can stick down. You can see it if you examine the keyboard as it
will be lower than the other keys. If one letter appears on the command
line of the startup screen with a flashing cursor after it, or it starts
printing a letter repeatedly on the screen at startup, suspect that key
of being stuck down. If the key feels OK, there might be some debris on
the underlying PC board that simulates a key held down (shorted contact on
the PC board. Unplug any mice or joystick(s) if used and try it again. If
suddenly none (or only a few) of the keys work, check to see that none are
stuck. Press each one and see if the "feel" of the key is right. A sticky
one will not come back up like the others. A missing spring under a keytop
can produce intermittant or "stuck key" symptoms. That's usually easy to
spot just by looking. Beyond that, stuck keys or whole rows or columns
that don't respond may be due to a failed CIA interface chip in the
computer. If unplugging the keyboard doesn't resolve a "stuck key"
symptom, that's another indication of a failing CIA.
REMOVAL OF KEYTOPS
Removal of the keytops must be done with great care so as not to
damage the top or break the plunger underneath. The tops are a press fit
onto the plungers that stick up through the plastic housing. A spring under
each top makes the key come back up when not pressed. You might be tempted
to use a knife or screwdriver to remove a keytop, but that's not a good
idea. The top must be pulled -straight- up and off. If you twist or bend
it to one side, you may break the plunger, keytop shaft or both. Prying
against another key will likely mar or scratch it.
You can make a simple tool that will pop those keytops off easily.
Here's how to do it: With a pair of tin snips or metal shears cut a strip
of metal from a tin can (I used the top cut out of a dog food can) about
2 1/2 to 3" long and about 1/2" wide. Bend the ends over at a right angle
with pliers to form a tiny lip (1/32" or less) at each end of the strip.
Bend the strip into a U shape that fits over the keytop. The tool should
be big enough so your finger will fit inside.
The lip at each end must be small enough so the tool will fit between
keys, but large enough to grab the underside of the keytop. Note: the
cut ends of the tin strip are very sharp! Wrap tape around the strip if
necessary for protection. Now, press the tool over the keytop and,
squeezing the sides gently to hold it in place, pull up on the tool and
the keytop will come off easily. When it pops off, don't lose the spring
underneath. Don't remove more than a few tops at a time unless you know
where they go back. It's easy to get confused with keys that are not often
used. (Ask me how I know.) It's best to have the PC board in place (held
by a few screws, if necessary) so the plungers don't fall out when the
keytops are removed. Plungers are removed from the bottom of the assembly.
If you have a broken keytop and no spare parts handy, the shaft can
be glued back on with SuperGlue, but first you have to extract the broken
stub from the plunger. Don't be tempted to just glue it back without
removing the broken stub from the plunger. If you do, you'll never get
that keytop off again, ever. Also, that glue tends to wick its way into
other areas. So, you'll probably end up with a key that's stuck forever.
Make the extra effort and extract the broken stub of the keytop shaft out
of the plunger, I use a paperclip with one end bent into an L shape small
enough to fit into the small hole in the shaft. Insert the tool into the
hole and pull it out quickly. It may take several tries, but that trick
usually works and doesn't damage anything. Now, orient the broken shaft
bit over the bottom of the keytop so it sits straight. Note the position
and prepare to glue it back on. To reinforce the mend, trim a wooden
toothpick or the wooden shaft of a Q-tip to tightly fit the tiny hole in
the shaft and the broken stub. Force it into the bottom of the keytop
and then the broken shaft down over it until it fits snug. When you are
satisfied it sits straight and is a good tight fit, remove the stub from
the stick and apply the glue, then press the stub back over the stick.
Immediately wipe away any excess of glue that is pressed out with a Q-tip.
Let the repair dry at least an hour before you try to use it. Trim off
any wood sticking out of the hole if necessary. The repaired keytop
should now work when pressed into place.
DISASSEMBLY / REASSEMBLY... C64 Computer
Remove three philips screws along the underside of the front edge of
the computer. Lift up the top half-shell and unplug the keyboard and power
indicator connectors. The keyboard connector may be hard to remove. Grasp
the ends and gently rock it back and forth slightly while pulling straight
up on it. Don't pull on the wires! Note that there is a blank pin on one
end. That makes it difficult (but not impossible) to put the plug in
backwards. The power LED connector will go either way... it doesn't
matter, because it will work either way. The top half-shell can then be
removed from the lower half. The rear fasteners are sometimes very tight
and will "pop" when the top is folded back. That's normal.
Rest the top (keyboard) face down on a cloth or cushion to keep from
scratching the keys. Unsolder the ShiftLock switch wires: heat the
connection and just pull the wire out quickly when the solder melts.
Remove 23 tiny philips screws holding the PC board to the keyboard
assembly. The assembly need not be removed from the cabinet top, by the
way. Lift the PC board up and out. Examine it for dirt or liquid spill
residue, and inspect the rubber keypads for contamination or wear. Clean
or replace pads as necessary. To remove a keypad, the keytop must be
removed. (See REMOVAL OF KEYTOPS). With the keytop removed, the spring and
plunger (with rubber keypad attached) will fall out. Make sure you don't
lose anything! The plunger should slide out from underneath easily. If it
doesn't, look for residue or cracks in the plastic. If it's broken or worn
out, the plunger/keypad must be replaced. A plunger will fit in any
slot... they are all the same. When installing a new plunger, don't forget
the spring under the keytop. Temporarily install the PC board when you're
ready to press the keytop back on. It gives the plunger something to push
against, since you must apply pressure to the keytop to get it to "snap"
back on. Don't use your finger to hold the keypad... oil from your fingers
will contaminate the pad.
When you're finished cleaning/repairing the assembly, reinstall the
PC board and put the tiny screws back in. Don't forget to resolder the
wires to the ShiftLock switch: heat the connection and poke the wire in
when the solder melts. Plug the keyboard back into the motherboard, and
plug the LED connector back in. Lay the back of the two half-shells
together and fold the top back down. It helps to squeeze the cabinet
together at the rear to seat the fasteners. Put the three screws back in
the cabinet and you're done.
DISASSEMBLY / REASSEMBLY... C128 Computer
The procedure is very similar to the C64, but the C128 has three
keyswitches to unsolder. These switches are plastic and can melt from
too much heat on their terminals... desolder them quickly!
On the "flat" 128: remove six screws (standard philips or Torx) from
the bottom of the 128 case. Note: the 128 may have special screws holding
the cabinet together. The newer star-shaped type is call Torx. The screws
require a special T-10 size driver to remove them, but a small flat blade
screwdriver can be used if you're careful... just make sure it fits
tightly in the screwhead. Lift the top half-shell, starting at the front
edges. Two small plastic "snaps" hold the case together on each side, and
so may make it difficult until you press in slightly on both sides near
the top row of keys on each side to pop it open. With the top shell loose,
lift up on the left side of the top shell, reach under and remove the
power LED plug from the motherboard. Now grasp the top half with both
hands and shift it to the left to expose the wiring (still connected)
between the top and bottom shells. You need to remove the keyboard
connnector and a small philips screw. The keyboard plug is usually very
difficult to remove. Because it is recessed, you can't get enough of a
grip on it to remove it with your fingers. Instead, grasp -all- of the
wires tightly between thumb and forefinger as close to the plug as you
can and pull up, gently rocking back and forth to loosen the plug from
the socket. Next, disconnect the ground strap held to the motherboard on
the right hand side with a small philips screw. You can now remove the
top half-shell and set it face down on a cloth or padded surface.
Unsolder the wires to the three keyswitches which are plastic and
will melt if the soldering iron is held on them too long! So, desolder
quickly! The easiest way is to get under each connected wire and when the
solder melts, lift the wire -gently- with the iron. If it's wrapped too
tightly, you will need to unwrap it from the switch while the solder is
molten. All I can say is: work quickly and carefully. I use tweezers to
move the wires while the solder is molten. You can cut the wires but may
not have enough to resolder them when it's time to reassemble the keyboard.
There is no need to re-wrap them around the switch terminals when you
solder them back on. "Tack" soldering is good enough... just let the wire
touch the switch terminal and solder the two together.
Remove the 27 tiny philips screws and lift up the PC board. Notice
the tiny spring off to one side... don't lose it! It will fall out if you
turn the panel over. That spring provides a ground connection between the
metal frame of the keyboard and the PC board. It's there to discharge
static electricity safely and harmlessly to chassis ground. You will not
see any difference in normal operation without it, but leaving it out can
open the computer CIA chips to possible damage from static electricity.
Clean and replace parts as needed. The assembly is similar in design
to the C64, but parts are not interchangeable between the two. Additional
note: there are two versions of keyboards for the flat C128 but they look
identical, and the PC board has the same CBM part number: 310401-01.
The difference in the two versions is the length of the plungers under
the keytops. In one version, green and white plungers are 3/8" long and
in the other, blue and black plungers are 7/16". They are therefore not
interchangeable. A longer plunger might fit and work as a replacement for
a shorter one, but that keytop will stick up higher than all the others.
Follow the previous instructions as far as parts replacement and cleaning.
Reassemble in reverse order of disassembly. The keyboard connector will
only go back one way. Be gentle. It's possible to bend pins over if you're
not careful. The power LED plug will fit and will work either way. Don't
forget the ground shield at the right side of the motherboard and don't
overtighten any of the screws. The plastic can't take much torque before
it strips out.
DISASSEMBLY / REASSEMBLY... C128DCR COMPUTER KEYBOARD
Unlike the keyboard in the C128, the 128DCR keyboard is inside of its
own housing. It only requires the removal of six philips screws to get to
the internals, but the screws are hidden beneath the rubber "feet" on the
bottom of the keyboard. Those feet are held with double-sided tape and are
easy to remove. I like to permanently remount them close to but not
covering the access holes. That makes it easier the next time I need to
open the case. Rather than "peel and stick" tape, I use glue to remount
the rubber feet. Silicone rubber sealer works well as glue.
As with the 128 keyboard, there are 27 tiny screws that hold the PC
board to the assembly. The interconnect cable is soldered to the board and
a plastic wire tie secures that cable to the board. The board can be opened
and cleaned without cutting that tie. Before you disassemble the keyboard,
it's a good idea to blow the dust and debris from the top. Lint and hair
will be trapped under the keys, so if there is a lot of it that doesn't
respond to blowing it out with an airhose, you may need to remove some of
the keytops to get at it. If you do that before you remove the board, the
plungers will not fall out when keytops & springs are removed. Wide keys
such as the ENTER key have a wire clip that is rather difficult to put
back correctly. Note how it comes off, and work carefully so you don't
break the plastic retainers that hold the clip in place.
The keys are all part of an integrated metal and plastic subchassis,
and if that main deck is damaged, it may not be repairable. However, most
of the time failures are due to bad contacts, a broken plunger or a broken
keytop. The plastic subchassis is mounted over a "motherboard" where each
plungers conductive pad touches the corresponding board contacts when a key
is pressed. Don't use abrasives or rub on the conductive surfaces when
cleaning the PC board. Those contacts are just a carbon powder coating and
are easily damaged. Use a mild cleaner such as Windex or rubbing alcohol
on a cloth to clean up any spills on the board but -don't- rub hard. Wipe
dust and debris off with a dry cloth or just use compressed air. .
The three keyswitches (Caps Lock, 40/80, & Shift Lock) wires connect
to the board and must be unsoldered carefully (see instructions above). As
with a C128 keyboard, all keypads wear out with use and then require
increased pressure on the keys to make them work. If cleaning doesn't
restore normal operation, replace the pad. If you don't have spares, swap
the pad or the entire plunger with one from another key that isn't used as
often. The pads just snap onto the bottom of the plunger but that requires
removing the plunger from the subchassis.
Note that the plungers and springs under the keytops are
interchangeable with some C128 keyboards (there are two versions of flat
128 keyboard, each with a different length plunger). Although they look
identical, C128DCR keytops have a longer shaft to fit their plungers, so
not all C128 keytops will work. DCR keytops may therefore fit some C128
keyboards. My DCR keyboard is a Mitsumi and uses the longer keytops.
THE SX KEYBOARD: DISASSEMBLY AND CLEANING (from the SX64 article)
This keyboard is completely different than previous CBM designs.
Therefore, some of the previous information in this article will not
apply. You need to know how this one works to be able to safely take it
apart for cleaning. The main component is a conductive rubber bladder
separated from the PC board underneath by a white or grey plastic sheet
with holes in it. The plastic sheet is an insulator. Until a key is
pressed, the insulator holds the bladder away from the PC board contact
points. If the insulator gets damaged or distorted, some keys will not
work or may make contact all the time. Removing the bladder from the PC
board requires great care. The insulator tends to stick to the board
near each contact point. If pulled away quickly, it will most certainly
be torn. I use a dental pick to slowly nudge the tiny areas that stick
as I gently pull the bladder off the board. The last one took me about
10 minutes to remove. That work cannot be rushed. Some areas of the
elastic insulator deform as they pull loose and must be pushed back into
place before reassembly.
The keyboard comes apart for cleaning by pressing in on the four
plastic snaps on the front edge while separating front and rear panels
with fingers. The two center ones are easy... just press in and up on
the brown plastic top panel and it will bend upward. The two side snaps
need a bit of pressure... use a small flat tool like a screwdriver
blade inserted into the slots to make them release. With the top off,
the keyboard is visible as several rows of keys over a conductive
rubber bladder, which in turn is mounted over a PC board. Each key
plunger assembly can be removed for cleaning if one or more are sticky
from a spill. They would -all- have to be removed to gain access to the
PC board under the bladder. None of the keyboard parts are
interchangeable with a C64. Note that the shift-lock key is not a
latching switch as in a C64 or C128, but a momentary contact
pushbutton like all the rest. A circuit in the SX forms the "latching"
device and an LED on the keyboard key comes on when ShiftLock is on.
The following is a report from a fellow user who sucessfully cleaned
his SX keyboard:
"I am pleased to report complete success with my efforts to clean the key
pads on my SX64 and the keyboard is working like new again. Although not a
terribly difficult operation, care must be taken.
After removing the upper keyboard case half I removed the five screws along
the front of the circuit board that holds it in place. I then removed the
two screws holding the keyboard cable connector in place. At this point I
removed the keyboard from the lower case half and removed the keys one at a
time by pressing the two tabs together from the back side of the circuit
board that hold the key assembly in place. I didn't bother to keep the keys
in order as I would use a C64 keyboard as a guide when replacing the keys.
I then removed the two tiny screws that hold a small strip of metal to the
black rubber like bladder on the right side. At this point I was ready to
lift the bladder from the circuit board. Under the bladder is a grayish
colored membrane that is somewhat fragile and care must be taken not to tear
it. On several occasions this membrane stuck to the circuit board as I
attempted to lift the membrane from the circuit board. I used the tip of a
very small thin flat screwdriver to ease the membrane off the circuit board
when it would stick.
Once the membrane was removed I cleaned all contact pads on the
circuit board using cotton swabs (Q-tips) and alcohol. I also cleaned the
contact areas on the bladder in the same manner using VERY LIGHT pressure.
I replaced the membrane onto the board and attached the metal strip with
it's two tiny screws. I attached the cable connector and mounted the board
in the lower case half with the five screws. Take care not to over tighten
the screw that goes through the bladder to avoid damage. After the board was
secure in the case half, I replaced the keys. They simply press into place.
The space bar will require a little care to line up all three connectors.
Replace the upper case half and that's it. Not that difficult but very time
consuming and it does not pay to hurry. While the key assemblies are off the
keyboard it is a good time to clean the keys themselves and to blow any dust
or debris from the underside of the key. If you find this information useful
please feel free to pass it along to others." Charles Houck
All I have to add to what Charles has said is some cautions when
cleaning the contact points. Never use anything abrasive on the bladder.
The black dots are the contacts. They are just a conductive coating on
plastic, so rough treatment can easily destroy the bladder beyond repair.
A -light- rub with a dry Q-tip is all that is needed on each black spot.
For the copper keyboard contacts, I use a #2 pencil eraser to remove
oxidation from each contact point, then rub each with a dry Q-tip to
"polish" the surface. The resulting contacts should be shiny with no
spots or blemishes. I don't use solvents unless the board and/or bladder
is contaminated because of a liquid spill.
Ray Carlsen CET
If you have questions or comments, or more importantly if you spot any
mistakes here, please let me know. Thanks!