THE SX KEYBOARD: DISASSEMBLY AND CLEANING
               latest updates and corrections 09-24-2022

Note: I've only worked on half-a-dozen and I don't know everything...

     This keyboard is completely different than previous CBM designs.  
Obviously none of the SX keyboard parts are interchangeable with a C64. 
You need to know how this one works to be able to safely take it apart 
for repair. The main keyboard component is a conductive rubber bladder. 
A tiny spring inside the bottom of each key touches the bladder so its 
conductive pad of carbon film presses onto the PC board contact. If the 
bladder gets damaged or distorted, some keys will not work or may make 
intermittent contact! If the key spring is too short (recessed from 
fatague in use), that key will not work or it may be marginal and take 
a harder hit to produce a response. If the spring is too long (stretched 
and protruding from the key), that key will make contact all the time 
and other keys will not work! Adjusting those springs under the non-
functioning keys is the first thing I try before taking the keyboard 
apart further when only a few keys don't work. 
     If you don't have a keyboard test program to check key operation, 
boot the computer and just tap each key lightly several times. I write 
down the ones that don't respond properly so I can attend to them later. 
Most of the keys should show some character on the screen. A few that 
don't such as the F keys do affect the blinking cursor, so those can 
be checked as well. Ones like the C=, CTRL and SHIFT keys affect the 
operation of the other keys when they are held down. 

     The keyboard case opens for cleaning by using a coin to pry on 
the sides of the keyboard just to the left of the left arrow key and 
to the right of the F1 key between the top and bottom half-shells. 
The rear of the top cover will pop loose and you can pull the top up 
and off. The internal keyboard is then visible as several rows of 
keys over a black conductive rubber bladder, which in turn is mounted 
over a PC board. 
     The PC board assembly is held in place with four screws. The 
fifth screw on the bladder is a connection point to ground the bladder
to the circuit board underneath. That screw only has to be removed if 
you intend to remove the bladder. Remove the connector at the rear of 
the case (two longer screws) and the PC board assembly can then be 
lifted out of the case bottom. 
     All keys have to be removed to gain access to the PC board under 
the bladder. 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 provides the "latch" needed and an LED on that 
keyboard key comes on when ShiftLock is on. 
     To remove individual keys, use your fingernails to squeeze the 
two tabs of each key on the underside of the board assembly while 
pushing them forward. The key assembly will pop out. Don't lose the 
tiny spring inside the bottom of the key. If you drop one on the floor, 
you may never find it, so be careful!!! I have "lost" a few springs.  
I used a large magnet on a string to search and retrieve one's I've 
dropped. It's helpful to arrange all removed keys, in order, on your 
tabletop. If they get mixed up, you can use a C64 keyboard for a
visual reference. I put a white towel down on the table so the keys
do not move when set down. Springs will be more visible too. 
     Before you decide to pull the bladder for cleaning, if only a few
keys are not working properly, it may be that the springs under those 
keys are weak from use. (see photos) If that's the case, individual key 
functions may be restored by extracting each key, removing the tiny 
spring and stretching it -slightly- to provide more pressure on the 
bladder when the key is pressed. It is important to inspect the spring 
afterwards to see if it sticks out from the key bottom. If it does, 
that key might make contact all the time or it will be too sensitive 
and make contact when you don't expect it! The correct position of the 
spring is: very slightly recessed within the plastic when the key is 
not pressed. 
     Assuming there was a liquid spill, or re-tensioning the springs 
did not help, removing the bladder from the PC board requires great 
care. The bladder 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 bladder may deform as 
they pull loose and must be pushed back into place before reassembly. 
If you don't -have- to pull the bladder from the keyboard, don't do 
it, but if you must... 

     One area that can prove problematic is the connection points 
between the bladder and the PC board. There are nine painted "lines" of 
conductive material under a strip of metal (which must be removed, two 
screws) before the bladder can be lifted off the board. Those contact 
points will require rework of some kind to make contact again when the 
original bladder is put back. If any connections are open, the keyboard 
will have many keys that don't work! To ensure the conductive lines 
make good contact, there is a carbon based "paint" that can be used to 
restore the contacts. You can test the conductive lines between board 
& bladder after rework by installing a few keys: F1 S T V 9 P > 2 LOCK. 
Pressing those keys should produce a response to verify all lines are 
good. Note: such restorative paints should not be used on the key 
contact pads since they would then stick up higher than normal and make 
continuous contact. 
     If you must clean the bladder, never use anything abrasive. The 
black "dots" are the contacts. They are just a thin 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... if necessary. For the copper PC board contacts, I use a 
#2 pencil eraser to remove oxidation from each contact point, then I 
rub each with a dry Q-tip or cloth 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. Sugary drinks like soda pop or coffee can be 
cleaned with isopropyl (rubbing alcohol) and dried, then the board 
should be polished as indicated above. Isopropyl alcohol can leave a 
white powder residue that must be cleaned off. Hot water works OK. 
Make sure there is no dust or debris on the board or the bladder before 
reassembly. Plastic parts have a tendency to attract such particles, 
especially when the air is dry (low humidity).

IF THE BLADDER IS BEYOND REPAIR:
     There is now a replacement bladder: http//www.rwapsoftware.co.uk
Normally the plastic insulator sheet that comes with it is used with 
the replacement bladder. If you have a keyboard where all contacts are
good but some make contact and activate when the board is flexed, a 
spacer alone may allow that original bladder to be used. I recently 
used such a spacer sheet to correct that keyboard problem. To prevent 
having to remove the bladder from its board contacts, I cut the plastic 
sheet in two near the connection points and fitted the separate ends 
between the bladder and board. 
     If damaged and not serviceable, a replacement bladder with plastic 
spacer sheet will restore your keyboard. I found that the new bladder 
already has good contacts (no paint needed) and there is a pressure pad 
on the top that ensures good contacts when the metal plate (two screws) 
is put back. Just make sure the board contacts are clean. There is no 
need for the old paper spacer... it can be removed and the board area 
cleaned. Don't forget to bend the ground tab of the new bladder over 
before intalling the grounding screw (observe the original bladder) so 
the new bladder is properly grounded. 

     Re-install all the keytops and test the keys before you put the 
assemblly back in the case. If any keys don't respond correctly, pop 
that key out and retension the spring as necessary. Reassembly into 
the case completes the repair. Don't over-tighten any screws! One final 
test I do before reassembly is to grab the ends of the board (or case 
if already assembled) and twist it gently back and forth to see if any 
keys make contact when not pressed... they shouldn't. 

Ray Carlsen CET	
CARLSEN ELECTRONICS