Archive for September, 2016

Commodore 128 Keyboard Repair

Sunday, September 18th, 2016

My Commodore 128, (a “flat” C128CR), has suffered with a few temperamental keys on the keyboard; the symptoms being that keys are either slow to return after being pressed or don’t return at all and stay pressed resulting in undesired input, i.e. they’re “sticky”.

This can be caused either by spilling something, usually sugary, into the keyboard which leaves a sticky residue, or a failure of the plunger for each affected key. As I’m fairly sure I’ve never spilt anything I assumed it was the latter.

To get at the keyboard the machine needs to be opened up first which is simply a case of unscrewing the six screws on the underside; three along the front edge with the middle one usually underneath a warranty disclaimer sticker, two at each of the rear corners and one in the centre. The top half of the case will pop off with a bit of persuasion which allows you enough clearance to reach in and disconnect the power LED and then the top of the case can be hinged open along the right hand side of the machine which allows you to gently disconnect the keyboard cable and unscrew the grounding cable from the RF shielding of the main board. This should leave you with the following:


You might not necessarily need to remove the keyboard from the case but the size of the case makes it more unwieldy to work on. To remove the keyboard, just unscrew the six visible screws. Note the four black plastic spacers and their orientation for the screws along the top edge:


Once the keyboard is loosened the power LED will likely drop out along with the small plastic part that holds it in place. Keep all of these bits safe, ready for reassembly. You should now be left with just the keyboard itself:


Next the hardest step, which requires the use of a soldering iron. You need to desolder the wires from the three stateful keys; “Shift Lock”, “Caps Lock” and “40/80 Display”. It’s important not to let the soldering iron heat the keys up too much to prevent damaging them and thankfully the wires are not twisted together so with some tweezers and the quick application of the soldering iron the wires should separate easily:


There are now 27 small screws holding the circuit board in place to undo and then the circuit board should just lift away. Don’t miss this tiny spring that is situated above the ‘+’ key on the numeric keypad:


The circuit board can be gently cleaned with something like isopropyl alcohol if it’s dirty. To get at the plungers for each key, you just need to pull off the keycap on the top side of the keyboard, put that and the spring underneath it to one side, and then the plunger should just drop out:


The keycap snaps into the top of the plunger which rides up and down through a hole in the keyboard chassis with the spring making the plunger return and stay up. What happens is that the plunger can develop a split which means when the keycap is fitted the diameter of the plunger grows enough that it doesn’t move smoothly in the hole and becomes an interference fit causing the key to stick. Here’s one of the damaged plungers from mine with my nail showing where the split is:


It’s simply a case of replacing each damaged plunger, (I ended up replacing five along with two springs), and then reassembling the keyboard. Leave the re-soldering of the stateful keys until last and that you’ve checked all the other keys travel properly. You should now have a reassembled keyboard:


Fitting the keyboard back into the case is fairly straightforward, the trickiest bit is keeping the power LED stable while you drop the keyboard in place as it’s just sandwiched in, here’s how it should be fitted:


Reconnect the keyboard cable making sure not to bend any pins, reattach the grounding cable to the RF shielding and finally reconnect the power LED, (apparently it can be reconnected in either orientation), as you close the two halves of the case together. Then it’s just a case of powering on the machine and testing the keyboard. Make sure that you also test the stateful keys work correctly after you’ve de- & re-soldered them.