First of all: I have an old 12C with no LCD but perfect working keyboard, if you are interested get in contact with me!
This procedure workes for me, maybe others have better ways of doing it:
-take out the batteries
-open the case (four screws under the four rubber feet)
-take care not to lose the two tiny springs!
-cut the many black melted heads of plastic carefully with sharp knifeblade
(take care not to slip and if possible go underneath and lift up. This way you have a chance to end up with enough plastic to melt again afterwards
-lift up the PCB carefully and out of the case
-underneath the rubber mat are the keys
-clean! Maybe thats all that is needed, otherwise replace parts from an other 12C and put back together in reverse order
-to melt the PCB back in place, put it firmly and tightly back into the case, use a solderung gun to warm the plastic and press it flat with one finger.
Do this for all the plastic rods.
If possible take some ESD precautions!
Maybe after all, it's just easier to buy an old working 12c from EBAY? Just don't throw your old buddy out: the LCD is still a great replacement part for other Voyagers! (Now you know why mine has no LCD anymore! It's in a 11C :-)
Cheers an good luck
