Casio SA-21 with a few faulty keys... fixable?

[peanutismint]

I have an old Casio SA-21 keyboard synthesiser which is still in semi-working condition, except that some (3) of the keys have stopped working. I've opened it up and cleaned the contacts with contact cleaner, but it has no effect.

The keys seem to push a small rubber 'nub' onto the circuit board contacts (which are strangely black rather than usual copper) which causes the note to sound, only three keys don't work and there is no noticeable difference between those and the keys which do work... anyone know anything about the workings of old toy keyboards?!

It works fine, it's just annoying to have to play things in a certain key or certain area of the board where I know i'm not gonna need one of the three broken keys......Plus, once I fix the keys, the bending can begin!!! :-)

[Gordonjcp]

They're conductive rubber, with carbon stuff screen-printed onto the PCB.  The rubber wears out.  You can paint over the rubber pads with silver-loaded paint - it's not exactly cheap, but Halfords sells little bottles of it for fixing heated rear windscreens on cars.

[peanutismint]

Ah right, thanks for the tip!  I did actually think that might have been the case, so tried sticking little bits of tin foil to the rubber - it didn't work - not conductive enough?

I'll give the silver stuff a go! :-)

[djsynchro]

Maybe measure with an ohm meter from the keyboard contact to where it leads maybe you have broken tracks on your PCB? Just a thought.

[computer at sea]

Those keys all go to diodes, right?  I had a keyboard once where one of the diodes got crushed somehow.  Maybe take a look at that.