Recently I bought an Apple IIc with german keyboard and after some days of use the RETURN, ENTER and ARROW UP keys stopped working. I opened the Apple IIc case, unconnected the keyboard flat cable and visually checked for some damage, but all seemed to be correct. I applied Contact Cleaner (zero residue) to all the switches and after re-installing the keyboard again all the keys started to work.
Today, after a couple of weeks using the Apple IIc, the problem has come back. What can be cause? Flat cable or keyboard controller chip maybe (which one in this case)?