Make sure you open up those A500+ units and remove/replace the internal clock batteries. They have most likely leaked acid onto the motherboard by now, but if you are lucky, maybe not. :-) In any case, it's be wise to replace them as through time, due to old age, the NiCd batteries that Commodore used on most of the Amiga computers out there tend to leak acid which slowly starts to eat up traces on the motherboard.