> I found that CPU's rarely go bad (unless you force them to).

I acquired an HP A4/A3 plotter at no cost because it had a faulty CPU.
The company had two of them, one went bad and someone had been through
the loop of changing all pluggable chips, but they were all marked with
the HP stock number. HP insisted that they wouldn't supply a chip, the
plotter had to be sent back to them for repair, and they would send it
back to the USA at great cost !!! The price was far too steep to be
worth it, so it sat around the workshop doing nothing. Eventually it got
to a point where it was in the way, so I said if they were going to
throw it out, to throw it my way.

On duly taking possession of it, I checked out the chip they said needed
replacing. Sure enough the top bore the standard HP style part number,
but the underneath had a mark that said 6820. OK, we have some other
gear around that uses those so raiding an MC6820 chip, and fitting it to
the plotter resulted in one fully working plotter.

In reality CPU chips are just as likely to go AWOL in operation as any
other chip, assuming out of limit voltages are not applied. In practice
other chips probably fail more often as they may have more opportunities
to get rogue voltages applied.


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