Re: 7501/8501 Processors (was RE: 6510 and 8500 differences)

From: Nicolas Welte (
Date: 2000-08-02 10:42:42

Hi Nick,

> [COPLIN, Nicholas.]  I would have to agree that the 7501/8501 would have to
> be to most failure prone semi-conductor ever made. 2/3 of my Plus4 machines
> are toast and the remaining one works for all of 5mins before it
> "overheats", whilst the 3/3 of my C16's seem to work!
> Is the problem model specific? Does anyone know of any fixes to help prolong
> their life (Heat sinks? cooling fan?)

A heat sink is always a good idea, but I don't think overheating is the
main problem for that CPU. I think it is the same reason why the CIA died
so often in the old C64: the serial bus. On newer C64 and all C128 it is
protected by diodes, and I rarely see a CIA die on them. The service
manuals on funet describe how to add those diodes (at least four, sometimes
six) to the old C64, C16 and Plus/4, and I also did it to my C116. 

> Can a more robust 6502 be fitted?

Hardly. You have to add the CPU port and the AEC functionality, plus the
R/W gate if you want to have a 100% perfect replacement. I did a few
measurements on that R/W gate input on the 8501 and while it does have the
expected effect on R/W (it is somehow gated with the clock signal), the
machine is still running fine with the gate disabled and R/W not being
gated with the clock.

But at a time I was thinking about making such a CPU replacement (also for
8500 and 8502), but that would've required a complex I/O chip like the 6522
just to have an I/O port. The 6529 would be a nice replacement, but it
doesn't have a separated direction register as we all know.

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