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. Nicolas - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail email@example.com.
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