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Commodore 64s - tips and techniques for repairs?

PostPosted: Wed Sep 29, 2010 5:07 pm
by Carcenomy
I don't know a single vintage enthusiast who doesn't have at least a few Commodore 64s hanging around, but the reason they almost always have more than one is that by now, the Commodore 64 isn't exactly the most reliable machine ever built. Poor storage and poor maintenance over the machine's life are the general culprits I've seen, especially with all those nice exposed edge connectors...

Why start a topic about it though? Because I have five Commodore 64s and none of them work. One is a whole lot closer than the others to functional though and after a careful strip and check operation the board is in significantly better condition than the others. It also boots... sometimes. Now I've run through Project 64's C64 diagnostics and tried a few of the fixes, specifically the 6510 and the 6567 with others from my fallen comrades - the fault appears to have persisted past those chips.

So the fault is basically that when it powers up, I either get a black screen, the BASIC display with no text, garbled text or sometimes all there. IF it gets to that point, I either get half the keyboard not working, total freeze or it works properly - if it does work properly, it will load software. I DID manage to get it to load and start GEOS, although once it did that it froze up immediately. I haven't been able to get anything to load prior or since.

Everything I've read is starting to point at the CIAs. If anyone else has anything to glean, let me know... I don't really want to have to desolder a pair of CIAs from one of the donors if I don't have to. Good thing the donor board is fully socketed!

Re: Commodore 64s - tips and techniques for repairs?

PostPosted: Wed Sep 29, 2010 7:03 pm
by tezza
Well, I was going to point you to Ray Carlson's website, but I see much of his wisdom has been distilled into that Diag file. Not all maybe so here is the URL. You probably have found it already.

I've fixed one C-64 and also one SX-64 in my time. The C-64 though is one of those machines, that are so cheap and plentiful sometimes you wonder if it might not be just as easy to throw the board away and get a cheap scruffy model that goes for a few bucks.

Of course the challenge of fixing it might be fun :D

Sounds like it might be heat related if you get it working for a few seconds and then it comes to grief. Of course that doesn't really help with diagnostics as most things with current running through them will heat up.

Re: Commodore 64s - tips and techniques for repairs?

PostPosted: Wed Sep 29, 2010 7:50 pm
by Carcenomy
That's the thing that's really got me stumped - nothing's getting overly hot. The two chips I replaced are the only two that are actually getting warm at all, and only lukewarm at best...

Could it even be an IC if it's not getting hot? If it turns out to just be flaky power, I'll be upset :)

Re: Commodore 64s - tips and techniques for repairs?

PostPosted: Wed Sep 29, 2010 8:59 pm
by tezza
Yes, but if a chip is getting old and starting to develop a fault, it may express that fault just at normal operating temperatures (warm) but not when cold. I've found this with RAM chips (e.g. the System 80 posts) and capacitors also.

Power is the first thing I'd check. Are voltages what they should be on the board?

Re: Commodore 64s - tips and techniques for repairs?

PostPosted: Fri Oct 01, 2010 9:28 pm
by matsondawson
I feel your pain. I'm in the same position about 5 broken c64's with no VICs or SIDs in them. I can't bring myself to biff them as I may need the parts one day, but they take up space I don't have.