Re: very strange C64 fault

On Dienstag, 18. April 2017 13:34:58 CEST you wrote:
> >> Hi all,
> >> I have found a real tricky fault on my C64C. I'll try to describe the
> >> symptoms, in case anyone is curious enough to help in troubleshooting
> >> it. I am not a newbie in repairing C64s, but this one made me just
> >> wondering without even giving me a clue on where to start probing for
> >> possible faults (if I had a logic analyzer I would know what to do,
> >> but I haven't got one).
> > 
> > Logic analysers may not help in some edge cases like this one.
> > 
> > The nouns you mention (VSP, sprites, ..) seem to be pointing towards the
> > VIC. I'd check its voltages, frequencies and eventually swap it.
> no VSP errors showed from a tool that's supposed to check for their
> existence on a C64, so yes, the VSP was mentioned and likely excluded.
> The crashes are very repeatable infacts, all happens every time at the
> same point of some programs. Most (all I could try) programs show no
> random or repeatable errors.
> I don't have any other MOS 8565 VIC-II chip to try swapping it. I only
> have old 6569s

From your description of a sprite "almost?" touching the bottom of the 
screen... And the fact that it's always at the same position, so that it's 
really repeatable points to a logic error, not a timing error.

Maybe it's a stuck sprite collision bit, that once set is not reset and keeps 
the interrupt low?

You could write a simple test program for that.

Regards,
André


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