Re: very strange C64 fault

From: Francesco Messineo <>
Date: Wed, 19 Apr 2017 22:39:05 +0200
Message-ID: <>
Just an update but not much progress

On Tue, Apr 18, 2017 at 1:56 PM,  <> wrote:
> 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.

I'm sure this is the perfect job for a logic analyzer: the fault
happens every time at the same point on all programs that suffer this
I could record a few data+address bus on a working C64 and then on the
faulty one and spot
the difference easily I guess.

> 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.

no, the sprites all travel along the bottom border and cross a
background/foreground pattern.
It's the program that I always use to test C64s, you can see the
sprites traveling around here for example:

On the faulty C64C, once they are traveling on the bottom side, just
before the first sprite touches the last black bar, the machine
From the scope traces that I have collected, it seems that the CPU
goes into an illegal instruction or crashes for some other reasons. It
can be restarted with a reset and the program is still there and not
One question I have (I'll check tomorrow maybe on another C64):  is
the DRAM supposed to be refreshed when /RST signal is low on a C64? I
don't see the reset signal going to the VIC-II, so it should keep on
refreshing the DRAMs. However, if I hold the reset button long enough
(a few seconds) the DRAM gets corrupted.

> 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.

may be. Every game I could try works fine however. Only that test
program and a couple of demos are affected so far.

Best regards

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Received on 2017-04-19 20:39:05

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