very strange C64 fault

From: Francesco Messineo <>
Date: Tue, 18 Apr 2017 13:05:43 +0200
Message-ID: <>
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).

1) Lunatico demo from LFT freezes if started from the "note and vsp
tool" (pressing F8 from the note loads some more bits from disk, then
freezes). No VSP errors ever show while running the VSP tool.
2) Lunatico demo started with a LOAD"*",8:RUN works every time and
never freezes or malfunction.
3) Concert demo (from Performers) freezes after the RUN command
without even trying to load anything else from disk.
4) IC BURN-IN test program freezes always at the same point: when the
rombic sprites go around the screen, it freezes when the first sprite
almost touches the black bar at the end of the lower side of the
screen. Hitting reset button makes the test re-start and it never
shows any error on anything, but it freezes at exactly the same point
every time.
5) No game that I could find on my vintage floppies collection ever
showed any problem, I could play for hours everytime.

It could be RAM or a slightly bad CPU (or what else?), but I hate
desoldering things randomly.
No RAM test that I could write or run ever showed any bad location.
I was tempted to use an emulator with built-in cpu single-step option
to try to pinpoint the freezing conditions, but as a matter of fact,
I've never ran any C64 emulator so far (and time is always less than
half of what I'd need to have).

Anyone can come up with a reasonable hint?
Frank IZ8DWF

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Received on 2017-04-18 12:00:02

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