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C64 Characters get messed up after running for a bit.

J

Jentralith

Member
Joined
May 2, 2024
Messages
26
My c64 works fine for playing games and such but anything that displays characters has issues. When first turned on it will look fine but after a minute or so certain characters will start to look a bit off and the bottom half of the screen will have dots. The characters are still readable and the rest of the screen is fine. Once it happens it will happen right away when the c64 is reset. I saw a post with a weird text issue possibly caused by heat (https://forum.vcfed.org/index.php?threads/char-corruption-gets-worse-with-time-heat.1248510/) but that is way worse than what mine does.

This happens in the main screen, with the diagnostic cartridge.

I tried using a disk (1541 test disk) and the text there was unreadable.

Diagnostic cartridge says everything is fine. Dead test however fails. It gets past the color ram test and then melts. The characters all slowly disappear, even the red box. The border remains.

Initially thought it could be a bad character rom but from what I understand the dead test doesn't use it.
 
The character melting thing with the dead test cartridge is normal. It's part of one of the tests, you just have to wait a bit and it'll reappear and continue

As for the other issue, it does appear to be heat related, but it's hard to know for sure. Try feeling the chips to see if they get hot and if the issue matches the temperature. If you could provide pictures of exactly what's going on, that would be helpful
 
Screen when first started

PXL_20240706_183500686.jpg

After a while we get this....note that some letters like t and h are weird
PXL_20240706_183855482.jpg

In this pic only the lower line....then one in the dot area is fully affected
PXL_20240706_185212442.jpg

The dead test ran a displays no issue (possibly pointing to the char rom, it does feel slightly warmer the the basic and kernel ones (but that might just be me)
PXL_20240706_191010158.jpg

The diagnostic cartridge is affected...

PXL_20240706_191232014.jpg

Bet seems to slightly clear up during sound test.....which sounds right.
PXL_20240706_191147104.jpg

The only chips that get notably hot are the usual ones, the vic, sid and pla.

I tried replacing the vic and pla chips with ones from another machine (which currently does not work so no guarantee on the chips) with no change.


The character rom is soldered on both boards, so if it is bad I would like to better confirm that if possible before removing the chip.
 
The photographs show the dead tests as OK though. It is the cartridge diagnostic test that is failing.

Dave
 
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