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KevKelley

Graphic Issue Troubleshooting

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It's been a while since unpowered up the C64. I immediately noticed some issues. I thought it might have been the cable I have so that I can use modern televisions so I tried an RF adapter too but it still persisted. I am not sure if this would be a bad or dying chip or something else. I had replaced the SID chip with a SWIN Sid Ultimate a couple years ago and I don't recall these glitches to exist then.

 

It seems to only impact halfway down the screen.

 

Here is what they look like:

 

IMG_20200801_210433.jpg

IMG_20200801_205751.jpg

IMG_20200801_205543.jpg

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Looks like a bad counter somewhere in the video circuit, could be the VIC II. Is your machine socketed and does this also affect sprites or only character objects ?

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I haven't checked to see if it is socketed, as I have read to try reinserting it. It looks as if it happened to sprites as well as characters - the image with the dolphin may be more difficult to notice but as the dolphin swims in the area you can see the effect below.

 

I am not to familiar with the C64 too much (I only play around with it every once and a while) but I'll have to take it apart to see what's going on. Hope it is an easy fix.

 

Curious. Is there an alternative to the VIC-II? I only see pulled chips online. 

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The VIC-II chip exists in a number of versions and revisions within each version. It depends some exactly which motherboard you've got.

 

If it isn't the VIC-II chip, could it possibly be a cause of the PLA chip, or does it only manage memory in general? If you want, you could obtain a couple sets of POKEs for e.g. changing the memory bank for the VIC-II chip to see if that causes the problem to disappear, though I doubt it would.

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I don't really know much about the workings of the c64. I'll definitely have to do some research on this. I found one site that had common graphical errors but did not see anything matching this. 

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On this page is a short program (4 lines) which will reconfigure VIC-II from default bank 0 ($0000-$3FFF) to bank 2 ($8000-$9FFF). My thinking is that if you run it, and then enter text on the screen like you did before, you will see if the tearing still remains (probably a VIC-II issue). If it disappears, something with the RAM or PLA would rather be my suspicion.

 

https://www.c64-wiki.com/wiki/VIC_bank

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You could also go a step further and type in this basic program which copies the character set from the character rom to the RAM. 

 

https://www.devili.iki.fi/Computers/Commodore/C64/Programmers_Reference/Chapter_3/page_110.html

 

 5 PRINTCHR$(142)               :REM SWITCH TO UPPER CASE
 10 POKE52,48:POKE 56,48:CLR    :REM RESERVE MEMORY FOR CHARACTERS
 20 POKE56334,PEEK(56334)AND254 :REM TURN OFF KEYSCAN INTERRUPT TIMER
 30 POKE1,PEEK(1)AND251         :REM SWITCH IN CHARACTER
 40 FORI=0TO511:POKEI+12288,PEEK(I+53248):NEXT
 50 POKE1,PEEK(1)OR4            :REM SWITCH IN I/O
 60 POKE56334,PEEK(56334)OR1    :REM RESTART KEYSCAN INTERRUPT TIMER
 70 END        

 

Once, you type in the above listing and run it, then type the following statement to tell the VIC II to fetch the glyphs from ram.

 

POKE 53272,(PEEK(53272)AND240)+1

 

List the program or type some stuff on the screen, do you still see tearing on the bottom half of the screen ? 

 

In some cases character rom can go bad and glyphs can have additional artefacts or missing pixels, so you can also change the letter T to some other shape to confirm that. In your case, I'm confident that the character rom and ram is OK.

 

10 FOR I=12448 TO 12455: READ A:POKE I,A:NEXT
20 DATA 60, 66, 165, 129, 165, 153, 66, 60

 

 

 

 

 

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My noodle tells me that the VIC-II is fried up or one of its supporting timing circuits.  It appears to be sending scan lines before it reaches them, then sending them again, or sending the proper lines then sending them again.  Not a failure mode with which I have worked, but I swear I have seen this problem and the fix to it, just buried deep down in the recesses of hardly used information which has been archived in a warehouse of wooden crates like the Ark of the Covenant.

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