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C-128 Garbage screen in 40 column mode
- Thread starter rglenn
- Start date
ClassicHasClass
Veteran Member
How did you test the VIC?
ClassicHasClass
Veteran Member
How about the CIAs or MMU?
Can you burn a diagnostic cart and post the results: https://www.zimmers.net/anonftp/pub/cbm/firmware/misc/c128diag/index.html
Sure - it does test the SID at the end so it appears to be running the diags, I just can't see them.
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ClassicHasClass
Veteran Member
It's interesting that the entire screen is covered with the same character (the colour comes from the RAM at $d800). Since you said the CIAs also tested good, if you type blindly at the C64 prompt POKE 56576,6, what do you see? It should be garbage, but it should not be all the same character.
If you check the C128 diagnostic instructions PDF (https://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c128/servicemanuals/index.html), page 3-4 has a list of symptoms, yours could be letter I., signal on VA14?
You could find a closer description to your fault or measure the signals that match a symptom similar to yours. Or swap the PLA and see what happens if you got a spare and it happens to be socketed
You could find a closer description to your fault or measure the signals that match a symptom similar to yours. Or swap the PLA and see what happens if you got a spare and it happens to be socketed
Here is the result of the Poke
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Replaced PLA. No change.
dabone
Veteran Member
Are you able to run some sprite commands in 128 mode? What do the sprites look like? Are they corrupted?
SPRDEF results in this -
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dabone
Veteran Member
The sprdef command is just a text mode 40 col program.
Try this program, it will draw a X sprite near the center of the screen and move it horizontally across the screen to the right.
Try this program, it will draw a X sprite near the center of the screen and move it horizontally across the screen to the right.
Zippy Zapp
Experienced Member
It could be a few things. Without seeing it and the board in person I can only guess. RAM would be my first guess because if something is borked in the RAM it could display corruption. I think the C128 also only tests half the memory so there could be an issue with the other half. Also char ROM but I am not sure if that would equal a normal screen on 80 column.
dabone
Veteran Member
You can also swap the ram banks using the resistor trick shown in this video.
Char ROM would not display complete characters as seen in the post with all the pictures. I would say something wrong with the RAM or the supporting logic chips, as most of the computer seems to work. Since VIC, PLA and CIA are tested good, my guess is something on the VA14/VA15 line or a RAM chip
You could:
- check/replace U63 (7406 are known to fail from time to time)
- check/replace U26 (LS257)
- piggyback/check/replace RAM, but I would do that last
Anything of those get particularly hot? When in doubt, blame the MOS chip
- check/replace U63 - de-soldered, tested chip, test passed, added socket, replaced with another good chip
- check/replace U26 (LS257) - de-soldered, tested chip, test passed, added socket, replaced with another good chip
- piggyback/check/replace RAM, but I would do that last - I got some screen changes when piggybacking but not anything useful
I left the system powered on for 40 minutes. No chips got hot or even very warm. I did notice the screen change when I pressed on the CPU. I also noticed the CPU (6502) was somewhat loose in the socket. I was able to pull it out with my fingers.
I'm not sure if that is an issue or not as it boots up in 80 column mode fine every time I try but I'll likely replace the CPU socket and test.