• Please review our updated Terms and Rules here

C-128 Garbage screen in 40 column mode

rglenn said:
- 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.
Click to expand...
Fat fingers - CPU 8502
 
Of course I meant 8502. The CPU socket has been removed and replaced. Nice and tight now..... and same issue.
 
Do the swap between ram banks like the video above shows, completely swaps banks 0 and 1, and then you may be able to see what's going on.

It's just lifting one side of 2 resistors and running 2 small wires.
 
Done. No change.
 

Attachments

  • 20240426_131822.jpg
    20240426_131822.jpg
    1.2 MB · Views: 4
  • 20240426_131828.jpg
    20240426_131828.jpg
    1.9 MB · Views: 3
The garbage characters have more of the other characters though... does it still run through the diagnostics?

It could be that you have a broken trace, cracked solder joint or bad socket. Does the pattern change when you flex the board on different spots or press on the socketed chips? Especially CIAs or VIC
 
powerlot said:
The garbage characters have more of the other characters though... does it still run through the diagnostics?

It could be that you have a broken trace, cracked solder joint or bad socket. Does the pattern change when you flex the board on different spots or press on the socketed chips? Especially CIAs or VIC
Click to expand...
It does still run through the diags. I'll press on the chips and look for changes.
 
more information - may not be relevant
The tape drive works and will load a program. The disk drive errors with device not found.
 
powerlot said:
The garbage characters have more of the other characters though... does it still run through the diagnostics?

It could be that you have a broken trace, cracked solder joint or bad socket. Does the pattern change when you flex the board on different spots or press on the socketed chips? Especially CIAs or VIC
Click to expand...
pressing chips and flexing board produced no screen changes.
 
powerlot said:
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
Click to expand...
Newbie question - how would I check the signal on VA14
 
rglenn said:
Newbie question - how would I check the signal on VA14
Click to expand...
Do you have an oscilloscope? You can try to find out if the signal is inverted and / or multiplexed correctly, or if some chip is marginal and doesn't output the correct singal level on one of its pins.
 
You also need to triple check all of the previous repairs with a meter to make sure you don't have any broken or bridged traces. And check all connections using the schematics.
 
You must log in or register to reply here.
Back
Top