Well, I checked virtually any line involved in video output and couldn't find any continuity problems or shorts. However, I checked the exact pattern on the screen and found out that there are 6 pixels working, 2 pixels dead. I reckon that the first column on the screen also is the first bit in the video RAM, so the problem might be with the two highest bits somewhere. Given this, it leaves two possibilities:
- The dual multiplexer UA8 is dead, as it triggers RA(6) and RA(7) in both video RAMs (UE6/UE7)
- The UE8 shift register has some issues
Somehow, I find it somewhat more likely that the UA8 is dead as this would leat to 2 missing bits somewhere rather than the UE8 having issues with the uppermost 2 bits only.
Now I have to find out where to get 74F253, they seem to be somewhat rare :-/
I'll keep you posted!
Steffen