Problem
Four-pixel-wide vertical bars covering the screen every four pixels.
Diagnosis
Suspect an issue with the character video circuit. MAME was useful for identifying which ROMs store the character data, helping me to quickly identify the relevant part of the schematic:
- Character ROMs and shift registers
No sign of damage or prior repairs to this area, so let’s probe some 74194’s with the logic analyzer.
7K looks reasonable. Data in, data out:
7H appears to have its outputs stuck high:
Fix
Replaced 74LS194 at 7H.
