Since my last post I've put this project aside for the moment.
Summery of what I've done:
• Replaced all caps on the logic board
• Adjusted the voltage rails on the analog board
• Changed to another pair of RAM modules — computer now actually started to boot normally, but with intermittent issues. I SHOULD’VE STOPPED HERE!
• Soaked the logic board in vinegar and rinsed with water — BAD IDEA! Now it didn’t start at all... (the start of this thread).
• Soaked the logic board in vinegar again, rinsed with IPA.
• Replaced a missing/damaged micro cap on the back of the logic board
• Removed and cleaned the Egret chip — computer started again! PHEEEW
• Replaced several caps on the analog board
• Re-adjusted the voltage rails
• Replaced the onboard RAM chips — twice!
• Replaced all capacitors (except 2 × 1 uF) on the logic board — again! Better soldering job this time
Issue is virtually the same. Just another pattern on the screen (after the swapped RAM chips I guess). Pattern is less garbled, though.
One day I will remove and clean the Egret chip again. Then I'm out of ideas. I've beeped the most obvious components and traces with the multimeter, and everything has been normal.