I spent a good 12 hours on this board today. I'm just out of ideas. Everything north of the lower ground plane near the ports has 5v shorted to ground. So all the capacitors north of it are shorted, and diode D10 reads both directions (if I pull the ADB filter and isolate the lower ground plane, everything reads normally, which is how I know the short is north of the lower ground plane ... which means the other 90% of the board.)
I also replaced today (with no change in the problem) the corroded ICs at UH1, Q2, and UA4. I also removed, cleaned, and resoldered (to no change) VD11, UA13, and UA7. They were also corroded.
Each time I removed any of those components, I tested for shorts, but it was still shorted.
I also tested every single resistor on the back of the board. They all tested fine except R66, R64, R42, and R36. Those all read 0.5 ohms regardless of their values. If I take them out of circuit, they read normally, and the short is still there.
Because 5v is shorted to ground, after I had initially replaced UB13, UD13, and UE13, the computer would start automatically no matter what. No ADB activity. So I had replaced diodes D3, D4, D5, D6, D7, D8, and transistors Q4, Q5, and Q1. The computer is back to not booting, and 5v is till shorted to ground.
I have completely exhausted my ability to diagnose this board, unfortunately. Unless anyone has any suggestions.