Spent some quality time with this PCB last night & tonight. Still don't really know what I'm doing, so any guidance or advice would be appreciated.
First thing I did was check voltage into CPU pin 8 (Vcc), 4.77vdc. Tweaked AR1 to get 5.00vdc at the +5 test loop. This brought Vcc up to 4.95, but didn't change the symptoms.
Checked RESET pin on CPU. High, not pulsing, indicating that the watchdog is not triggering. All address bus (AB) lines are low. All data bus (DB) lines floating. Clock output 1 is high, 2 is low, and clock input is low, explaining why the board won't boot.
Moved to the clock circuits. No clock output on the 9316 counter @C4 pins 12, 13, or 14, which all measure low. Moving to the input on pin 2 shows this low as well, pointing to Y11, a 12mhz crystal, or something in its circuit.
Checked Y11, which looks like it has been replaced at some point. Found input low and output floating, moved to the LS04 inverter @B5. Vcc & GND test fine. Schematic shows pin 13 (6A) grounded through R32, which should bring 6Y on pin 12 high. However, pin 12 measures low. This output feeds the crystal, explaining the lack of clock.
I suspect replacing the LS04 at B5 will solve my problems, but I don't have one handy. I'm also not sure if I need to remove some components (or legs of components) from the circuits to accurately test them. Advice on whether I should try other tests before attempting to replace B5 would be greatly appreciated. This is my first board repair, and I'd prefer to at least destroy all the traces & pads replacing the correct component.