You have the issue described and the procedure you list above should allow you to isolate where the fault may be? With all unplugged, D20 and D21 on solid is incorrect. It appears to be your CPU board.
I don't think the issue is low 5V? 4.93V should be OK.
You could try........(from pinrepair guides by Clay)
a. Re-seating the U9 WPC chip.
Or for the more advanced......
b. Replace/test U1, U2 (74LS244) and U3 (74LS245). Then U5 (74LS14). Check resistors R95 and R99 (1 meg ohms).
c. U10 (a MC34064 transistor that is part of the startup circuit) can be replaced.
d. Using a logic probe, also check for a good clock signal on pins 34 and 35 of U4 (6809).
Memphis, TN
