Sorry if it’s already been suggested but to eliminate “environmental” factors (bad grounds, power, etc) that others have mentioned, are you able to put this board in another 80B machine and confirm that it also doesn’t work in another machine? At least that should support the theory that its a board issue.
Or do you have access to a known good board that you could put into this machine to confirm that it boots?
Edit- I reread your original post and it sounds like the board is somewhat booting. Is PROM2 all the way in? It looks like it’s somewhat out of the socket in your post but it could just be the angle. I WOULD get a small sticker or something on the PROM window to protect it from being corrupted by UV light however.
Also, your reading for the reset line sounds consistent with what I’ve seen in the past. I think that reset signal will usually only last about 50ms (1/20 sec) LOW (0) then will go HIGH (1) and remain there