Not sure on the polarity of the IRQ for that cpu chip, but typically IRQ's are meant to be high, and only pulled low when controlled from the other chips onboard. (of course your cpu may be the opposite of everything else).
Regarding your AC measurments on pins 15/16 (0v) did you try measuring with both MM probes on 15 AND 16?
It looked like you might have measured from pin15-to-ground, and pin16-to-ground, which would probably measure 0v, misleading you.
Any chance of running your display board down to Mark's to plug into a known-working pin to be SURE your displays are ok?
I also find it odd that only 60V is mentioned on your displays, typically Vacuum displays require pos & neg volts, typically +/-50v upto +/-100v etc....
just my 2c 
MM.
Thanks MM,
I think I've got the display issue sorted. From everywhere I've read pin 4 of U1 (CPU) the IRQ should stay high for the first five seconds, then drop to pulse (at 3.5v I think).
Mine is stuck at 5v and not pulsing - hence the board not booting.
I have just about given up on this. Its nearly got me beat. I cannot work out what else can be causing issues with the IRQ.
