Display boards are fairly easy to troubleshoot.
Refering to the display schematic.
U5, U1, U6, U2, U13, U12 and U9 are simply drivers for the displays - converts low voltage to high voltage to drive them. If you have a signal going in, it wil drive the corresponding segment.
U13 = player 1 & 2 display (extra characters), U12 = player 1 & 2 display (numeric characters only), U9 = player 3 & 4 display. U13, U12 and U9 are driven by non-inverting buffers for the inputs to the drivers (voltage in = voltage out).
U5, U1, U6 and U2 basically control which display is being updated.
U3, U4 ,U7, U8 are a TTL logic buffered NOR gate (datasheet here: https://www.fairchildsemi.com/datasheets/CD/CD4011BC.pdf ) NOR gate truth table here: http://www.ee.surrey.ac.uk/Projects/CAL/digital-logic/gatesfunc/index.html#norgate
Basically if you have both inputs low you will get a high output, so for U8, pin 12 and 13 have to be low to geta high output on pin 11.
So measure voltage at each input and see if the output corresponds - if not bad chip (make sure you check the voltage supply is present on the IC's as well).
You also need to check to make sure you have high voltage for the displays as well: R10, R12, R14 and R17 should have +90v while R11, R13, R15 and R16 should have -100v.
Hopefully that makes some sense.