The likelyhood that it's on the individual display boards is low, since they're all doing the same thing, right? It's probably the display controller board.. and something that is common to all the displays.. I gotta go look at the schematic..

OK.. Looking at the schematics, since it seems to be ALL five displays (4 scores, plus credit/ball) it is likely something that is in common to all of them.. Try reseating/checking the bottom middle cable on the master display board (4J7) to the left of the power connector someone circled in the picture above.. Also.. on the CPU board try reseating PIA I (IC 18, part number 6820 or 6821). This chip seems to control all of the segments EXCEPT for the comma, which would make sense. You could swap IC18 with IC36 right above it and see if the symptom changes. The one above (IC36) controls the sound and the commas on the displays. If the trouble moves and you have displays but no sound then you have a bad 6821. For what it's worth, 6821 can be used in place of a 6820 They're apparently the same except the 6820 can't drive as much current. If the trouble doesn't move, then maybe there's an address decoding problem on IC18? IC18 also controls the 7 segment diagnostic display on the CPU board, so if you dont get anything showing on the diagnostic digit, then you have a clue there that it's probably IC18, or the address decoding for that chip. Do you have a logic probe? Even something like this would help:
http://www.newegg.com/Product/Product.aspx?Item=9SIA5D52NZ9151

If you can see with a logic probe whether pins 2-17 on IC18 are strobing (counting across the bottom of the chip from left to right starting pin 1 on the bottom left corner) that would help.

Troubleshooting this stuff can be fun!

Oh.. and in case you haven't found it, and dont have a manual.. I was too lazy to go downstairs and get my hardcopy, but I found this online:
http://gamearchive.askey.org/Pinball/Manufacturers/Williams/pdfs/sys7.pdf