Excellent - as I said I have never seen U2 go bad, but it looks like that board has had a hard time

Your next questions:
1. who cares!  Check the transistor on the SDB, check coil is not burnt out/broken wires.
2.The connecting cables are often broken - check for continuity. Probably worth getting a replacement off Ebay or making up your own. Check the cables are connected properly - pin 1 to pin 1, no gaps between the two separate cables. Check the header pins on the MPU and the sound board.  Check voltages are all at the test points.  Very rare for these boards to go bad, so check the other things first.
3. Can't get a credit up? go into switch test and see if the coin switches are registering, if not try reaseating the switch connectors on the CPU (Right hand ones).  Credit button does not register?  go through the same process.