If it won't credit up or start a game, but you get the 7 flashes/chimes, and it goes into attract mode, I'd check the connectors on the MPU board. Remove them, clean them, reseat them.
If you haven't already, consider reflowing the solder on all the connector pins. Several of mine were broken loose and free floating in their holes in the board. Also, probably not a bad idea to replace all the electrolytic caps on the two sound boards and the big filter caps on the solenoid driver board. Those things did the trick for me, anyway.
Have you figured out what they were trying to achieve with that relay they put on the power supply?