Check the large plastic connector (J3) on the rectifier board (as seen in photo below) for browning or discoloration. Especially at pin 8 terminal. When counting from left to right, pin 8 is the one directly after the blank (key) spot. That is the connector that sends 11.9VDC to the solenoid regulator board. I found that wire connection loose on my supersonic pin. It caused the connection to draw high amperages, which in turn, caused it to burn/melt the plastic connector which then caused it to loose the connection entirely. My game stopped working just as you described, so it might be worth a shot.
If nothing there then, check the connector at the top right of the solenoid driver board (as seen in photo below). Again, connector J3. It carries the 11.9 VDC from the rectifier board's pin 8 on the J3 connector to the solenoid driver board's pin 12 on the J3 connector. Check for complete continuity between pins or connector malfunction. Check for cold solder points on the back of both boards. Especially behind the pins.
In addition, as suggested, removed the battery from the MPU immediately. Carefully remove the connectors fom the MPU, and unscrew the board from the metal clips. Take the board to a well lit place, and carefully inspect the solder side of the board. Take note of trace flaking, corrosion, and component acid saturation. Repair as needed. Mine was so bad I had to replace it. The entire ground traces were lost, and multiple component were saturated in acid. I replaced my MPU with the Universal Bally/Stern board from arcadeshop.
As a suggestion, I would completely rebuild the solenoid driver board. I found both capacitors on my board testing at 0 microfarads. In the solenoid driver board photo below you'll observe that it has been rebuilt.
I am tired now. Hope this helps.
Greg-
RECTIFIER BOARD
SOLENOID DRIVER BOARD
UNIVERSAL BALLY/STERN MPU BOARD
http://www.arcadeshop.com/mpu-old/New_Board.jpg