The game boots with seven flashes and runs the tests, but the displays blank and it locks up when stepping into bookkeeping.

Game locks up entering bookkeeping after a battery change

Cause: The contents of the 5101 RAM are corrupt.

Checks

• Try clearing it by shorting all the 5101 pins together with the chip out of circuit.

Fix

• Replace the 5101 RAM if clearing it does not help.

Systems

• Bally MPU AS-2518-35

Related tips

• Seven flashes but no attract mode
• Get the MPU booting before you change any ROM jumpers
• Garbage in the bookkeeping totals
• Seven flashes, then it starts over endlessly
• Four flashes only: U11 PIA, R134 or a bad display
• How the first-flash ROM test works
• Garbled player 1 display and strange play from a bad 5101 RAM
• Flicker but no first flash

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)