No score displays work and the controlled lamps act strangely; both are fine with the sound board unplugged.

Faulty sound board blanks the displays and confuses the lamps

Cause: A faulty sound board (seen on a Cheap Squeak) loading the shared MPU lines.

Checks

• Unplug all sound board connectors and see whether the displays and lamps return.

Fix

• Repair or replace the sound board.

Systems

• Bally MPU AS-2518-35

Related tips

• Sound module -32 and -50 test points
• AS-2518-51/-56 sound board jumpers
• Bench-test a -51/-56 sound board with a test EPROM
• MPU stops before the 7th flash until sound board power is unplugged: replace CR3
• Sound Plus -56 and vocalizer test points
• No sound at all: start with the amplifier hiss
• Which -51/-56 sound ROMs respond to the board test button
• Game only makes chime sounds after a battery or RAM change

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