The game plays the wrong speech for its callouts, and sound test plays the wrong samples (for example a speech line where a sound effect should be), although the ROMs test good and have been reburned and reseated.

Wrong speech and sounds play for callouts: battery-corroded CPU board

Cause: The CPU board, which sends the sound commands, had battery corrosion; the sound select signals it sent were wrong.

Checks

• Inspect the CPU to sound board cable and header pins for pitting or oxidation, since a bad select line can do this.
• Look over the CPU board for battery corrosion near the battery and the logic around it.

Fix

• Repair the corrosion damage on the CPU board, or replace the CPU board.

Systems

• Data East CPU Version 3

Games

• Back to the Future

Related tips

• Combining two-chip CPU code into one 27512 for 5C
• Where the TIP122 drivers are on the CPU board
• Free-running the CPU to check address lines
• Where the main chips sit near the CPU board batteries
• Missing VMA signal from corroded traces
• New CPU board locks up when the coin door buttons are plugged in: reseat the PIA
• Other CPU jumpers and what they set
• Bench-testing a CPU board on +5 V

Source: Pinball Repair help group!! (Facebook group)