Mid-game every sound became a strange wrong sound that lingered, and the background sound stopped. Later even the sound board's own test button gave wrong sounds or none.

All sounds suddenly wrong and background gone: bad 6802 on the sound board

Cause: A failing 6802 processor on the sound board.

Checks

• Unplug the sound select connector from the sound board and press the board's test button; if the test sounds are wrong too, the fault is on the sound board.
• Swap the 6802 with one from another Williams board to confirm.

Fix

• Replace the 6802; consider a new socket too if the old one is tight or worn.

Systems

• Williams System 7

Games

• Firepower II

Related tips

• System 6/7 sound board can take a 6802 in place of the 6808 and 6810
• A 6802 CPU chip works in place of the 6808
• CPU chip types: 6800 on System 3/4, 6802 or 6808 on System 6/7
• Replacement CPU board keeps reporting a U38 PIA fault: try another 6802
• Transistors, diodes and chips to keep for CPU and driver repairs
• CPU still will not boot after the basics
• Test program does not run at all
• The sound board is its own computer

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