After fitting aftermarket CPU and sound boards there is no game sound, only background hiss.

No sound from a replacement sound board: its 6808 CPU socket was empty

Cause: The replacement sound board came without the 6808 processor in its socket (U24); the old chip had to be moved over from the original board.

Checks

• Look over the replacement board for empty IC sockets.
• If moving chips over from the old board, try each spare: the first one tried here did not work.

Fix

• Fit a working 6808 in the empty socket on the sound board.

Systems

• Williams System 9

Games

• Space Shuttle

Related tips

• System 9: is the sound fault on the CPU board or the speech board?
• CPU chip types: 6800 on System 3/4, 6802 or 6808 on System 6/7
• CPU still will not boot after the basics
• U15 CPU and U25 RAM substitutes
• Electronic spares worth keeping for System 11 boards
• System 9 sound and speech boards fit System 6/7

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