The sound board test button plays everything, but the game is silent or misses sounds.

Sound test works but sounds missing in play

Cause: Usually the 6821 PIA on the sound board, sometimes the 6810 RAM, or the input lines from the driver board.

Checks

• Logic-probe the five sound inputs while stepping through solenoid tests 9-13: each should pulse low.
• Ground each input at the sound board and listen; watch the matching 6821 pins.
• Sound board IC6 (4068) pin 13 should pulse high whenever an input is grounded.
• The 6821 IRQ pins 37/38 should go low when an input is grounded.

Fix

• Replace the PIA or RAM, or repair the input path.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• No sound effects in play though speech works: check the sound board PIA
• Sound effects work but speech is missing or very quiet: replace the speech board op amps
• Sound board test button gives no sound: normal on Joust
• No sound in play or solenoid test, and grounding inputs does nothing
• Missing words or sounds from cracked connector pins
• Speech board faults
• Garbled or partial sounds from bad ROMs or sockets
• Sound self-test passes but some game sounds never play: bad PIA on the sound board

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)