Voltages and amplifier are fine but the board makes no sounds.

Sound board powered but not running

Cause: Reset circuit, clock, Scanbe sockets, EPROM, PIA or RAM.

Checks

• CPU pin 40 should be high (reset section working).
• Logic-probe pins 38 and 39 for the clock; if missing, the crystal or its capacitor is probably bad.

Fix

• Replace every Scanbe socket, on the speech board too if fitted.
• Burn and fit a new sound EPROM.
• Swap in a known-good 6821 PIA, then a 6810 RAM.

Systems

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

Related tips

• Sound CPU pin voltages with the EPROM removed
• Which sound board each game uses
• Sound board jumpers for 2K sound-only games
• Using the sound board test switch
• How System 3-6 games trigger sounds
• The sound board is its own computer
• Type 1 and type 2 sound boards and their switches
• Setting up a sound board for speech

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