After the CPU board was refitted, there is a hum and then no sound at all, even after the plugs were swapped back to where they were thought to belong.

No sound after board work: 1J15 and 1J16 swapped (they aren't keyed)

Cause: The plugs at 1J15 and 1J16 had no key and went on the wrong headers. 1J15 is the output to the speakers. 1J16 carries the CPU board's sound and speech signal out to the background sound board and the mixed signal back in.

Checks

• Check the wire colours on 1J15 and 1J16 against the manual, not against memory or an old photo.
• To test the main amplifier, touch its pins on the back of the board and listen for amplified static.
• To bypass the background sound board for a test, plug the volume control connector into 1J16 and leave the speakers on 1J15.

Fix

• Put each plug on its correct header and add keying pins so they can't be swapped again.

Systems

• Williams System 11

Games

• Grand Lizard

Related tips

• Testing the CPU board sound section with Leon's sound EPROM
• Where sound is made on System 9 and System 11/11A
• CPU sound diagnostic switch SW1 results
• No sound from SW1 although sounds play in diagnostics
• Whole lamp column works only intermittently: reflow the board header
• Loud constant hum that the volume control does not change
• Special solenoids dead after fitting an early System 11 CPU in a later game
• How System 11 A/C switched solenoids work

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