Pressing SW1 gives no sound, though the sound test in diagnostics works.

No sound from SW1 although sounds play in diagnostics

Cause: Missing sound select signals, a low or noisy -12 V supply, or a fault in the amplifier, volume control, speakers or cables.

Checks

• With a logic probe, check that U9 pins 2-9 pulse during sound test 01.
• Check the gray and gray/green transformer wires for about 19.4 V AC.
• Check CPU filter capacitor C26 for -12 V DC, and for AC ripple above 0.75 V AC.
• Turn the volume fully up and briefly touch the tip of a corded, powered soldering pencil (40 W or less) to the centre tap of the volume control. A hum means amplifier U1 (TDA2002), volume control, speakers and cables are fine.
• No hum: turn the volume down slightly and repeat to see whether the volume control is at fault.

Fix

• Repair the failed supply, amplifier, volume control or cable connection found.

Line voltage is present in the cabinet; unplug the game before working near the transformer, line filter or wall wiring.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B

Related tips

• Testing the CPU board sound section with Leon's sound EPROM
• No sound after board work: 1J15 and 1J16 swapped (they aren't keyed)
• Loud constant hum that the volume control does not change
• Sound circuits on the 11A and 11B CPU boards
• Where sound is made on System 9 and System 11/11A
• Coin door test buttons won't enter diagnostics: jumper test at the CPU plug
• Random slam tilts: replace the SRC6 resistor pack on the CPU board
• CPU board test switches SW1 and SW2

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Two, Clay Harrell (pinrepair.com)