The speaker hums, especially on Flash.

Hum from a type 1 sound board

Cause: Ageing audio capacitors C10 and C30, or poor grounding: the type 1 board sits on wood in the lower cabinet.

Checks

• Check the sound board ground braid and the ground strap between backbox and cabinet.

Fix

• Replace C10 and C30.
• Make sure the ground braid and strap are connected.

Systems

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

Related tips

• Reducing hum on a type 1 sound board
• Type 1 and type 2 sound boards and their switches
• Sound EPROMs work in either sound board type
• Which sound board each game uses
• Using the sound board test switch
• Powering a sound board for testing
• How System 3-6 games trigger sounds
• The sound board is its own computer

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