A loud hum from the speakers that stays the same whatever the volume, or minor hum and static.

Hum or static that doesn't change with volume: filter caps

Cause: The large sound board filter capacitors or cracked solder joints on them: C24 and C25 (4700 mfd 35 V) pre-DCS, C20 and C21 (10,000 mfd 35 V) on DCS, C36 and C37 (10,000 mfd 35 V) on WPC-95.

Checks

• Inspect the cap leads for cracked joints.

Fix

• Reflow, and solder jumper wires from the traces straight to the cap legs; replace weak caps.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Deafening hum at power-on: cracked ground trace at the DCS sound board filter caps
• Loud hum at power-up and the cabinet speaker burns: shorted audio amplifier chip

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Three, Clay Harrell (pinrepair.com)