Steady hum or strange noise from the speakers while everything else works.

Constant hum or odd noise from the speakers

Cause: A faulty LM7809 regulator on the aux sound board, which can give the right voltage yet still be noisy.

Checks

• Find the LM7809 on the A20 board.

Fix

• Replace the LM7809.

Systems

• Gottlieb System 3

Related tips

• The A20 auxiliary sound board
• Both sound board LEDs dark
• A5, A6, A8 and A20 plug identification
• Gottlieb System 3: constant hum from the speaker at idle

Source: Repairing Gottlieb System 3 Electronic Pinball Games from 1989 to 1996, Clay Harrell (pinrepair.com)