Sound is very low and a 60 Hz hum is mixed in.

Very quiet sound with a 60 Hz hum: bad capacitor on the A5 board

Cause: A failed filter capacitor (C6) on the A5 auxiliary power supply board.

Checks

• Inspect the electrolytic capacitors on the A5 board for bulging or leaks.
• Test the capacitors out of circuit with a proper capacitance/ESR meter.

Fix

• Replace C6 on the A5 board.
• Because the board is from the mid 1980s, replace all of its electrolytic capacitors at the same time (there are only a few).

Unplug the game before working on the power supply board; large capacitors can hold a charge after power off.

Systems

• Gottlieb System 80B

Games

• Hollywood Heat

Related tips

• The System 80 sound board generations by game number
• Sound boards listed for System 80 to 80B
• Sound board MA-490 main chips
• Sound board MA-766 main chips
• No sound though the sound board boots: 5 V reaching it is low, adjust the trim pot
• Sound/speech power supply MA-149 parts

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