Sound is very quiet and turning the volume control has no effect, even with different ribbon cables and another sound board.

Sound very low and the volume knob does nothing: bad crimp on the volume pot wire

Cause: A poorly crimped terminal on one of the wires to the volume pot connector, so the pot was not connected.

Checks

• With the plug off, measure the pot's resistance across its two wires while turning it.
• Check continuity from each pot wire through the connector to the sound board pins.

Fix

• Recrimp the wire on a new terminal.

Systems

• Stern MPU-200

Games

• Galaxy

Related tips

• No sound and not even a speaker hum: bad crimp on the sound board's 12 V feed
• Sound module -32 and -50 test points
• Sound breaks up into static after fitting a replacement MPU: do its master reset
• Game locks up after the seventh flash
• A display not working from MPU or sound board faults
• Never use a TMS2716 EPROM

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