4.10.2 Loud Hum and/or Distortion at Low Sound Levels
Stern released Service Bulletin 133, which discusses a fix for a loud hum and / or distortion when the master volume is set below level 4. The machines that are specifically effected are Monopoly and Austin Powers. However, it has been over 10 years since these games have been released, and boards get transplanted to other games over time. So, it is important to check the two resistors (R106 and R110) in the amplification section of the CPU / sound board to see if the correct values are installed, regardless of the game. The correct resistor values should be 33K ohms 1/4 watt for R110 (ORG-ORG-ORG-GOLD), and 10K ohms 1/4 watt for R106 (BRN-BLK-ORG-GOLD). Consult the service bulletin for more details, and the layout of the amp section. The R106 and R110 control the analog volume. When these resistors are broken (open) the analog volume goes to 100% and can't be controlled via the buttons (digital volume) on the coin door. The higher the resistor value the higher the volume.
Another method of accomplishing the same thing, without removing and replacing a resistor, is to add a resistor to the back of the board, across R106 and R110, creating the equivalent resistance as mentioned above. A 4.7K resistor seems to be a sweet spot.
Yet another method used is to clip a 10K trim pot across each resistor. The trim pot can be adjusted to a good compromise between hum and volume.