I have been trying to get rid of the constant idle hum from my Riverboat Gambler Tried everything and did find a fix. I tried the ferrite chokes and was waste of money. LEDs looked great but did not help humming. Moving wires did not help. Adding capacitance to light matrix cap did not help. All boards were screwed down. Unplugging roulette wheel did not help. Adding larger caps to sound card DC input filters did not help.

I found three things that did help:
1. Had Rottendog power supply. I placed large capacitor on +12V line to ground. This got rid of higher frequency noise form PS but made no difference in power line 60Hz buzzing.
2. Put nylon washer on lower left screw on sound card. Need to have screw loose and card pulled away from bracket. Makes huge difference. Went from loud buzz to something that could only be heard for 15'.
3. Added resistance to speakers. I put four 220 Ohm 1/4W resistors in parallel to give 55 Ohms. Added series resistance for backbox speakers and also added for bottom speaker. This made humming barely audible from 3'.

Before the humming was unbearable. Had to switch game off when not in use. Now can hear at 3' at idle but now I can leave game on. Hope this helps other Williams System 11 owners. Be sure to post your results.