Sylvania, OH
Game: The Who's Tommy Pinball Wizard
I have (un)fortunately read every post in this thread. I have tightened, loosened, isolated all the boards. Have not put in a separate power supply.
When I read about that, I think is it the isolation that solves the issue(s) or is it the cleaner(more regulated) voltage??
Last night, I pulled my DMD/speaker panel out completely, I still have the attract mode noise. I never noticed the DMD hum also discussed in this column. When I pull the CN4 off the power supply this noise stops. Which makes sense as there is now no power for the light circuit. When I disconnect the lamp driver connector(CN7)(Lamp Columns) again no sound issues. Again makes sense because there is no power drop. Again, all noise is coming from the boards, not any speakers as they are disconnected. I do not remember if I disconnected CN6 (Lamp Rows).
Back to my initial question is it the isolation, grounding loops? or is it cleaner voltage? From all(nearly)500 posts, no one seems to have a consistent solution(other than power supply).
If it is dirty voltage, couldn't we replace the bridge rectifiers, to make sure they are not degraded, and add some inductor/ferrite bead/line filter between the F1 on the backbox and the bridge. This would help suppress the poor AC voltage quality, and therefore the new Bridges would be able to supply cleaner DC voltage?
If it is a grounding issue, could we not solder a grounding strap across the back of the CPU at the CN5 connector location(all pins) and tie that to the chassis?
I am an ME not an EE so I am asking questions.
If we used inductor what type would be recommended? What type of ferrite bead would you suggest? Any line filter would work correct?
Any input is appreciated as long as it is objective and helpful. You can keep your ME jokes to yourself 
