Quoted from Brunsie:
My Stargate has a slight hum also. Noticeable when you walk in the room but not while in play. I disconnected the coax from the aux sound board to the aux power supply board and shorted the audio input to the aux power supply board with a little wad of aluminum foil in the connector. Still got the hum - but not the power up "thunk". So my hum isn't from either of the sound boards. And the power-up "thunk", from what I've read on line, is due to the sound board D/As not being held in reset during power up. Back to the hum... Seemed to mainly be coming from the cabinet speaker. The cabinet speaker does not have a dedicated signal return - it runs to the center tap (CT) of the power transformer and the CT goes to the aux power supply. Not good practice to mix power and signal grounds so I ran a dedicated speaker cable form the aux power supply board to the cabinet speaker by splicing into the back box speaker signal return and the cabinet speaker output right at the cable connector going into the aux power supply board. See photo. Hum was significantly reduced but not eliminated. Can't hear it at all when you walk into the room until you're a few feet from it. Also completely masked when central air is on. Wish I had a spectrum analyzer to have more quantitative results to share.
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Having the same issue with the hum from gottlieb games and went ahead ahead and took some DB readings from inside the cabinet.
Inside I saw 56db in normal attract mode.
With the cabinet speaker disconnected it went down to 47db
Applying the @Brunsie fix the noise level is now 50db. 6 db difference and yes it is noticeable! You still hear it humming away but only when getting close. I have four gottlieb games all in the same corner of the game room and it was pretty loud. Its much better now. I did it to Stargate, Gladiators, Wipeout and Waterworld. It made a difference on all of the games, right around 5-6db each. I also have a subwoofer on each game and this helped a lot with cleaning up the sound there too.
Its easy to do and takes 15-20 mins each game. Really, it could be less I'm sure but that darn 'testing' extends things out a bit.
As a side note, 2 of the games have colordmd's (Single color) in them and it still makes a difference - Those games are just quieter than the plasma DMD games is all.