A final post mortem on this problem. Well the sound and spinner went out later. After I replaced all the capacitors minus the ones on the monitor, replaced the oil capacitor, added new solder to all the connectors on the sound/io board, opto board, grounded everything, had CDJump repair the...
Quick final update. The aftermarket power supply went back to it's previous shenanigans and the sound and spinner went out again. So, I bought a Suzohapp power supply and new MCR adapter board and now the sound and spinner control work every day for the last 4 days.
So.... I gave up trying to figure out what was causing the humming sound and went back to the switching power supply that also came with my game. Whatever I did to rehook it up fixed the missing sound and spinner. Now the game is back up!
Here is another data point. I used a friend's Fluke multimeter that allows you to see the Hz of the electrical signal. I checked J2 on the Dual Amp Board and each speaker is 120 Hz.
I hooked up the grounding strap to the control panel since I saw some other posts saying that is where it should be connected. Unfortunately it didn't stop the hum.
Another thought has come to mind regarding the grounding straps in my game. There is one that is next to what I think is the line filter and it goes to the top left corner of the coin door frame. There is another group strap that connects to the same spot on the coin door frame and isn't...
Reflowed the solder and added a little bit to all the connector pins on the power board and it didn't clear things up. I also did the same for the new caps I put in... No luck with that either...
I repinned the cable for J1 on the dual power amp board, however the hum is still there. I wonder if I need to relook at the soldering of the caps on the power supply board...
OdinM, I've repinned all the connectors on going to my power supply board, but will work on the dual amp board next and then work my way around the rest of the connectors.
Mecha, since I don't have a scope, do you have a recommendation for me to buy?