I've found there is a fairly complicated path for sound power on System 3 games. Mine is borderline, seems noisy when the game is idle and in attract mode...but not as noticable in gameplay mode.
At first I also thought it was restricted to the cabinet channel and replaced a number of components including amp, caps and others in the output stage. It seems the noise is also present in the backbox channels, but "masked' by the characteristics of the backbox speakers. My ground mods are also done.
Due to the way the +/- 12VDC is created, split and trucked off the aux power board means there is no simple way to power the audio amps off an external supply. I basically stopped investigating the problem at that point. It just hasn't been a priority for me.

I'm not certain the noise is originating in aux power board in my game. There's all kinds of noise problems, and not all have the same root cause or symptoms. I'm thinking this might be a ground loop problem. If you replace your aux power board and that solves it, let us know. I'm sorry I don't have a solution at this time.

I have a Rescue 911 with the exact same symptoms. I too was going to replace the aux power board. Please let us know if this fixes the problem for you.

I went ahead and replaced the aux power board and sadly the buzz remains. I am not sure where to look from here.

Thanks for posting.

Anyone think it would be worth changing the filter capacitors on the transformer? Thought it might help with the buzz.

Measure the AC ripple. If it is less than half a volt, I wouldn't bother.

Spectrum analyzer app pics - CBW

Game on (idle in attract mode)
28_Apr_2018_06_41_35 (resized).png

Game off (background noise)
28_Apr_2018_06_39_33 (resized).png

Any updates on this? Mine has the hum also and I would love to fix it.

I already tried the aux power supply which did not work. I ordered one of those power supplies that will do system 80 or system 3 for my genesis. I am going to put it into my R911 first just to see if it changes anything. I'll post the results.

The main power supply also did not fix the problem. There has to be some way of getting rid of this super annoying hum/buzz. I don't even turn this game on unless I am going to play it because the hum is so loud!

woodmedic. did you do the ground mods? also carefully wiggle the connectors into the metal transformer plate. Sometimes these need repinned. I had a particularly bad one on the right side, after repair the hum was reduced dramatically.

I have not done the ground mods on this machine. I did wiggle the connectors on the transformer with no luck. I'll gets the mods done and post back.

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.

Speaker Cable in Back Box (resized).jpg

Speaker Cable in Cabinet (resized).jpg

Brunsie. Thanks for posting
that's a significant amount of troueshooting. I thought my noise was cab speaker only, it sent me on a wild goose chase of replacing components only related to the cab speaker channel. Upon further analysis, the same noise is on the backbox speakers as well, just much noticeable because the speakers there have different response characteristics than the cab speaker.
Just good for thought.

Posting this, as it may help some one down the road. Had the very loud buzz issue on the lower speaker. This is a tricky one to diagnose as all the sounds are present, everything appears to run fine, but with that loud buzz in the lower speaker. We had even tried the new board as well. Made the mistake of assuming we must have all the voltages as we had all the sounds, could control the volume, etc. Turns out there are two separate and fused 12.6 vac inputs into the Aux power supply board (A5) . F11 was bad, not supplying 12.6vac (one of the two) to run the board properly. This causes U3 to overheat and damages it rather quickly. Replaced U3, fuse F11 and all is good. Back to the minor audio hum that all these late machines have. I suppose the same issue can happen for the upper speakers as well if F10 was bad. Hope this helps someone down the road.

Thank you. I'll look into that.

I just got a Stargate with a loud hum coming from the 8" cabinet speaker. I rigged up a quick test for @brunsie 's fix using speaker wire. It was a huge improvement and made the speaker hum less noticeable than the DMD noise. I will be making the fix permanent next time I get a chance.

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.

Glad to see this fix is working for others and that someone actually measured the improvement!

excellent that i find this post because i was tearing my hair out!
i had a sf2 with shorted tantalums on the main sound board and one of the 12s out , replaced them all and recapped the aux psu/amp module
now just this damned hum! was thinking about some sort of hum / low frequency rejection filter on just the base speaker, but the dedicated speaker wire
is a stroke of genius. i may yet do the filter to to shut this ahole thing up, and guess what? a cueball wizard with an even worse hum coming up.
i salute you Brunsie
- performed mod and hum is greatly reduced.
other earth modding would be beneficial even including around transformer panel mounting baseplate flange extra to line cord earth (doubling up)
and removing the silly group of white plugs and crimp on ring lugs
i also earthed the common lead on the amp module to headbox backplane (even though that is just foil on partical board)

Does anyone have a link to the Ground Mod?

I have a "static" noise on mine through the backbox speakers with some of the music, not all music/call outs. I need to investigate it a little further still.

I have a hum all the time on my wipeout but it’s not bad, seems on par with most older machines but every couple days when I turn it on the hum is waaay louder. Even if I turn it off and back on again it stays like that and then the next night it might be gone again for a couple days and then back.

I have the buzz mainly coming from the cabinet speaker in my Operation Thunder, I’m going to add a sub filter and a new sub in there that should filter out the buzz. Had the same problem in my LOTR and that fixed it.

Wow, hello folks. I have a Stargate. It has the hum too. When I first got it. It was pretty loud. I replaced a few cap by transformer. It did help at that time. I will try post on Stargate thread. We have some smart people here. Hope they can help.