High voltage to displays is probably marginal. Replace the caps.

Very common problem with this era of Bally games, yes, the buzzing changes pich as the high score alternates, & does tend to get worse as the game warms up too, annoying.

Replace.the caps on the sound board may fix it, or for a quick fix, if you simply loosen the two bracket screws on the sound board a bit to disconnect the ground contact from the bracket to the board, this usually stops it. I have had mine like this for years, has worked fine. In the end I put a small section of cardboard in between the board & bracket as a buffer & tightened the board screws back up.

Board gets a ground connection to function from the a plug connector wire, so breaking the board to bracket ground connection doesn't seem to create any problems.

Sometimes it works and sometimes it doesn't (or only partially reduces the noise). For the long-term I prefer to fix the issue though if possible.

The way I do the temporary fix you describe it is to remove the sound board and take two pieces of heat-shrink tubing and shrink them onto the grounding areas on the mounting brackets (where the screws thread into) and then just use the plastic snap locks to mount the board.

I finally fixed the humming sound coming from speaker in attract mode high score to date display. Bally Supersonic. I added 470uF 25v capacitor + at TP3 and ground at C24 on the solenoid driver board 5V regulator output. This lowered the buzzing hum by huge amount and the annoying buzz is gone. Here is photo of fix:

Bally Display Hum Fix (resized).jpg

Thanks for posting this, Im gonna try it out, maybe start doing it on all the boards I repair here on out!

Knxwledge Adding the capacitor and lifting the lower left corner of sound board off ground is the full fix. I just got a Lost World and this fix worked there too.

Why lower left specifically? Dont have a board in front of me to look at it. Could add a pinball post rubber under it I think, would be easy

Knxwledge I cannot explain why. Lower left and upper right are both same ground plane around the board. The metal brackets are same ground. But I have tested this multiple times on two different machines. If I tighten lower left I get loud buzz. I can loosen and tighten upper right and it has little effect and even seems better tight. So I loosen lower left screw and pull board away from ground bracket and noise stops. Ground loop? Trace hitting the bracket? Don't know. I just tried putting kapton tape on audio traces near by to rule out shorting and did not help. Both machines so exact same thing. Post a answer if you find one.

Ground loop. You are breaking the loop by isolating the sound board circuit board ground from the ground on the mounting bracket. I do this by using nylon screws and a nylon washer between the board and mounting bracket.

Thanks Kent_Diego for posting this solution. I had this issue on my Xenon and it was really starting to bug me.. I had already isolated the sound board with nylon screws and washers and it helped some. The put the ground wire under two head mounting bolts, which helped more but I could still hear the last score switching back and forth from high score. I had purchased a cap kit for my sound board which I had not installed yet that had a 470uF 25v capacitor. Popped that unto the solenoid board like you suggested and the hum was gone!

I also upgraded the dual dwarf crappy paper speakers with a CELESTION Eight 15 Guitar Speaker T5813 (only $32 from Amazon), and now Xenon breaks the silence with a "Try me again" that sends a tingle down your spine!

Oh man, my Paragon was driving me crazy and I’m so glad I found this thread! Lifting the lower left side of the sound board basically made all the noise go away. I put some electrical tape on the back of the board for guaranteed isolation. I don’t think the capacitor fix is needed but I’ll go back to that if I notice any more noise.