I found this old post on RPG from 2011
Peter G
Tech: Weird Bally SS Sound Board AS2888 Issue
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Game is Star Trek
So i was getting a loud hum through my speakers each time the four
score displays light up in attract mode. After checking over
connectors and having no obvious issues, i connected a sound board
from another game (without mounting it) and the display hum goes away.
I then reconnect the noisy board back in the game without mounting it
and no hum...
I then move the board onto the mounting bracket and all of a sudden
the hum comes back (this is just while the board is on the plastic
mounts no screws installed). It appears that when the sound board gets
close to the back of the head (or the main harness) that this
interference issue occurs. Anyone else ever see this and know the
solution ? Right now i have the board mounted an extra half inch off
the back and there is no issue, but i would like to know what causes
this and how to fix it properly.
Thanks
Peter
It's a ground loop, and there's no solution other than to isolate the
ground points from the board from the head, like you have when it's
hanging. All early solid state ballys do this to some degree when the
sound board is mounted to the head (WMS, too)
-scott CARGPB#29
Yup.
All you need is electrical isolation. Even electrical
tape over the mounting bracket will work. Just so you
don't connect the ground on the sound board to the chassis
through the mounting holes.
-Mark
As Scott said, it's a grounding problem, producing 60 cycle hum.
You may be able to eliminate some if not most of it by going through
the machine from the power plug to the backbox, and making sure that
all
ground points are making good ELECTRICAL contact.
Ground check the plug, filter, grounding braid, all pins/connectors
that
are associated with ground.
I did this with an 8BD with success.
Jim