Stern Moon War loud audio hum

TOK

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Hey guys, I'm working on my first Stern game. It actually had a ROM problem which I resolved. The game now plays fine but has a wicked audio hum, to the point where you can't hear game sounds.

Here are the two things that are weird about it:
First, and this may be the biggest clue -the hum starts as soon as the game is plugged in, even with the power switch off.

The second thing is that the pitch of the hum as well as the loudness of it can be varied with the volume pot, you can't turn it down enough to hear game audio though, its always pretty loud.

The first thing I tried was pulling power for the marquee light, and that didn't affect anything. The volume control looks like it has been worked on. It is 3 wires soldered to the board and a remote 10k pot.

Is this a normal Stern setup? Just want some suggestions before I start taking it apart.
 
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