audio board for Sanyo 20-Z2aw monitor

I have recently obtained a Nintendo Donkey Kong game that has a 14" version of the Sanyo 20-Z2AW monitor in it. The audio amplifier is part of the monitor.
The audio has a hum in it that is not controlled by the volume control. I have checked every part on the board - everything is OK.
When I remove the board from its mounting position and move it away from the yoke, the hum disappears. When properly mounted, the audio board is very close to the yoke.
My question - is there supposed to be a "top cover" on this assembly, to shield it from the magnetic interference that is undoubtedly being generated by the yoke? The board was mounted to its mounting bracket with two screws; I suspect that there may be a top cover that would mount to the other two screws.
Any help would be appreciated.
Dave Edmonds
Jackson, MS