I always thought this was a good video.
The schematics will help you know what AC voltages you should have at the transformer itself.
-Mike

Humming from speakers,soundboard or transformer itself?

Speakers humming could be a bad ground. If the humming is in tune with the playfield lights it could be a ground loop.

Sound boards can give off an odd hum also indicating a bad ground or ground loop.

Replacing the very outdated capacitors is another idea to freshen things up and not have any leakage and get the signals smoothed out again.

I have seen transformers somehow loosen up and give off a hum from the plates.
If that is the issue it's as simple as unplugging the machine and tightening the bolts.

-Mike