so is the hum bar actually the fuse holder?
"Hum Bar" is just the name that they nicknamed your problem over the years. It hums a little bit, and the bar rolls across the screen... so the problem you're having is you have a "hum bar".
The reason it does that is because the power doesn't make good contact from the wall, to the board. Before it gets to the board, it runs through the fuses, the fuse holders in pac man are known to be pretty crappy.. if you mess with them, sometimes they break.
Over the years, this has been a problem in literally tens of thousands of these machines, so people have figured out that to get rid of the hum bar, the easiest thing to do is to replace the entire fuse holders.
You can do this several ways, you can get a nice pretty one like they linked to, or you can get fuse holders from somewhere else, as long as you replace the fuse holders in the cabinet with new ones, and new fuses, you will probably get rid of your hum bar.
AFTER you do that, you may still have an issue, but not as bad... then it could be that the connector on the edge of the gameboard where the power goes in is dirty... sometimes you can get away with just cleaning it.