As nem said all DJ MAIN Pop'n cabinets shipped with Toei monitors. It's possible that someone replaced it down the line with a Nanao (probably an MS9) at some point since the Toei tubes have a high failure rate. When you say it's a Toshiba monitor I'm assuming you mean that it's a Toshiba tube?
If the picture you've attached is presently what the machine is displaying, I'm doubting it's monitor failure. MS9s tend to have less neck glow than something like a K7000, and that tube wouldn't be showing an image like that if there wasn't any neck glow. The screen brightness is way too high on the chassis flyback. There are two adjustments on the flyback transformer itself - one for brightness and one for focus. You'll want to turn the one labeled "SCREEN" (brightness) down until the horizontal white lines disappear, and then adjust your colors and check your signal cable from there.
If you don't know what you are doing it is highly recommended that you seek some help from someone who knows this stuff locally. Monitors have lots of high voltage going on when they're powered up, and it's very very easy to make things worse by adjusting the wrong things.