A variety of things. First off, it could be the flyback transformer with a screen setting that is just to high. Do not touch the joints of that thing, it is extremely dangerous and can kill you. It might be as simple as just adjusting your contrast and brightness pots on your board right in the control panel. I don't want to offer the advice you discharge it yourself unless you really understand how to safely do so. When I work on a monitor chassis, I have a basic idea. I discharge the monitor, *look up arcaderepairtips monitor discharge on youtube* I remove the anode cup off the crt, unscrew the chassis, pull it out. I am gonna assume your cab is running a nanao ms8-29 or ms9-29. Those aren't a nightmare to work with. I just redid two recently.. I really recommend having the unit unplugged and making sure it is, take a flash light and look under that flyback and make damn sure none of those solder joints are dry or cracked away. Then try and see if others in other sections might have that issue. It might be tough to see, but if that is the case, a reflow of that might fix your issue, and again this requires a discharge, which is very dangerous if not handled properly. If visually it all looks great, I suggest giving the cap kit idea a try in the future if it becomes something you cannot stand.
