Sometimes, you can have problems with a CRT where you see a flash, a line on a screen, and hear a zap. It's scary!
Usually replacing the flyback transformer, putting grease on the anode cap suction cup, replacing capacitors, or re-flowing solder can fix the issue. Sometimes, it's a faulty tube and you need to replace the whole thing, but thet's extremely rare to have zaps and flashes as symptoms of a faulty tube.
First, make sure the brightness isn't turned down (in software or with brightness knobs.) You'd be surprised how many people think they have a faulty tube and it ends up just being a brightness control issue.
Another common cause is bad capacitors. Over time, capacitors explode or leak from use, wear, and tear, and sometimes they get too old and leaky to hold a charge and produce a picture. Check for leaky caps on your analog board.
Usually, the most common issue is just an old worn-out tube. Just like humans when they run, CRTs eventualy get tired and weak and don't produce as bright and crisp of a picture as they used to.
You can try and fix a worn-out tube, but usually it's not worth it as you will usually break it even more.