Sparking In The CRT Tube Neck
If you come across sparking (blue arc) internally in the CRT tube neck, there could be two causes. It could be the high voltage generated too high by the flyback transformer (due to a higher than normal B+ voltage) or the picture tube itself have problem. A leak (usually a mild crack at the neck of the picture tube) would cause the air to rush in. For your information, inside picture tube is a vacuum and assuming you accidentally break the tube neck you will hear a sound of air going in and NOT gases that are coming out.
First make sure that the anode voltage is within the spec of about 24 Kvdc. Test the anode voltage with a high voltage probe. If the anode voltage tested good then suspect abad tube. If the anode voltage is higher than normal then check the B+ voltage or even the internal capacitor. In some Monitor design the feedback produced by the internal capacitor will send to the B+ circuitry and if this internal capacitor have problem, the B+ voltage will shoot up thus causing the anode voltage to increase. Again in some Monitor design, the high voltage shutdown circuit is not sensitive enough and will not shutdown the high voltage and this will cause arcing in the neck of the picture tube.
By the way, i did not catch any big fish, only small fishes but I’m equally happy be able to fellowship with some of my buddies.