[compucat]

As I understand it, it goes something like this. When a picture tube wears out, the emissive material, that is the coating on the cathode element is diminished. It gets slowly consumed over the lifetime of the tube as a result of the act of boiling off electrons when heated by the filament. The filament is like a light bulb in that it either works or it doesn't. This reduced emission results primarily in a dim picture with poor focus. There are fewer electrons in the stream therefore brightness is reduced and focus is not as sharp because the other elements in the tube have little to work with.

The electron gun fails primarily from, open filament(s), weak cathode emission or shorted elements. Clolor tubes can also suffer from bent shadow masks which affect purity and convergence. I hope this helps.