What things can cause a blurry arcade monitor screen?

[adder]

hey guys, just generally speaking, can anyone tell me what things can cause an arcade monitor to have a blurry picture

assuming the glass screen itself is clean and so not covered in a layer of grime/soot which many people overlook and end up with less of a sharp image;

first of all I know there is the focus knob found on the flyback which can be adjusted, - if that doesn't help am I right in thinking that a cap kit can sometimes fix the problem?  also, the tube itself, can that result in a blurry screen if the tube has had heavy use, in which case, is there much that can be done (I think I heard of something called tube rejuvenation? I havnt had a look around the internet yet regarding that, but will do so.. that said is that something someone can do themselves or does it involve exclusive/specialist/expensive equipment)

are there any things or other parts of the arcade monitor which when well used can start to give you a blurry picture (the flyback itself? dirty connectors, etc)
thanks for any advice

[jennifer]

   Don't sell yourself short on those expensive tools/equipment, Thing is its only pennies on the dollar since the market pretty much only caters to collectors anymore.... A good pattern generator will show you monitor issues you wouldn't notice otherwise... a rejuve is usually reserved as a last ditch effort to save the tube, Its not hard but in researching that (and in practice) be aware of the dangers.