you have a shorting filament-maybe some crap in it,seen this only a few times
only way to sort this is use of tube rejuvenator or a new tube

you cannot take the tube apart,its a sealed vacuum
now although i would try the following myself i will not be held responsible if you knacker it completely,o.k
take the entire monitor out of the cab,put it face down on protective surface(towel or bubble wrap) and then(real gentle) tap the neck of the tube all the way around with the handle side of a screwdriver(not a hammer)-do it too hard and you will crack the neck and your tube is then screwed for good.I did this with an operation thunderbolt about 20 years ago and it did the trick(dunno if it still work though)
like i said if you break the neck then don't blame me,but tbh if you do not possess a tube rejuvenator then what have you got to lose because you would need to get a complete new tube/monitor anyway