screen shrinks?

I have this tv, which is mitsubishi make and a cathode ray tv. it has been working fine,but since 1 month ago the tv does a wierd thing. once u switch it on,its fine but after like 2mts the tv screen display shrinks to the centre of the screen.so all u see is a 10cm wide line of display stretching through the centre of the screen. How can i fix this my self.please help...
Please email me at dushkath@hotmail.com