if it is a crt, might just need the focus adjusted on the flyback,,,or wherever it may be. You have to take the cover off AND that will expose you to VERY LETHAL VOLTAGES....In the range of 2KV OR MORE. You may want to have someone professionaly evaluate the crt for you and possible adjust it.
...and then again , crt's are cheap as LCDs are also, might be cheaper to get a new one