Does it get brighter or dimmer as it warms up? Does the focus drift in and out, like, starts off fuzzy then gets clear after the game warms up? Those are the symptoms of a bad focus divider network on the flyback. Usually you will see hairline cracks around the control knobs too.
But... if the picture is correctly focused and stays that way, and the brightness never changes, it might not be the flyback, but a weak tube. Do you have a tube tester? Does the SCREEN control have any effect?
-Ian