White Lines

[VA561]

I have a 1950 philco, model 50T-1404. The set runs great except for about 5 or 6 annoying diagonal white lines that will not go away. They are thin lines that remain no matter how I adjust the brightness/contrast. They start from the top right corner and go to the bottom left. When I start to turn the vertical adjustment knob, the lines will move before the picture starts to roll. If I move the vertical up slightly the lines move up. If I turn the knob down , the lines move down. Except for this annoying problem the picture is great. Any advice would be greatly appreciated.