Panasonic 32

Question

Recently I have noticed that the picture size on my 6 1/2 year old Panasonic CT-32D11E appears to be shrinking. I am wondering if this is just a sign that the tube is beginning to fail or is there a way of adjusting the picture size on the screen. It is most noticeable in the horizontal with black space appearing on both sides of the picture.After removing the back I only see adjustments for Focus and Screen and the one for Screen does not seem to control the size of the picture.