If it has a white line in the middle running left to right the problem is in the vertical circuit.
This can come with age as solder connections (this is the most common cause) come loose or break at the chip in that circuit.
Much of the time with set on a good hard rap on the top or side and the line will try to open and return a full picture.
Someone who can get to it and can solder can fix that---the chip will usually have 7-13 pins and be mounted on a metal plate that is fairly small(it keeps the part cool)---fixed a lot in less than ten minutes.
You will have to open the set, locate the chip(usually a location number on top of board by it will be something like IC301 or 401).
Most of the chips will have a generic number on them a LA------ or such. You will have to remove the board to look at the bottom where the chip solders in.
I attach a photo of a typical chip.
Let me know if questions
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