These sets had problems with bad picture tubes that cause this exact problem, but there is something you can try that may help you get some more use out of the set.
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Here is what you need to do.
1.) You need to remove the entire plastic piece below the front of the screen.
You most likely will find 3 screws on either side in back that you have to
2.) The grill will not come out easy, pull from one side and then when it
starts to come loose from the other side(there are several plastic pegs that
hold it in.) Then from the bottom pull out( most of these sets have square
plastic tabs at the top that tuck in.)
3.) With the grill removed locate the inspection plate behind it (it usually
has 6 screws.) You will have to tilt the grill to the left and on its side as
several cables are connected to it--they are held in place with trash bag like
ties that you can release to get some room.
4.) With the inspection plate removed you will see a fairly small (about 3 inch
square) BLACK PLASTIC BLOCK WITH SIX CONTROLS-usually they are marked with
whiteout so someone can tell if they have been adjusted.
5.) You need to look very close at the block with a flashlight----usually the
top three controls are FOCUS--don't touch those.
The lower three are usually marked Screen and if you look very close you will
see b, g, r(blue green red)
6.)Usually the GREEN will be in the middle.
7.) Once you locate the BLUE control have the set on and turn it
down--if the set is shutting down you may have only a few seconds to try to
turn it down---usually to the left with the Screen control.
8.) Best way to do this if the set is on is on a video input
with nothing connected--you turn
the blue down until the set stays on and then very slowly turn it up until you
see a slightly blue screen with or without diagonal trace lines----turn it
down slowly until the blue screen and trace lines just disappear.
Let me know if you get it to work--Like I said if it is shutting down you may
have to slowly turn the screen down a few times to get it to stay on.
If this works you may only be buying some time and if it is the tube sooner or later it may return with the same problem.
If this has helped you rate this solution as I do this here for free.
Regards, SD TECH