I feel you have a failure in the high voltage regulator circuit on your set. At lower brightness levels, the picture appears out of focus and things are not distinct. Right? When you advance the brightness and contract controls, things get sharp and distinct? <br /> <br />Let's talk about high voltage circuits for a second. It takes a voltage anywhere between 25,000 and 45,000 volts to the screen of your TV picture tube to allow the tube to make a picture. This voltage has to remain constant so that the picture does not change during scenes of darkness and scenes that appear in high brightness. So, the manufacturer puts in a a regulator circuit to ensure this voltage stays at the same level. When the regulator circuit fails (and they do!...now and then), you have to adjust the brightness and contract to a level where the voltage is optimum to get a decent picture. <br />Another note is the fact that the high voltage circuit provides a "focus" voltage to the picture tube and any change in the high voltage causes this focus voltage to change. <br /> <br />Shutting off? Well, that is protection circuit provided by the manufacturer. If the high voltage regulation is removed, the high voltage can reach values outside of the safe parameters. When it does, the set is designed to shut off. It saves you from damage to the TV as well as fires. <br /> <br />Call your repair shop and discuss what I have given you for information. <br /> <br />
try this link also
http://www.fixya.com/support/t14213-tv_turns_off_itself_correct

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you have to change the voltage regulator. if you cant handle it please get it done by some repairer.

Thanks