I turn on my TV and after a few sec the screen goes black?

bad High voltage NPN power transistor
throw out the tv - get another one at a thrift store for $20 - I have seen sony wega at this price or free on craigslist

You may need to do a hard reset. Hold the power button for 30 secs and then wait a few secs and turn the system on. Hope it helps.

If your laptop appears to boot up and the hard disk LEDflashes, then the problem is probably a back light fault. This can be a faultyinverter or a faulty CCFL lamp. Please click on this link and follow myinstructions to identify the fault and possible fix :- http://www.fixya.com/support/r3598095-faulty_laptop_lcd_screen

Now u turn on the tv the tv pictures only flashes a few sec.than dies,but the tv still have audio sound left only?Yes,mean ur tv have a 1 or 2 long boards that are call a LCD inverter boards are the causes and the problems.These 1 or 2 long LCD inverter boards only job are to provided the tv LCD screen panel for a backlight for the tv to have picture only.Tries websites like Shopjimmy.com,Ebay.com to buy a 1 or 2 the tv long LCD inverter boards for the replacement.

Now,tv turn on u have a picture few sec.than dies have sound left only?Yes,than those 2 tv long boards call the LCD inverter boards are dead.Tries websites like Shopjimmy.com,Ebay.com to buy those 2 refurbishes long LCD inverter boards for the replacement.

Tv turn on now,picture flashes only few sec. just dies and have sound left only?Yes,than a long board call the LCD inverter board is dead.This LCD inverter board help the LCD screen to have a backlight to produced a picture.Tries websites like Shopjimmy.com,Ebay.com to buy a refurbish a long LCD inverter board for the replacement.

More likely to either be the backlight inverter board went out or it is not getting the 24vdc from the power supply, need to chek out the voltage at the inverter fuse.
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It sounds like you have a power supply problem, this powers the inverter! Where you just replaced the inverter, we could assume it is good! Look for some bad caps (capacitors) on the power supply board. They will look bulged on the top an may be a little leaky (brownish stain). The power supply can fail on some output and be good on others. Was the back light inverter new? If all else fails, see if you can return the back light inverter and get another! I hope this is helpful!!

it is your HD box. get another one.

do you get sound while screen does black? what do you mean by "until eventually it has turned black and will not come back"? is it black from the minute you turn the tv on untill you turn it off? if sound is good but picture is black, it may be the resault of a fried monitor tube, in this case replacement will cost alot. if sound is dead also, them it is probobly power supply related issue. get out your DMM and measure the voltages in the power supply. When the screen goes black, is there still regulated B+ going to the circuitry? Do you still have HV? Is the filament of the CRT lit? Does the set still have a raster (you may have temporarily increase the screen setting to see it)? Do you measure any of the flyback derived B+ voltages that go to the video processing, tuner, Vertical Deflection, and other circuitry?