Samsung UN65J620DAFXZA LED tv - the screen goes very dark shortly after turning on.

SOURCE: LED POWER ONLY LIFE, SCREEN DISPLAY DARKNESS

theres only one solution to the problem you have to replace the whole power supply board .its easier to replace them than finding and soldiering the defective parts like the capacitor on the board .u can just replace the power supply board if u think the power has a problem. if it doesnt work then u have to replace also the main board or the board connecting the rgb cable.its more expensive to replace the board but its worth it. ur lcd will work better and its easy to put them on the lcd casing .

SOURCE: turn on TV, screen dark at first then vertical

i think you should buy a new vertical IC and replace the old one.

SOURCE: Toashiba 1005-S157 won't turn on.

remove the batteries for 2-3minuty, check if it works,
Enter the BIOS, set the boot order of devices on the 1-HDD, 2-CDROM

SOURCE: screen goes dark after a few minutes of tv on

It has bad lcd inverter and Westinghouse don't sell the parts either.

SOURCE: New Samsung LCD TV won't turn on or off

This is the best solution for your problem as a 40 year experience with wires etc. Go to the source of your cable signal connector . Open it and clean the terminals with emery paper , sand the end of the cable shiny clean from corrosion , get a new barrel connectors with gold terminals if possible lengthen the ends of the cable , insert and tighten good, use thick rg-16 or rg 40 cable with good insulation ends crimped professionally with a crimpier or buy ready made , not from dollar store. Get a gold plated professional splitter if you need one or two , it will cost about 15 or more each . Make the cable runs as short as possible, using the thick good cables. If needed get a signal magnifier. From radio shack or the source etc. Connect them good and tight . Ground the splitters . Problem solved , you will get crystal clear channel 2 to 7 including 3 and 4 . Isolate the TV cable from other cables and power wires. Vitally