Hello everyone,
My laptop (NP515-51-86BZ) can be turned on, but sometimes can be turned on with the keyboard light up and black screen (the external keyboard can find that the Caps Lock light is not on. Confirmed that it is not the problem with the dispaly.) I use the adjustable current power suppy to find that it should take at least 1.5A to light up the screen normally, when it runs to about 1.0A at the dark display status.
Sometimes it would turned off and restart to the black screen with keyboard light up automatically during the boot process (the condition that can light up the screen) or a few seconds or dozens of seconds after access to Windows system. Then I can start and light the screen again after a long forced shutdown (pressing the switch for three seconds) in the black screen condition, or it may have to wait a few days to start and light the screen again. It can reboot to enter the black screen status with keyboard light up any time. It is quite irregular and random.