hi, please help me

when i power on, i can see clearly the hp logo then all it's dark.

if i enter into the bios i see it, but very very very dark

if i put an external monitor i can't see the bios, but when windows boot, i can see clearly the screen and use the pc without problem!

another strange thing, soon before windows boot, the arrow point appears for some seconds than disappear

another strange thing: when i disconnect the external monitor, some times the internal monitor power on and i can see clearly!

if i disconnect the battery and the power chord, holding power button and the i reinsert the power chord, i can boot into the bios and see the bios for 2/3 second

i've done a small video, can i upload it on youtube and then put a link on the forum?

may be problem data cabel system board to notebook monitor. open close case and try.

i don't think it is the problem because i've the problem everytime in the same way!

if i disconnect the battery and the power chord, holding power button and the i reinsert the power chord, i can boot into the bios and see the bios for 2/3 second

i've done this test like 10 times, and overtime it has the same behaviour! so i dont' think it's a cable...
Posted after 1 hour 2 minutes 47 seconds:
i've discovered another thing: if i disconnect and reconnect the screen cable to the motherboard i can perfectly see the bios for 3-4 sec, then i've no backlight.

i've try another inverter and another screen: same thing

Did you try to exchange KBC?