iPhone 6s No Image (no water damage) Display completely Black

Hi Guys..

I m here again for expert advises and for learn reparing :)

My client sent me iphone 6s wth no image.

The phone is second hand, so they dont tell me anything about phone history.

I check with new display but the screen remain black, itunes recognize phone.

I search on web and found this video

https://www.youtube.com/watch?v=dYbW35dl...

Try to putout C4200.

fortunately the first component (c4200) is shortage and i solve the problem.

Obviously you are thinking, if everything is ok, why i m here, :)))

I m here because i want to learn and understand how find the problem on the phone and not just solve the problem searching in net or putting out every component.

My question is :

In this case, which is the first step.

Before searching on the web i check if the backlight is present,

i check all backlight filter, check FPC connector if damage.

MY proper question is this line DISPLAY POWER PP5V7_LCM_AVDDH must have 5 volt???

so i check a phone who run perfectly (without lcd / touch panel)

If there is no problem in the phone, i find 5 volt here (c4200)?

Which is the right way to proceed in this case, if i have only schematic and not INTERNET :) Thanks a lot for attention.