I've been trying to diagnose a problem with mid 2017 A1534, board 820-00687.

Doesn't turn on.

When trying to turn on, LCD backlight comes on for about one second, and nothing happens after that, also there is no image while that happens or after (when shining a light on LCD).

Negotiates and takes 20v properly from charger.

Gives a chime when connecting the charger.

I'm a complete novice in board level troubleshooting, so don't be surprised if I don't know something obvious. I did read up everything I could find.

So far I think I've narrowed it down to something around the L8410 inductor.

Schematic says that after the inductor voltage should be around 45v, while instead it's 8.6 exactly like before it.

I assume something should pulse the input into it to boost the voltage for the backlight, but there is no pulsing. Just stable DC.

Probably that "something" is U8401 which controls the Q8406 MOSFET which should be doing the pulsing.

Are there any preconditions to U8401 initiating the pulsing? Or should I assume that it's dead?

Edit: BKLT_EN_R coming from CPU to U8401 does go to 3.3v and stay there.

As for the differences, I'm using a brd file for 820-00244, so far the only difference I found was at the XW8401 slot, which shows a component in the file but not present on the board.