Unable to fix backlight -macbook pro 2017

[imcool]

This A1708 macbook pro 13 2017 , 820-00840-A has no backlight but image is there,.

I cant find the problem with backlight not working,
Watched youtube videos as well for troubleshooting,

Replaced following parts in backlight circuit
- u8400
- q8400
- D8401
- C8400



still no backlight, only shows image
I have 13V but no voltage on L8410, please check picture below to see measured voltage and resistance



thanks

[mon2]

Re: Unable to fix backlight -macbook pro 2017

Check the voltage on the Backlight ENABLE pin @ R8442. What is the reading?

If that is on, most likely U8400 is defective and needs to be replaced. Wait, you have done this already.

1) Remove the LCD display. Does this allow for the voltage to step up?

2) What is the resistance to ground on L8410 ? Do this with the power off / battery removed.

3) The voltage at the junction of R8401 and R8402 is not correct. This is a voltage divider that when R8402 is GROUNDED, the junction in the middle will LOWER the voltage from the source of 13v. That is, once this voltage divider is ENABLED, only then will Q8400 ENABLE. With the gate @ 13v = source voltage, Q8400 is remaining OFF.

You can use one of the many online resistor voltage divider calculators to calculate the expected true voltage at the GATE (middle junction of this resistor voltage divider).

4) How confident are you that U8400 is soldered correctly ?

[imcool]

Re: Unable to fix backlight -macbook pro 2017

i am pretty confident with my soldering as i repair cellphones(not screen) tristar audio ic and dead issues.
I will check all that you suggested and return the results

thhanks