a1707 820-00281-a no backlight

[Nicus009]

Hi all,
I have a macbook a1707 with a board 820-00281-a. The screen backlight does not work. I first replaced the capacitor after the d8410 diode. The voltage rose to 10-12v. , the backlight does not shine. There is still no voltage on ISIS_LCDBKLT_P, although there is a voltage of about 11 volts on ISIS_LCDBKLT_N.
Can you tell me how to solve this problem with the backlight.
I am attaching a photo with notes on the voltage at the backlight control area.

 

[Nicus009]

I'll add that I replaced the Q8400, D8410 and Q8401 taken from the donor, I also changed the R8400 from the donor, it didn't help

 

[rjenkinsgb]

It's a boost regulator circuit; IF the CPU is enabling the backlight and the controller IC U8400 is OK, you should see a logic-level PWM drive signal on pin 4 of that, feeding through to the gate of Q8401

If there is no drive signal, either the IC is dead, not being commanded on from the CPU, or is sensing a fault, eg. such as excess current (from the voltage across R8400).

 

[rjenkinsgb]

Have you tried all the keyboard reset sequences for PRAM etc? some corruption in the settings can lock out the backlight control.

How to reset the NVRAM, PRAM, and SMC on any Mac

If you're experiencing weird glitches with your Mac, resetting the NVRAM or SMC might help. Here's how to do it.

www.macworld.com

 

[Nicus009]

If i understand you correctly,i need to check PWM drive signal on pin 4?

 

[Nicus009]

Have you tried all the keyboard reset sequences for PRAM etc? some corruption in the settings can lock out the backlight control.

How to reset the NVRAM, PRAM, and SMC on any Mac

If you're experiencing weird glitches with your Mac, resetting the NVRAM or SMC might help. Here's how to do it.

www.macworld.com

No,if i remember i just try only one combination on keyboard,i'll try this
If it helps,i'll reply
Thanks

 

[rjenkinsgb]

i need to check PWM drive signal on pin 4?

Yes; if that is present but you still only get 11V, something is wrong around Q8401.

If it is not present, the backlight power IC is not enabled, locked out by a fault or failed.

 

[Nicus009]

Yes; if that is present but you still only get 11V, something is wrong around Q8401.

If it is not present, the backlight power IC is not enabled, locked out by a fault or failed.

Ok,i'll check and reply after )