820-00928-04

[iFix]

Hey Guys.

I have this A1708 no backlight. On Q8400 pin i have 12.6v and pin 3 12.6 also. I checked C8400, R8401(80.6k) and R8402 all good so the voltage divider works good no issue there and when measured direct pin 3 to 4 on Q8400 it's 80k. I have input on U8400 on pin 5 and 18 i don't have output. All resistors around U8400 are good expect R8432 which should be 150k but it's 24.1. I removed it but then when measured directly it's 150k and i get diode value on LCDBKLT_FB. Also i noticed that R8440 which should be 1m it's 0.86 on 1m scale. Ideas?

 

[2informaticos]

Be aware, high value resistors will always indicate lower value measured onboard.
Is normal, because of influence of other components.

You've said no backlight, do you get image at least?
Test external video and USB activity.

 

[iFix]

Yes laptop working fine for a few months after loss of backlight the client had time to come now for repair they were using on external display

 

[2informaticos]

U8400 requires enable signal on pin 17, but backlight is adjusted through I2C bus, connected to LCD assembly.
You must try known good compatible LCD assembly.
Apple got a lot of problems with them on the new machines.

 

[iFix]

Yes many came for a flex repair A1706 and A1708 but this one i already tried with another screen and it's the same

 

[2informaticos]

Check R8450-53.
Post voltage and diode mode at SCL/SDA lines.

Do you get image for sure on internal LCD?
BKLT_EN_R is high?

 

[iFix]

On both line 5v and 0.500 diode mode

 

[iFix]

i replaced U8400 and Q8400 still same

 

[2informaticos]

"Do you get image for sure on internal LCD?
BKLT_EN_R is high?"
???

 

[iFix]

Yes i get image and 3.3v on R8442 and 3.3v on R9826 and R9827

 

[iFix]

I found very low reading on D8410 pin 2 side 0.012 diode mode

 

[2informaticos]

Disconnect LCD and check again.
If goes high, change LCD assembly.
If still low, then should be a filter cap.

 

[iFix]

It was C8471 now testing . Thanks 2informaticos

 

[karate88]

Hi, i have same model 820-00928 with backlight issue. The problem is a bit weird. When you connect the charger it display fine on the screen the battery level but when you power on the device it display on the background but no backlight. I have tried to plug external screen with usb c adapter but no luck. The backlight IC system seems to be working fine gives 51v-54v on D8410 when its on but doesnt want to give backlight image on. i have tried to go on recovery mode but it doesnt seems to be working that either. Any suggestions?

 

[2informaticos]

First of all, welcome to the forum!

Please open new thread in the future.

"The backlight IC system seems to be working fine gives 51v-54v on D8410 when its on but doesnt want to give backlight image on."
Because there is no connection with LED strip.
Check with Apple if machine is covered by "flexgate" program.
Don't touch motherboard, unless LCD connector is bad.
Your problem is on the LCD assembly.

 

[karate88]

First of all, welcome to the forum!

Please open new thread in the future.

"The backlight IC system seems to be working fine gives 51v-54v on D8410 when its on but doesnt want to give backlight image on."
Because there is no connection with LED strip.
Check with Apple if machine is covered by "flexgate" program.
Don't touch motherboard, unless LCD connector is bad.
Your problem is on the LCD assembly.

Could it be that the customer has reduced the light down to 0% ? Customer doesnt remember the password for it and cant access to profile to enable the touch bar to work and try increase the lighting . Is there any other way to confirm if its the screen the issue? Why it display fine the charging ICON when you plug the charging cable and only has dim light when its on?

 

[2informaticos]

Correct backlight voltage is 44V aprox.
Move/rotate the screen when the machine is turned on, the screen will probably flicker.

 

[karate88]

I have attached the photo below. As soon as you lift the LCD you get that small battery icon that displays LCD light fine then when you turn on the MacBook it keeps staying on the backlight problem. If you move it up and down slowly or faster it remains the same. It's an a1707 model 15".

When the MacBook is on the backlight power stays on 54.5volt but when you lift the screen and it displays the battery logo its 44-48 volt

Do you still think it's an LCD issue?

 

[2informaticos]

Backlight voltage goes high because connection with LED strip is lost.
Try another LCD assembly...