820-4924 No Backlight

[StraightToThePoint]

Hello, 820-4924 with some water damage on the LVDS connector, replaced and still no backlight, pin 1 was frayed and bent, so I replaced it. That is the only known history on this board. Backlight circuit looks untouched.

Tested with a 100% known working display, and the display had the bad LVDS connector, both show same symptoms.

Display is getting an image, tested with flashlight behind, flashing question mark folder shows.

I get 0V on pin 1 of the LVDS connector, no short to ground on anything as far as I can tell. F7700 is good.

Q7700 has proper 12.6V on pin 4, pin 3 always has a little bit less voltage (around 12.0-12.1V). Pins 1,2,5,6 sometimes have voltage, sometimes dont, dont know if this is my shitty multimeter however im noticing Q7700 does open but voltage will drop from around 6-8v to 0v slowly (visibly decreases on multimeter) never seen this so not too sure what it means. When I say slowly, I mean it fully decreases in no more than 15-20 seconds.

LCDBKLT_EN_L is not being pulled down, otherside (BKLT_SD) is also 12V.

U7700 is getting 5V on pin 5.

SMC_LID is 3.3V

EDP_BKLT_PWM is 3.3V

R7700 and U7700 have continuity on pins 9 and 10 with 3 and 4.

Stuck on this one, anymore info or needed voltages please let me know!

edit: Also to clarify, 100% known these are both 2015 screens

 

[2informaticos]

Apart of BKLT_EN_R, U7700 requires correct communication through I2C bus with LCD screen.
PCH sends the PWM signal to LCD assembly being encoded into T-CONN, going then to U7700.
Check R7750-53 and diode mode to ground on SCL/SDA lines.

Be sure D7710 is not shorted (A-K) and has good continuity until J8300.
Check R7731/2 and traces.

 

[StraightToThePoint]

Diode mode to ground on SCL/SDA lines, red probe to ground.
R7750 - 0.530 (Measured again, goes to 0.526)
R7751 - 0.526
R7752 - 0.526
R7753 - 0.526

D7710 is not shorted, diode mode to ground reads 1.155 (pin 2), and 0.848 (pin 1). D7710 has continuity to J8300.

Feedback trace and R7731 and R7730 look good, no scratches, or water damage, or anything out of the ordinary.

R7731 reads 13.30 K Ohms
R7730 reads 149.2 K Ohms

These readings seem correct as im comparing them with a good board I have here.

Going to check continuity with I2C lines to display.

I2C lines all have continuity throughout and going to J8300. Going to boot to OS to see if communication is happening with brightness keys.

edit: Could this be a bad U7700? Also with further inspection, top left of back of the board was hit with water damage, U3210 has 2 pins that were corroded, but not too bad, could one of those capacitors (C8358-59) possibly be causing communication issues if they got hit with water? No corrosion on them, they look fine though. edit 2: TP for BKLT_SDA looks like it possibly had some corrosion, or something is it, very very small though, doesnt affect continuity however, or any reading, just throwing this out there though.

 

[2informaticos]

I meant to check R7750-53 values in ohm; not diode mode to ground.

Should be good to change U7700; just in case, nothing to lose.
It should pull down BKLT_SD, when has correct communication with T-CONN.

 

[StraightToThePoint]

R7750 - 0.5 Ohm
R7751 - 0.5 Ohm
R7752 - 1.5 K Ohm
R7753 - 1.6 K Ohm

These seem normal?

When I booted to OS X, Ive noticed very weird symptoms. It also told me how old and used the machine is, battery has 1200+ cycles lol.

OS X Registers brightness keys, and keyboard backlight brightness keys, Keyboard backlight does work, HOWEVER, when tilting the display 70 degrees or higher, brightness goes away, and keys do not register anymore (Circle sign) When putting it below 70 degrees it returns to normal. Display backlight is always out though.

I thought the keyboard backlight connector was caught on a hinge somehow, or connection was weird, it wasnt. Keyboard backlight connector is completely normal, no tear, no rip, solid connection and only does it when the display is open higher than 70 degrees. Is this an ALS issue or something? Could this be related to the issues im having? Very wierd. System preferences and information both recognize the keyboard backlight and display perfectly fine too.

Im going to replace U7700 as these issues are very weird, and im suspect of this chip as everything else is perfectly normal.

Also last question, would a bad inductor (possibly L7710) cause any issues like this?

 

[2informaticos]

I never heard about backlight inductor failure.
Doesn't mean not possible.

You have flexgate problem.
Not only backlight flex can suffer of this problem.
Seems that (at least) camera flex is damaged in this case; ALS connection is affected.

 

[StraightToThePoint]

I wonder if display is damaged internally then too? Very strange. This has basically been a very weird rabbit hole. I replaced U7700 and still same issues.

Any other advice?

Backlight flex (Do you mean keyboard backlight cable) was not being bent however, in anyway. This is an A1502, I know 2016's and up suffer the same problem with display, however that is connected to the hinge, the keyboard backlight flex is not, only connected to the backlight itself, not flexing when the display is open or closed.

 

[2informaticos]

Yeah, webcam is connected with wires here, as video too (no flex).
In fact, you've said have tested board with two good screens; shouldn't be video, or webcam cables issue.

Tried classic 2-3 times PRAM reset?
Be sure any trace and component in the area is good.

Check if EDP_BKLT_PWM reacts when pressing backlight adjust keys.
An o-scope will help you to detect reaction on the I2C_BKLT bus at same time.