820-3476 No backlight, missing BKLT_EN_R

[nickalith]

Hello!

Somewhat new to this and just trying to get some direction.

MacBook turns on, can use with external monitor, confirmed no backlight/can see an image when I use a flashlight.

I have 0V on PPVOUT_S0_LCDBKLT

Missing the enable signal for U7700. I checked where it gets the signal from and it looks like it's an OR gate, and needs either: EDP_BKLT_EN or LCD_PSR_EN to generate the enable. Both are missing. Both signals come from the SMC (I think). Anything I can do?

 

[2informaticos]

First of all, welcome to the forum!

Check diode mode to ground at PPVOUT_S0_LCDBKLT , w/ and w/o LCD connected.

If image is present, display is correctly recognized and EDP_PANEL_PWR is present.
The other two input signals of U8330 come from CPU.
You should check their level in the first seconds (for pulse at least), when hear the chime.

 

[nickalith]

Thanks!

Diode mode showed an Open Line for PPVOUT_S0_LCDBKLT when the connector was connected and disconnected (tested with ONLY the connector inserted, no other cables or power was connected to the board when I tested). I should have mentioned I tested the same thing earlier with resistance mode and confirmed it was not a bad cable/display.

EDP_BKLT_EN and LCD_PSR_EN both pulse at about 1 or 2 mV very briefly when I supply power before going down to 0V (had only the power supply connected, nothing else) You mention that I should check for the pulse when I hear the chime, I feel like that entails that I need to have more than just the power supply connected (like the speakers, at least) for this particular test, but I'm not sure so I figured I'd include that information

 

[2informaticos]

No need to connect speakers.
Was only a time pointer; need to check in the first 5-10s.
Of course, internal LCD MUST be connected.
Do not waste time searching for mV pulse; there is no pulse if you cannot catch more than 1V peak.
An o-scope can show you peak value for any fast pulse.

 

[nickalith]

After plugging in the display, I actually AM getting my 3.42V on EDP_BKLT_EN. U7700 has its enable on pin 17, and is getting its required 5V on both pins 5 and 18. However, I am still getting 12V to Q7700 on the LCDBKLT_EN_L. Replace U7700?

 

[2informaticos]

So your first post was without LCD connected???
Very strange, as you said to have image on internal LCD; at least is what I understood.

Enable signal is not sufficient.
Backlight level is controlled through I2C bus.
Check R7750-53 resistors.
Also post diode mode and voltage on SCL/SDA lines.

I recommend you to test known good LCD assembly: as I2C bus comes from display.

 

[nickalith]

Yeah I explained it in a confusing way. I tested the computer for no backlight with everything connected, and confirmed the issue was no backlight. Then I took the board out of the macbook and started testing, not realizing the display needed to be connected (cuz I'm a dummy). Then I connected the display and found out I was getting the enable. I don't have an extra LCD assembly, if we can confirm its a bad LCD I'll probably just move on, no biggie.

R7750 = 0.2 Ohms
R7751 = 0.2 Ohms
R7752 = 0.801K
R7753 = 0.801K

both R7752/3 are supposed to be 2.4K per the schematic?

Diode Mode (no power to board while testing these)
BKLT_SCL = 1.63V
BKLT_SDA = 1.63V

Voltages
BKLT_SCL = 5.039V
BKLT_SDA = 5.039V

 

[2informaticos]

The resistors usually show lower values when measured onboard.

Check always diode mode with red probe at ground.
And forget the "V" when post the values.

Do PRAM reset 2-3 times, just in case.
A known good LCD assembly is the best test.
You can waste time in a board without problems, just bad LCD.