[SOLVED]820-3437 No Backlight

[elcapitan]

Hello everyone, I'm working on a liquid damage macbook air 820-3437b. It got 3 pads (Pin2 NC, Pin3/4 PPHV_S0SW_LCDBKLT) missing on the lcd connector. I already ran a wire from pin 3&4 of the LVDS connector to pin 1 of c8317 but i still don't have a backlight. My output voltage at PPHV_S0SW_LCDBKLT is 8.42v. According to Louis sticky post this might be related to U7701 not boosting the backlight. I proceeded with troubleshooting this area since the problem is most likely on this circuitry. My readings on u7701 are the following: BKL_PWM 1.46v, BKL_EN 1.38v, PP5V_S0 5.09v, LCDBKLT_BOOST 8.55v, PPHVS0SW_LCDBKLT 8.40v. Should my voltages on BKL_PWM & BKL_EN be higher? Do you think my U7701 is no good?

Thanks!

 

[dukefawks]

Don't know where to start here, did you read the sticky on backlight repair?
WHY IS THERE A 2010 LCD CONNECTOR ON THAT BOARD????!!! WTF!

 

[elcapitan]

I never said i didn't know were to start LOOL I just wanted some clarification so I understand. To answer your question yes I read the sticky note. I just had a few questions. I didn't know the 2010 model connector wasn't compatible with this model. I thought they were all the same. Maybe thats my issue. I guess Im better off answerign my own questions. Thanks for the answer lool

 

[dukefawks]

The LCD cable doesn't even properly fit in such a connector, how? They don't even look the same.
1 Connector is wrong
2 BKL_EN is too low
3 LP8550 is probably fucked

 

[larossmann]

BKL_EN has never been lower than 2.7v for me. If you ever want to know what you should expect at a voltage divider, use this calculator. http://www.raltron.com/cust/tools/voltage_divider.asp If you put 8.4v as starting voltage, 300000 for R1, and 100000 for R2, it will show you 2.8v is what to expect. See R7731/R7715 in the schematic? This is a voltage divider.

There is a chance the feedback via is broken between D7701 and ball A5 of LP8550(u7701). Check under LP8550 for what ball A5 for feedback looks like.

Check the sticky for more info and let us know what you find.

 

[elcapitan]

Thanks for the info Louis!! I will install the correct connector and take it from there.

 

[elcapitan]

BKL_EN has never been lower than 2.7v for me. If you ever want to know what you should expect at a voltage divider, use this calculator. http://www.raltron.com/cust/tools/voltage_divider.asp If you put 8.4v as starting voltage, 300000 for R1, and 100000 for R2, it will show you 2.8v is what to expect. See R7731/R7715 in the schematic? This is a voltage divider.

There is a chance the feedback via is broken between D7701 and ball A5 of LP8550(u7701). Check under LP8550 for what ball A5 for feedback looks like.

Check the sticky for more info and let us know what you find.

Im happy to say that I was able to fix this macbook air with no backlight. I had to reinstall the correct LVDS connector and make a jumper from the bottom of the board to the top to fix the blown trace. This was a awesome repair. It taught me a lot of things. Thanks for all the help!!