Macbook Air 2015 no backlight

[vieuxchoub]

Dear friends from badcaps.net!!
May I request some piece of advice for a Macbook Air 2015 820-00165 motherboard, waterdamaged, with no backlight....
I have cleaned the board in the ultrasonic bath, then investigate its electronics. The Macbook boots up, chimes, displays the log in screen and, after cleaning the keyboard is responsive. There is no backlight and its circuit voltage, measured at the connector, diode and output capacitors is 8.20v. The BKL EN resistor R7731 was burnt and desoldered. I have replaced it and the BKL EN line shows 2.8v... but the backlight output is always at 8,20v. The fuse, the diode are good. The vin is at 5.20v. The grounds are good. All the voltages around the backlight IC are good: SCLK, SDA at 3.3v except the PWM which is between 0.9 and 1.5v. I have changed the backlight IC, LP8550, new and original three times with still the same result: no backlight and still, 8.5v on its line....
What can I do more or should I change the board?????
Thanks for your advices!!!

[zpi]

Re: Macbook Air 2015 no backlight

Hello,
Did you cheked LVDS socket?

Also remove LP8550, and check feedback and boost pads continuity to other side of board on Coil. After liquid damage often test points are eaten away for Boost and Feedpack Via's.

[vieuxchoub]

Re: Macbook Air 2015 no backlight

Hi ZPI
I have forgotten to say that I have checked the feedback line continuity which was good... I had 8.2v on the track leading to the A5 pad of the backlight IC when AC and screen plugged in. The LVDS socket was slightly oxydized but not seriously, not burnt....
I have read that the PWM line comes straight from the CPU and is responsible for dimming but also activation of the backlight IC and that its supposed voltage should be 3.3v....

[zpi]

Re: Macbook Air 2015 no backlight

PWM line is signal line, your multimeter cant measure it correctly. But you can short bklt_en with pwm to be sure if you want. But 1.5 on pwm looks ok to me.

You need to check not only Feedback continuity, also check U7701 PIN 6,7 continuity to L7701 PIN 2.

If you have image on screen, it eliminates bad LVDS connector, because if connector was bad, you still would get 30-50v on output.

[piernov]

Re: Macbook Air 2015 no backlight

Also confirm SMC_LID is high, and make sure there are no missing components around LED driver IC.

[vieuxchoub]

Re: Macbook Air 2015 no backlight

OK, thanks for the advices... I will check tomorrow the continuity of U7701 pin 6,7 and coil L7701
May I ask to Piernov, my French colleague, what is SMC LID??? High??? I confirm that there are no missing components, resistors, either capacitors around U7701....

[zpi]

Re: Macbook Air 2015 no backlight

SMC_LID has to be 3.4v, it is magnetic sensor on your magsafe board. When your Lid is closed it pulls down 3.4v to 0v, and backlight disappears. So yes, SMC_LID has to be high, but i think if SMC_LID is low, you should not have BKLT_EN, no ? Or jus PWM will go low?

Just in case, measure smc_lid rail, to be sure.

[vieuxchoub]

Re: Macbook Air 2015 no backlight

Dear friends
SMC_LID is at 3,36v, measured at pin 1 of the keyboard-trackpad connector J4800 when the lid is open, at 0 when the lid is closed... which seems correct
BKLT_EN is at 2,79v measured at the divider resistor R7731 which seems also correct
I have checked the continuity between pads 6,7 of U7701 and pin 2 of the coil L7701. The voltage on the track close to pads 6,7 of U7701 is at 8,5v
But I still have no backlight
Thanks for your answers

[dellxps15]

Re: Macbook Air 2015 no backlight

i had fixed some backlight macbook air, and if i remember good, when u have 8 volt on diode, it mean lp8550 is not working properly.
check A5 pad if it is in good condition

[zpi]

Re: Macbook Air 2015 no backlight

You did not check continuity, you just cheked voltage ? You have to remove lp8550, and check how many ohms from pad 6 and 7 to L7701. Put probe dirrectlt on pad, also check feedback pad to diode in ohms.

[dellxps15]

Re: Macbook Air 2015 no backlight

on that video is well focused on ur problem, and after lp8550 was replaced he replaced the diode.
https://www.youtube.com/watch?v=-7_paJiwMO8

[vieuxchoub]

Re: Macbook Air 2015 no backlight

Hi zpi!!!!
I have scraped the solder mask on the mm closest to pins 6 and 7 of U7701 and my microscope showed that this track is continuous with pin 6 ball, I have checked the continuity with pin 2 of L7701 and got the bip, then I have checked the voltage of pins 6 and 7 , on the copper just close, and measured 8,5v....
This board went first to a professionnal shop in Greece: Computer Systems, in Thessaloniki, they changed the fuse which was good and the U7701 ( with a chip where nothing was written on..) and were unable to make the backlight work again.... They have sent it back unrepaired. Then, I have decided to check by myself. I have found no voltage on BKLT EN which I have fixed but this stupid board refuses always to light up its backilght.....
The diode is definitively good and let the 8,5V go to the led screen...
I have measured pins 3 and 4 of the lvds connector and I get 0.565V....

[vieuxchoub]

Re: Macbook Air 2015 no backlight

In diode mode......

[zpi]

Re: Macbook Air 2015 no backlight

Have you cheked continuity from pin 6,7 (or scrapped off via near) to L7701?

[vieuxchoub]

Re: Macbook Air 2015 no backlight

Yes, dear zpi friend, I have checked the continuity between the copper near pins 6 and 7 of U7701and L7701 and it bipped very clearly
I will remove my last soldered U7701 and check again . I will resolder a new LP8550, just in case

[vieuxchoub]

Re: Macbook Air 2015 no backlight

Good morning, friends!!!
I have removed U7701 and the pads either A5 of feedback and A6, A7 of SW look good and are in continuity with L7701, on the other side of the board.... I have resoldered a new U7701 with no luck..... I had my Quick hot air set at 420°C (blowing for a short time). I will try another time another chip with my Quick hot air at a lower temperature, 350°C, just in case that a too high temp could spoil the chip.....
Any other idea????

[zpi]

Re: Macbook Air 2015 no backlight

R7753 and R7757 check voltages on pins2.
Also next time checking continuity, better do in in ohms mode, then you can see even if continuity is not very good (like 5-10ohms).

[vieuxchoub]

Re: Macbook Air 2015 no backlight

R7757 and R7753 have 3.3V on each pins. They are 0 ohms resistors in good continuity on my board...
Thanks for your advices

[vieuxchoub]

Re: Macbook Air 2015 no backlight

Could it be that my LP8550 ICs are defective???? I have changed it again this morning twice, at 280°C, with a very good look of the bga balls soldering under the microscope....
I rechecked the continuity of the SW and FB lines, with voltages of 8.3v of the FB line and 8.5v of the SW line.....
I do not understand what could be the problem...

[mon2]

Re: Macbook Air 2015 no backlight

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