[solved]820-3209-A no backlight

[maazkamani]

i have a 820-3209 macbook air logic board. liquid damaged... no backlight. did ultrasonic cleaning and measured fuse. fuse is ok. damage was main on LED driver. i was wondering what would be the diagnostic steps to know where the issue might be .

thanks in advance

 

[aprendiz]

It is just no backlight?? do you see any image on display?? what voltage do you have on D9701??

 

[maazkamani]

7.74v. i put flashlight on apple logo on back and see harddrives when i press option

 

[larossmann]

I just made this a sticky.

https://www.rossmanngroup.com/board...acbook-air-no-backlight-troubleshooting-guide

If you can post your bkl_en voltage, bkl_pwm voltage, and tell me if ball a5 for feedback has continuity to backlight output, we can go from there.

 

[maazkamani]

thanks ill look in it

 

[maazkamani]

bkl_en measured on r9715 2.7v

bkl_pwm on r9704 - 3v

from a5 probe to c9796 there is continuity

 

[aprendiz]

Do you have PPBUS_SW_LCDBKLT_PWR? If no Check LCD_BKLT_EN and BKLT_PLT_RST_L......

 

[maazkamani]

3.3 LCDBKLT_PWR
8.36LCD_BKLT_EN
3.3v- BKLT_PLT_RST_L

 

[aprendiz]

but you have 7.4V on D9701??

 

[maazkamani]

yes d9701 = 7.74v

 

[aprendiz]

ok missing LCD_BKLT_EN = LVDS_IG_BKL_ON this signal cames from PCH, measure on diode mode red probe to ground pin 5 from Q9707, this could be a bad PCH....

 

[maazkamani]

diode mode red probe to ground pin 5 from Q9707 = 0.5to 1v in diode mode

 

[aprendiz]

diode mode red probe to ground black probe to pin 5 from Q9707 should give you a value about 0.400 or something like that..... are you sure it is just 1 ??

 

[maazkamani]

0.39 something to 0.5. i was putting the probes the other way. yes 0.39

 

[maazkamani]

r9741 is showing 5 ohms. i was wondering if that has relation to the no backlight or does that relate to something else

also i saw some corrosion on q9707... very minor but now pin 5 is getting 3.3v
PPBUS_SW_LCDBKLT_PWR=8.36v now. dont know what happen but these voltages are coming through. but still no backlight

 

[dukefawks]

If there is 5 ohm over R9741 then LP8550 is probably bad.

 

[larossmann]

Do you have PPBUS_SW_LCDBKLT_PWR? If no Check LCD_BKLT_EN and BKLT_PLT_RST_L......

Let's stop a second. If he has 7.74v on PPVOUT_SW_LCDBKLT, that means Q9706/Q9707 are doing their job. I am guessing 7.74v means his multimeter batteries are dying more than anything else. A bad LED driver is the most likely case here.

 

[larossmann]

duke beat me to it. I went to get my food, I eat, I finish my post and he beat me. that dude is fast!!

 

[maazkamani]

thanks guys ill replace it tomorrow

 

[aprendiz]

Sorry for that, my mistake, this happen when you answer the phone, repair motherboard and answer on forum, all at the same time, even I asked the voltage on D9701 twice.... Anyway, sorry...