Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

[xalu]

Macbook A1278 2011 MC700LL/a 820-2936

No backlight. There was liquid damage on the bottom of the board. L3895 was getting around 4-6 volts.

On the top of the board C3890 was getting the same voltage so I decided to pass 12v from PPBUS_G3H (off C3880). When I did this U3890 seemed to come to life and produced 15v. L3895 went up to 12.5V.

My question is this, I am still not getting backlight. I have BLK_EN (3.04V) and BLK_PWM at U9701. I am thinking maybe L3895 has gone bad?

Anyone else have any ideas? Anywhere else to check.


Thanks for any tip!

[dellxps15]

Re: Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

voltage on f9700 ?

l3895 is pch related..

[xalu]

Re: Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

F9700 = 12.55V

Do you know what voltage L3895 should be?

[dellxps15]

Re: Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

l3895: 8-13V Input .

try a soft reflow on u9701

edit: voltage on Q3880 ?

[xalu]

Re: Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

l3895: 8-13V Input .

try a soft reflow on u9701

edit: voltage on Q3880 ?

I tried a reflow by heating it up with some flux. No change.

I can't measure Q3880 directly since it's covered with the clear sealant that looks like hot glue. If I measure C3890 I get 12V
C3892 = 5.1v
R3891 = 12v
C3880 = 12v

[xalu]

Re: Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

If I remove the jumper I added from c3880 to c3890 then everything drops way down.

C3890 = 4.6v (crawls up to 5v
R3891 = 4.6v
C3892 = 3.3v
l3895= 4.5v

Correction::
C3890 = 0.46v (crawls up to 5v
R3891 = 0.46v
C3892 = 0.33v
l3895= 0.45v

[dellxps15]

Re: Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

voltage are ok to me.

focus on page 77 of schematic for backlight problem.

if u can post lvds voltage at backlight part pin 21-22.

[xalu]

Re: Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

12V at 21-22

Should I keep the jumper on c3880 to c3890, or trace the cause of this issue? I couldn't figure out what was a bad connection which is why I added the jumper.

I'll focus on page 77 like you said. It seems to me that the issue is that I am getting 12v when I should be getting 50v (places like C9799 I get 12v).

What converts the 12V to 50V? Is it u9701?

[dellxps15]

Re: Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

u are killink the board and lcd with these solder wiring.if i think so, u sent 12volt instead of 3.3 volt

50 volt ? these are not lcd but led display. and 50v never seen to a little small lcd. lcd work with small voltage. for backlight there is the inverter for high voltage , but these are led display.

ok ive seen the schematic and it say 50volt

[xalu]

Re: Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

I am providing C3890 with the voltage is says it requires? Look at page 36 PPVIN_SW_T29BST (Under page notes it says 8-13V). Are you sure it is suppose to get 3.3V?

After I provided it with 12v is when it worked...thats when I got:
C3890 = 12V
C3892 = 5.1v
R3891 = 12v
C3880 = 12v
and I was getting 15V at pp15V_T29

As for the 50V. I wasn't sure. On the schematic at L9701 it says VOLTAGE=50V. Also at PPVOUT_SW_LCDBKLT it says Voltage =50v....it's suppose to be 12v? Is the schematic just wrong then? Or am I reading it wrong?

I am not seeing any sign of overvoltage anywhere.

[dellxps15]

Re: Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

try another u9701

[TheSkualo75]

Re: Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

Hi Dellxps15,
i wrote from italy. I have same problem.
ppbusg3h after fuse is 11.96
bkl_en =2.9
lvds 21/22 to gnd =19.6
..my screw driver falling off my hands.Laptop was off, but the battery was connnecteed..(i forgot).........and backlight doesn't work.
what can i check?
thanks

[highpulse]

Re: Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

i there , 50 volts is the max voltage for the led lcd , meanwhile on the d9701 , pin 1 you should have the system voltage 8v to 14.5 depending on wich model . pin2 of that diode is the FB from the pump led charger driver , usually its a small bga or a normal QFN depending once again on wich model , usually this led driver broke or get damage due to sensivity or usuallywaterdamage issue forcing led driver to overcurrent and blow .
normal test , chek always CRITICAL parts , for EX main P fet to generate PPBUS_SW_LCDBKLT_PWR , chek carefully inpendance value on all sides , source drain and gate , and therefore also chek directly d9701 both sides to gnd , if impedance is high , then no short is found , and problem is on led driver . if no voltage from PPBUS_SW_LCDBKLT_PWR , then chek system fuse and PPBUS_S0_LCDBKLT_FUSED.

[techman1025]

Re: Macbook A1278 2011 MC700LL/a 820-2936 no Backlight issue

Hi
Do BOTH SW pads on the board have continuity to L9701?

Does A5 pad have continuity to LCD connector pin 21/22 and pin 2 to D9701?

Is 3v on BKL_EN?

Is backlight output a blink of 12-20v that goes down to 12v steady, or does it go down to 0.1 or 1v after a while?
Techman1025