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No backlight on LCD - 820-3437 Macbook Air
- Thread starter patrickribbsaeter
- Start date
Voltage on backlight output?
patrickribbsaeter
New member
2 sec im gonna chekc now
patrickribbsaeter
New member
1. No apple logo when adding light to the screen. The screen is pitch black still.
2. I get chime when pushing start button. And the apple logo on the back of the LCD is lit. But not front.
3. L7701 (I have 8,5 Voltage). On Diode mode I have 0.384.
4. C7710 (resistance mode I get 0.428) and Voltage mode 5.12V)
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patrickribbsaeter
New member
NO apple logo at all.. The screen is black.. Im following some examples now from step by step: REWA https://www.youtube.com/watch?v=-7_paJiwMO8
Will reply shortly after my finds.
I have: PPVIN_S0SW_LCDBKLT - R7731: pin1: 5.1v - pin2: 0.6v (Should be: Pin1: 8.5v, Pin2: 2.8v) According to REWA.
I have: PPVIN_S0SW_LCDBKLT - R7715: pin1: 18.5v - pin2: 00.00v (Should be i guess here same: Pin1: 8.5v, Pin2: 2.8v) ???
On: EDP_BKLT_PWM - R7704: I get 1.7v on both pins. (Should be 3.3v on both pins)
What should I do now, change the resistors or could it be a bad U7701..?
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Gmorb
Member
measure those resistors first if they are good then check u7701 on pin 5 sometimes feedback ball will be missing then you'll know you must replace the led driver and run wires. Either way if i were you id inspect the board and replace or reflow and clean anything that looks badly corroded and damaged before you go measuring. try reflowing led driver as well.
patrickribbsaeter
New member
Measure the resistors.. Do i need to take them off the board first? Because ive now measured them whilst on the board with voltage..
patrickribbsaeter
New member
1. Ok, thanks. Should it be diode mode? Resistance mode correct?
No worries. I found it on ifixit how to do it https://www.ifixit.com/Guide/How+To+...r/25632#s65006
2. reflow U7701 and LED driver.. How much temperature?
Ok. So now ive check the resistors: R7731 & R7715
R7731 Shows 120k (should be in schematic 200k), R7715 shows 99.6k (in schematic it says 100K, so that one is ok).
That means R7731 needs to be changed?
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Gmorb
Member
no r7731 is ok if it was higher than the rated resistance then yes it would be bad. make sure you reflow the led driver until you see it move (dance around) add a good amount of flux. i always have my hot air station at 400 degrees and 80 for air. whatever works for you though.
make sure you check if ball on pin 5 is even there.
make sure you check if ball on pin 5 is even there.
patrickribbsaeter
New member
Thanks u for helping me out really.. im very greatful.. Love to fix my first board.. Im going to stay on this thread until we have solved this board.. Will be my first.. hope thats ok with you?
Ive checked pin 5, the ball is still there under when lookin in the microscope. but its hard to measure it, since its under the IC.
1. Ok, i start reflow the LED driver first and then we see next step. back in a bit.. Ps. How do I find the LED DRIVER??
U7701 is the LED driver.. I found this video of louis: https://www.youtube.com/watch?v=xMhu4WyQZAA
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patrickribbsaeter
New member
Ok, so now i have reflow the LED driver(U7701).. Now what I do?
patrickribbsaeter
New member
Ok. will check that now: "make sure r7704 is measuring good too and check voltage on both pins of d7701"
patrickribbsaeter
New member
- R7704 measures correct: 33 ohm
- D7701 measures: 0v on both sides. DMM basically goes down slowly to 0v.. Must be the zener diode 7701 then?
Gmorb
Member
check if you get an image or backlight. if you still dont get an image either an apple logo or flashing folder by flashing light onto the apple logo on the back of the screen then check continuity on pin 18/19 of lvds connector and pin 2 of L8304 and check voltages on pin 18/19 of lvds as well.
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patrickribbsaeter
New member
I checked. Still NO light on screen or image.
1. Could it be the bad Diode(D7701) since its measuring 0v..? Its should measure 0v right, since its a diode..
Im stupid sometimes..2. Could it be the reflow not properly done?
I can feel we are several step closer though to get it working. Just need to lock down where the issue is. The laptop is coming on perfect. Ive tried with two working lcd screens. So its not the screen themself. Its the motherboard somewhere..
patrickribbsaeter
New member
Ok, im going to chekc it now. hold on pls. thx