[SOLVED]820-3437 no image / no backlight

ytesfay80

Member
This board was sent to me as no backlight after liquid damage. For some reason I didn't even check to make sure there was an image before replacing everything. I replaced that entire area around the lcd connector and the connector as well as the lp8550 and the LVDS cable.

I am getting 0v on backlight while U8300 has around 3.3v on pins 1-5. Here are some other measurements I got.
J8300:
Pin 3/4 - 0v
Pin 15 - 2.67v
Pin 16 - 3.28v
Pin 18/19 - 3.3v
I get 2.5v on BLK_PWM but 0v on BKL_EN
Diode Mode on Backlight output caps - 0.59v

For some reason when I turn the machine on I can see the LCD slightly changing colors as if the screen turned on and I can see a faint color change when I turn it off but I don't see any image on the screen.
I don't have another display to test with but I plugged in an 11" lcd only with another lvds cable and I was able to see an image on the screen either when putting a flashlight to it.
 

ytesfay80

Member
Okay I managed to get a known working display assembly to test with and the same thing happened. I can see a slight change in the darkness of the screen but no backlight comes on and I dont see an image when I put a flashlight on the apple logo.

Could this be an issue with the LED Driver because before I put another one on I accidentally ripped the pad for BKL_PWM but I ran a wire for it and BKL_PWM is 2.5v.
What would cause no image even though the lcd is recognized and getting voltage at the connector?

Update: I took a look at the led driver at an angle and I saw that the balls for BKL_ISEN1 and BKL_ISEN2 were melted together. I think the reason is because when I ripped the pad for BKL_PWM I scratched a little too deep so when I put the new driver on there was a slope making it hard for the chip to sit correct.

I removed U7701 and powered on the board with a 11" lcd only and I still was not able to see any image but again I got 3.3v on both sides of L8304.
 
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ytesfay80

Member
Okay I managed to get a known working display assembly to test with and the same thing happened. I can see a slight change in the darkness of the screen but no backlight comes on and I dont see an image when I put a flashlight on the apple logo.

No question mark folder ever appeared and I already tried replacing the connector on the board but still the same thing happens.
I tried again and waited over a minute and no question mark folder appeared. The cpu gets warm and I get a chime.
 
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dukefawks

Administrator
Check for missing caps in the eDP lines to LCD connector. Also check soldering of the connector and check continuity to the end of a separate LCD cable.
 

ytesfay80

Member
wow I don't know how this happened but C8320/21 were missing from the board. I put them on from a donor board and put a new LP8550 on and now everything is working fine.

Thanks a lot duke.
 
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