JESSA IS ALWAYS WRONG! 2012 13" Air - No Backlight

[wrbailey]

2012 13" Liquid damaged Air 820-3209-A

Has an image. No backlight. Went over Rossmann's no backlight sticky...

I have 8.4 volts at left side of L9701 and 0.033V on the boosting side.

I did have an F'd up LED driver. Half the pad was still there, but the connection between the pad and the trace was gone. I ran a TINY jumper to the pad and it looks perfect.

Still turns on and no backlight. I checked for continuity on all the broken trace suggestions in the sticky and I don't know where to go from here.

R9731 is fine and I do get 2.9v at BKL_EN.
Backlight fuse is fine (unfortunately)
Connector doesn't look damaged at all, and I've tried known (cracked) good working LCDs (at least my backlight test screens.)
I have 5v at PP5V_S0 and 3.3v to the LED Drvr

any help? I tried two different LED drivers due to swapping out to a smaller jumper to FB ball. No change.

 

[rvanpruissen]

What's the voltage drop to ground on backlight output (diode measurement, red probe on ground)?
Also, check again for continuity between pin 1 of L9701 and the connector.

 

[wrbailey]

If I recall correctly I did check the voltage drop to ground on FB (which is connected to output) and it was 0.583

I will check for continuity between L9701 and connector this afternoon.

 

[wrbailey]

Just to confirm, I do have 0.589 diode mode from backlight output @ C9797.
I also tested for continuity to pins3/4 on LCD connector from backlight output and the continuity is there.

 

[dukefawks]

I have 8.4 volts at left side of L9701 and 0.033V on the boosting side.

That just makes no sense coils like that have near 0 resistance. It would suggest an open coil but more likely bad measurements.
You should have checked the feedback trace A5 when the LP8550 was off.

 

[wrbailey]

I remeasured... again, I get 8.4v on "left side" pin 1 and 0.05v on right side of circuit, pin 2 of L9701

I have no problem removing the LP8550 again to take measurements.

 

[wrbailey]

HAHA!!

Here is one for Rossmann!!! IT WAS THE COIL!!

OL on the bad one I removed and 0 ohms on the donor board... and the backlight comes on!!

 

[wrbailey]

All tested and all working. You can mark this as complete/solved.

Thank you!