820-0165 No backlight

[rsiesta]

Video out to screen, no backlight, fuse was blown I have traced it back to Q7707
and all the pins look correct other then pin 5 EDP_BKLT_EN which is zero,
The schematic says this goes to a pin on the CPU, Could just that pin on
the cpu be damaged as it appears to work fine other then backlight. Or is
there something else that could keep the backlight from being enabled?
I checked the trace from pin 5 on Q7707 to R1470 and it is 2.3 ohms which seems a little
high but there is continuity. I can't see anyway to check the trace to the cpu. Pin 2 on R1470 is also zero.

History it was liquid damage mainly around the lcd connector which was shorted and replaced along with backlight fuse. Have 8.6v at fuse output now.

 

[2informaticos]

U7701 suffers a lot by corrosion...
Do PRAM reset first and check SMC_LID signal.

 

[rsiesta]

Did PRAM reset, no change. I checked SMC_LID and it was 3.4v. I replaced U7701 but still no backlight.
I did confirm that display port is working correctly and was able to boot up and login on external monitor,
I get apple logo and progress bar on lcd shining light through apple logo in back.

 

[2informaticos]

Check continuity between U7701 and D7701.
XW7720 is a known common failure.

Post BKLT_PLT_RST_L and EDP_BKLT_EN.
Check resistors and traces around U7701 and also to R5490.

 

[rsiesta]

BLKT_PLT_RST_L is 3.3v EDP_BKLT_EN is 0v on Q7707

 

[rsiesta]

Anyone else? I don't see anything that could affect EDP_BKLT_EN other then the CPU

 

[2informaticos]

Post resistance to ground for EDP_BKLT_EN.
Make a short 4-5 at Q7706.
Just for test, let see what happens...

 

[rsiesta]

It is 100k which matches what R1430 is to ground. No change shorting 4/5, should
that not energize the lcd backlight? I still only see 8.6v on pins 3/4 of the video connector.

Will I still see boost voltage on those pins with no screen connected?

 

[2informaticos]

LCD need to be connected, of course...

 

[rsiesta]

Ok, so still no backlight, I even tried another
screen. Still getting display, just no backlight. So
I have another problem besides not having the
enable signal? Seems like if I have no enable coming
out of the cpu then no reason to get rest of circuit working
with a bad cpu or is there some feedback circuit that is
telling the cpu not to turn the display on?

Thanks

 

[rsiesta]

Could the reed switch be stuck so the system thinks the lid is always closed? Any idea what that switch is called in boardview?,
Nvmd, it is SMC_LID and it is working correctly.

 

[2informaticos]

"I checked SMC_LID and it was 3.4v"
So is not that...

Remove R7704 and force 3V at BKL_PWM, from BKL_EN.

 

[rsiesta]

Ok, not sure what I did but I have 3.3 on EDP_BKLT_EN now, BKL_PWM and BKL_EN both are around 2.7v
I have 8.6 volts at pin 3/4 so it seems stuff I was missing is there, maybe bad lcd connection... anyway it now
appears I am just not getting boost. 8.6v on D7701

Pulling R7704 didn't change anything and it tested at 33 ohms.

 

[2informaticos]

Check traces for FB and SW from U7701 to L/D7701.
If OK, change U7701 and be sure is properly soldered...

 

[rsiesta]

What is XW7720, I don't see it on my board and I can't find a place to check or continuity between pin 5 FB and the diode on other side of board D7701, 6/7 to LD/7701 seem fine.

Could XW7720 be under D7701?

Ok after a little cleaning I was able to get continuity between D7701 pin 2 and pin 5 on U7701, but I don't see any XW7720

 

[rsiesta]

Ok is working, I scraped around the pin 5 pad it was a little corroded
and cleaned it then put new U7701 in and I now have a screen thanks
for your help.