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820-3115 No Backlight Killing Me

felixthecat

felixthecat

New member
Awesome work guys, just needed to say that. I usually find everything I need in this forum for my answers to the things I get stuck on. For the past two days I have been following all the 820-3115 No Back Light posts, and up until tonight, I'm at a dead end.

All voltages are correct, no break in any traces, parts all mentioned in other posts replaced, last part replaced tonight was SMC_LID which showed high so changed out the resistor and it now shows low on open lid. For the LIFE of me, I cannot figure out what I am missing, and I KNOW it's something stupid retarded, so I am finally reaching out for some help.

Board powers on on impact of magsafe green light, orange light, chimes, see faint image, boots... But I cannot for the life of me get the backlight to turn on. If any of you have any ideas on what on earth I am missing, please by all means see what I am missing. I built this board from the ground up with no green light, nothing and this is my last snag, the fun part was running a wire through the board for the RTC, if I can manage to find THAT what on earth am I missing on the backlight :(

**Forgot to mention I have tried the board in two different chassis....
 
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felixthecat

felixthecat

New member
BKL_EN is 0.11v
BKL_PWM is 3.3v
Pins 2,3,4 on LCD connector 3.3v
Pins 21,22 on LCD connector 0v and pins on both caps are going to ground
 
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felixthecat

felixthecat

New member
dukefawks, I greatly appreciate your time, I found out what is wrong, I need to re-do the solder mask for the LP8550, it didn't look to bad the first go around but I pulled it again and yeah, you can't make out where the pads go. So I have to make a blackout stencil and uv some solder masking so the balls don't decide to merge over.

You all keep up the great work. Inspiration to all of us in the repair business.
 
felixthecat

felixthecat

New member
Ok, this board is really upsetting me, remasked the area, put a new LP8550 and short to ground is gone from all BKL points, now Pin 21,22 on LCD connector is giving me 2.4v as well as Pin 1 on D9701
 
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Alan.L

Alan.L

Member
can you check for short on C9782, just had a 3115 fixed for no backlight, fused does give 12.5v but C9782 was shorted when i measured both side of this cap. good idea to swap Q9706 if that cap is shorted.
 
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Alan.L

Alan.L

Member
something wrong there. Pin 4 should have the same voltage as F9700 fuse.

#edit

btw, how did you measure C9782?
 
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J

jadao

Member
Pin 1,2,5,6 are tied together they should measure the same, please check again voltage.
Pin 4 is PPBUS_G3H witch you measured good at the fuse before.
Look if you don't have now a short on PPBUS_G3H because of the soldering.

You need to have ppbus_g3h 12.6V at pin 4 and Q9706 must open to let 12,6V appear on pins 1-2-5-6.
To have Q9706 open, Voltage divider must operate, this means pin 3 must be lower than pin 4.
​​​​​​
 
felixthecat

felixthecat

New member
jadao said:
Pin 1,2,5,6 are tied together they should measure the same, please check again voltage.
Pin 4 is PPBUS_G3H witch you measured good at the fuse before.
Look if you don't have now a short on PPBUS_G3H because of the soldering.

You need to have ppbus_g3h 12.6V at pin 4 and Q9706 must open to let 12,6V appear on pins 1-2-5-6.
To have Q9706 open, Voltage divider must operate, this means pin 3 must be lower than pin 4.
​​​​​​
Click to expand...


Let me check because I just realized I am so fed up with this board I was measuring U3990 UGH... Why isn't there a head bashing wall emoji! Let me recheck....
 
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