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Model A1502 / 2.6 GHz (Turbo Boost up to 3.1 GHz) or 2.8 GHz (Turbo Boost up to 3.3 GHz) dual-core Intel Core i7 processor with 4MB shared L3 cache.

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820-3476 No Backlight but image is there

Answering the usual questions:

Do you have 2.7v to 3v enable signal on pin 17 of U7700? YES 3.3v

Do you have 5v on pins 5 and 18 of U7700? >> YES

R7733 0 ohms? >>>> YES

What voltage do you have on backlight output, pin 1 of LCD connector? 0v? 12v? 20v? 27v? >>> 0v

crwdns2934089:0crwdne2934089:0 crwdns2934093:0crwdne2934093:0

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crwdns2853045:0crwdne2853045:0 crwdns2853046:0crwdne2853046:0
crwdns2889612:0crwdne2889612:0 1
crwdns2944067:018crwdne2944067:0

Pins 3 and 4 of Q7700 are the same voltage, 12.6V so all voltages after Q7700 are 0V.

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@pollywaffle did you rule out bad screen first?

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I actually plugged in a same year 15 inch retina lcd and it behaved the same, image viewable with a light behind the apple, but no backlight.

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sorry , i cant help you with your problem, but i have a similar problem with Intel Core 2 Duo 2008 and maybe my problems can helping you solving yours :).

I replaced all the i wrote that can help fix the problem (inverter cable, inverter board, logic board), but its still not working. when the screen turns black I can use the dim key back to zero and then again forward and sometime it works and the screen light up again, but not everytime, sometimes it only flare up a bit and then turn off again

crwdns2934271:0crwdnd2934271:0crwdne2934271:0

What about a PPVIN_S0SW_LCDBKLT shorted to ground?

This would lead to a high current through R7700, detected by both U7700 and U5450.

Can you run the AHT and see if U5450 raises an error?

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crwdns2931211:0crwdne2931211:0 crwdns2931215:0crwdne2931215:0 crwdns2934225:0crwdne2934225:0
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Check voltage on pin 2 of pull up resistors for i2c_bklt_sda and i2c_bklt_scl. Should both be 5v. If not 5v, test resistance of pull ups 2.4Kohms. If those readings are good, check the trace continuity from pull up resistors to connector.

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crwdns2853045:0crwdne2853045:0 crwdns2853046:0crwdne2853046:0
crwdns2889612:0crwdne2889612:0 2
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Bill crwdns2934231:0crwdne2934231:0
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crwdns2936751:024crwdne2936751:0 0

crwdns2936753:07crwdne2936753:0 0

crwdns2936753:030crwdne2936753:0 1

crwdns2942667:0crwdne2942667:0 1,217