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820-3435 Orange light, no power after attempted backlight repair

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AaronH

New member
My customer came in with his 11" Air soaked in milk. After drying it would power on with no backlight.

Under the microscope, there was heavy damage and corrosion around U7701. I started with a reflow but the chip was not behaving as expected on the BGA, so I removed it for inspection. Sure enough, both pins on the LCDBKLT_BOOST circuit were blown off.

I replaced the badly damaged U7701 from an 820-3437 donor board. This was the closest donor I had available - was this a mistake? I ran a jumper wire from B1 and B2 to the diode side of L7701 around the other side of the board. Now the board does not power up at all and the magsafe LED remains orange.

I will be home from ERC tomorrow and will be going over this board again - just wondering if my attempted repair could have caused the new problems? The board is heavily damaged from the milk, so it would not be surprising at all if something else crapped out in the meantime.
 
A

AaronH

New member
IS

I've been out of town since last Friday so I was using the past-tense just speaking from memory. I'll update the thread tomorrow with power rail measurements once I'm back in the shop.
 
A

AaronH

New member
I don't see any damage around the diagnostic port but removed it anyway. I've already run it through the sonic cleaner so it's hard to see problem areas. Testing a few power rails, I found the following measurements:

PPBUS_G3H = cycles slowly between 8.59V and 8.39V
PPDCIN_G3H = 14.98V
PP3V42_G3H = 3.421V
PP5V_S0 = 0.228V
PP3v3_S0 = 3.314V
SMC_RESET_L = 3.421V

Magsafe light still orange, fan does not spin
 
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D

dukefawks

Administrator
Well if 3v3_s0 is there then the machine is trying to power on. If 3v3_s0 is there then 5V_s0 should be there too, short on the rail?
 
A

AaronH

New member
Yup - dead short to ground, measuring 0.002V in diode mode on multiple 5V rails so far:

PP5V_S0
PP5V_S4RS3


I pulled the U7701 I had previously installed & jumped, short still exists. Also pulled several caps on each line which were pretty nasty looking. After removing U7701 I'm injecting 5V to the rail to search for the short, it is currently pulling ~850 milliamps.

Edit 2: It was U8080

Thanks for the tips, will update if anything else is weird
 
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