820-3787 Green Light No Power

[hambook]

I have an A1398 820-3787 board that came in not turning on. It pulls 91mA from the power supply and the green light comes on but nothing else. No signs of any huge liquid damage on the board, all the indicators are still white and I can't see any corrosion or obvious spots under the scope. Very new to this, any idea where to start on this one?

 

[2informaticos]

First of all, welcome to the forum!

Do you get 3V42_G3H, PPBUS_G3H and 3V3_S5/SUS?

 

[hambook]

Thanks!

I don't get 3V42_G3H at all, and just the smallest trickle (meter shows between 0.001-0.004v) on PPBUS_G3H, 3V3_SUS and 3V3_S5

 

[2informaticos]

Very strange to get Magsafe light, without 3V42_G3H.

Are you using original 85W Magsafe 2 charger?
This board will not turn on with 45/60W charger, unless battery is also connected (and holds some charge)

 

[hambook]

That's actually a good question, this cord end may only be from a 60W now that I think about it. I'll pick up an original 85W charger today and report back!

 

[hambook]

Just got back from the ol' Best Buy with a genuine Magsafe 2 85W, but even with the proper charger it still pulls the same amps and same measurements on those lines. Seems very wacky.

 

[2informaticos]

Post Q7010 voltages.

 

[hambook]

Measured from surrounding components
Pin 1/2/3 18.46v
Pin 4 6.76v
Pin 5/6 18.46v

 

[2informaticos]

I was thinking about leakage issue, is not the case.
Post now U7090 voltages.
Also basic voltages for U7100; pins 1, 2, 3, 9, 12, 13, 14, 15, 17, 19, 20, 27.

 

[hambook]

Here are those measurements:
U7090
1 1.24v
2 3.38v
3 7.03v
4 3.39v
6 17.86v
8 17.89v

U7100
1 0.004v
2 18.22v
3 4.35v
9 0.02v
12 3.32v
13 3.29v
14 0.02v
15 0.14v
17 1.94v
19 1.58v
20 3.44v
27 17.78v

 

[2informaticos]

"I don't get 3V42_G3H at all"
Despite of what you've said before, looks like 3V42_G3H is present at L7095.
"U7090
-----
2 3.38v"

"U7100
-----------
19 1.58v
20 3.44v"
Here is the problem.
Post diode mode to ground on both pins.
If low, check C7100/01 first; you may need to change U7100.
Be sure R7101 is still good.

 

[hambook]

Both were very low, the caps and were good but looking closer at U7100 revealed some nastiness under the chip. Replaced that and all is well! Thanks so much for the advice!