820-00045 not turning on

[[email protected]]

the little led lamp on the board lights up but and that's it, no image no sound.

 

[JohnB8812]

Any history liquid etc? PP3v3R3v0_AON present?

 

[[email protected]]

no liquid damage nothing it was updating OSX all of the sudden turned off and does nothing,
yes I get 3.32v on PP3v3R3v0_AON.
I thought that if the led lamp is on means that the board is powered on,

 

[2informaticos]

Post voltage for R7120 and F7140.

 

[[email protected]]

R7120 14.76v
F7140 8.59v

 

[2informaticos]

Do you get any S3/S0 voltage?

 

[[email protected]]

PP3V3_S5 changes rapidly between 1 volt and higher and 2 volt and higher. it's not steady even for a second
and the resistant to the ground on PP5V_S0 is only 5.3 ohms and in diode mode it's 0.008.

 

[JohnB8812]

So PP5v_S0 is definitely shorted then. Inject voltage and see what heats up.

 

[2informaticos]

Injection method should be good to find the culprit.
1A will give you 5W heating elemnt...

 

[[email protected]]

Thanks a lot for your replay. is the pin 1 of C7552 a good spot to inject voltage?
I put the current at max and begin with 0.5 volt. is this the correct way?

 

[2informaticos]

You can use any point connected in that power rail; usually the bigger pad you can find.
And yes, the method is correct; start with lower voltage and set amps to max.
In your case, is not a pure short, so even 5V is not too much.

In fact, when the lab power supply indicates overcurrent, then you can lower the voltage and try again.
This is a general rule for current injection...

 

[[email protected]]

I found it. The L7520 has gone bad. I'm wondering if I could use L7820 of a 820-3437 board?

The specification of L7520 is: 2.2UH-20%4.8A-0.105OHM

The specification of L7820 is : 2.2UH-20%-2.0A-0.10OHM 2.2UH-20%-2.0A-0.108OHM

 

[2informaticos]

What is the problem with L7520?
It cannot make short to ground...

 

[[email protected]]

when I inject voltage to the PP5V_S0 it gets warm and when I removed it the short is gone ( according to the multi meter)

 

[bjf]

Did you test both pins 1 and 2 of L7520 for a short after you removed it? It's just a pathway.

 

[2informaticos]

If you really injected in PP5V_S0 (pin1/L7520), means short is on P5VS0_PHASE.
Then is normal for L7520 to get hot.
This also means bad U7400...

 

[[email protected]]

Dear sir you are right after that I removed it. I see that pin 2 of L7520 still shorted to the ground. I assume that U7400 most gone bad.
Do you know where I can buy this chip? because I couldn't find it on ebay and Alibaba.

 

[2informaticos]

No idea if it has firmware; in such case you will need to remove it from the same donor board.
Maybe someone already changed it can tell us if used new...

 

[[email protected]]

Thank you for your reply, do you know what could cause this issue? I mean, even if I replace it with a new one without first taking care of the main issue, then it will destroy the new one as well.

 

[2informaticos]

U7400 has integrated many output MOSFETs for various buck converters.
If on old boards discrete MOSFETs got burned, do not be surprised if such high integrated chip got burnt too...
One stage fails and you need to change a rare chip, because Apple doesn't allow to repair these power supplies one by one.