820-4924-A - burnt Fuse F7140 - PPBUS_G3H shorted to ground?

[stef4n]

Good evening,

I have a 820-4924-A logic board with amber and green light on the charger. The board is currently out of the MacBook,and not connected to anything besides the charger.

I measured PPBUS_G3H on the fuse F7140.
Pin1: 12.68V
Pin2: 0V

I unplugged the board, tested the fuse with diode mode; probes on pin 2 and ground => Beep. I also measured 0.1 Ohms.
Problem seems similar to what was described there : https://boards.rossmanngroup.com/threads/820-4924-a-ppsub_g3h-shorted-ground.7292/

dukefawks and aprendiz asked the resistance to ground from F7140 to L7320. Original author didnt reply.
On my board, I have 0.5ohms on pin 2 of L7320, but it decreases to 0.0 in 5-6 sec.
I have 0 ohm on pin 1 of L7320

There is no sign of corrosion, no sign of liquid, board has not been cleaned; there is still dust a bit everywhere, but almost nothing.

Nothing get hot.

How do I narrow down the cause of the problem ? And if there was no liquid, then what might have caused a short?

P.S: Was I supposed to post under the old thread, 6y old, or create a new one ?

 

[2informaticos]

"If I measure from pin2 of F7140, to pin1&2 of L7310/20, I get 0.1 Ohm."
What do you expect measuring from pin 1, if F7140 is burnt (open)?
Please, put some logic on your job.

The forum purpose is to help in the repair process.
Personal training is different thing.
You can still learn a lot reading as many threads as possible.

Can't you see any path F7140 -> Q7320 -> L7320 -> CPU core voltage???

You are lucky if only Q7320/21 are the burnt components; maybe U7320 too (less probably).
However, was expecting resistance to ground on the other side of L7320.
To know if CPU may still be alive.

 

[2informaticos]

What can you expect from a board without F7140 (PPBUS_G3H shorted to ground)?
Did you think where 5V comes from???

As already said, try to put logic on your job...

 

[stef4n]

Regarding the short:
I had a very small resistance on pin 4 of Q7320; 0 on the other pins.
And I had 0 ohms on all the pins of Q7321. I therefore removed Q7321 first.

Now on the pins below Q7321 I have:
pin 1/2/3: 0 Ohm, ground, as expected.
pin 4: >800kOhms
"pin5": >200kOhms

And on pins of Q7320, I read >20kOhms on all pins.
So, short is gone and Q7321 was the culprit.

"What can you expect from a board without F7140 (PPBUS_G3H shorted to ground)?"
Did you think where 5V comes from???

Yes I did, and I hadn't the answer. I had wrongly supposed that it could be created from all the P3V3S5 currently at 0V, but I suddenly realized that I had completely missed the VIN pin 23, "PPVIN_S5_HS_OTHER3V3_ISNS" which comes straight from G3H...which is not there...

Jeez I feel shameful.

So. Will solder a new Q7321 on that board, replace the fuse and we should be good.

 

[2informaticos]

Didn't change Q7320 yet???
Was mentioned in the first post.

 

[stef4n]

Not yet, wanted to check everything I need prior to order the necessary components.

As I said, I'm just starting with the logic board repairs; I dont have donor boards to harvest for these 13" 2015. I'm checking the usual/common failures on these models and filling my cart. Will receive everything in 2 days. Any suggestion is welcomed.

 

[stef4n]

Almost fixed.
Now the MacBook is able to boot; but it takes 24 seconds. It's quite slow.
Furthermore, it does not see the battery anymore.
And at the end of the boot, the screen first displays an image on 70% of its surface, then switch to black, and then display the image on 100%. That's weird.