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820-00840-A short to ground

D

dudipupan

Member
Hello everybody,

I'm very new in rapairing boards. Just started some weeks ago with some old (2009 - 2011) MacBooks, which I could repair with help of the YouTube channel of Louis Rossmann (Thank You very much at this point ;) ) .
Now I tried something newer and more expensive :-D A MacBook Pro 13 (A1708) from 2017. The guy who sold me the MacBook told me that repairstore diagnosed a faulty transistor.
That's why I thought: "No problem, just change the transistor and everything will be fine." Well... thought wrong.
The faulty transistor (Q7960) was not hard to find (because the repairstore soldered it out and kept it)... but the problem was another...
So, the solder pads (pins) 3 and 4 (Net BANJO_SW3RVR) are shorted to ground (00.1Ω).
But that's not all... also Inductor L7370 is shorted to ground (00.1Ω).
Sadly I can't locate the shortcut :(
Some more informations:

-The board takes actually 20V at 140 to 150mA (fluctuating) (without transistor).
-PPBUS_G3H is present with 13.2V.
-I have 4 spare transistors for the Q7960 but didn't soldered any of them on the board because of the short.
-I have access to various equipment: hot air station, solder station, amp-meter for USB-C, multimeter, oszilloscope (in case this is helping for diagnosis).

I hope I will find some help here :)

Thank you and best regards from germany.
DudiPupan
 
2informaticos

2informaticos

Administrator
Staff member
Unfortunately, looks like.
You can inject voltage there (no more than 1.2V) and check if CPU heats.
Remove heatsink first.
You may need 20A lab PSU.
 
D

dudipupan

Member
I just injected 0.8V (>3A) on the pin 2 of L7960 - but nothing was really heating up... I'm confused. The CPU seemed to get slightly warm after some minutes but not hot...
 
D

dudipupan

Member
SO I just did some more detailled research about the temperature:
Initial temperature: 21°C
After 60s: 24°C
120s: 26°C
180s: 26°C
240s: 26°C
300s: 26°C
Seems to stay at 26°C.

I didnt' reflow it. Don't know if someone else did. But it doesn't look like... the rubber-frame around the CPU-Chip is not damaged.
 
D

dudipupan

Member
How would the CPU behave if there are just some faulty solder-balls on the BGA-Chip, shorting the surrounding pins to ground.
Would the CPU even get hot in this case?
 
2informaticos

2informaticos

Administrator
Staff member
The CPU die is small, you should note the heat.
Keep the amps lower than 5A; decrease the voltage a bit.
If not, your PSU will not inject voltage at all.

Maintain amp limit button at max.
Remember to buy a proper PSU for voltage injection.

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You don't need to adjust current, must stay always at max.
But sometime is good to see how much current board sinks.

The short is not caused by solder balls...
 
D

dudipupan

Member
I did retry the whole thing with 0.9V (resultuing in 4,98A with amp limit at max).
Same results regarding to the temperature evolution as in my post#9. :unsure:

Already ordered a new PSU with higher amp limits :)
 
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