820-3437-B PP3V3_S5 shorted

GoodguyFR

New member
Hello there !

I have a 3437-B from a MBA 2013. At the beginning I had no light on charger and no fan spin, no more historic. I found corrosion on PP3V42_G3H supply, I replaced the bad components and still no light on charger. I tried reflow SMC without success, and I finally had a light on charger after replacing the SMC with a working one. Now I have light on charger but still no fan spin. I finally found out something that PP3V3_S5 is present on the board but it's shorted...

I don't know if it's shorted since the beginning but I tried to remove the SMC, also U7501 and every component attach to PP3V3_S5 and I still have the short. I also looked for broken traces and bad resistances value, found nothing. I finally throw it in the ultrasonic cleaner but still shorted.

It's a dead board to me but if you have any advice I would be glad to hear it :) Thanks
 

2informaticos

Administrator
Staff member
Which is the exact ohm value of that short?
Not a good idea to remove ALL components from 3V3_S5.
I hope you didn't power on the board without U7501!!!

Lift L7560 one side and see where the short remains...
 

GoodguyFR

New member
I never plugged the board since I removed SMC and U7501. The short is 22ohms. When I lift L7560 the short is on PIN 2 and when U7501 lift up the short is only on PIN17, it looks like the short is on PP3V3_S5_REG_R. When I remove components I put them back in place after I checked the short still there, i do it page by page, don't know if it is a good methodology.... Every caps and resistors on page 54 are fine.
 

2informaticos

Administrator
Staff member
22 ohm is a bit high to heat something.
But you can try injecting voltage there, no more than 4V.
You may need to give it time; IR camera may help.
Can also have luck and maybe the culprit chip enter in thermal avalanche and heats faster.
Remove CPU heatsink first.
 

GoodguyFR

New member
Actually PP3V3_S5 is shorted but present, so could it heat with the MagSafe only ? I don't have any lab supply yet and I tried with the MagSafe and an IR camera, the only hot point is under PCH (or the other side of the board I don't know). CPU heatsink was already removed.
 

2informaticos

Administrator
Staff member
You have other volatges present too; that dowsn't help to locate one chip from 3V3_S5.

3V3_S5 goes directly to PCH and its big size distributes the heat on entire CPU.
So probably is the culprit...
 

GoodguyFR

New member
Alright so your guess is reballing CPU ? Maybe dead CPU ? No way that I am doing a reballing CPU for a 3437-B, neither a CPU swap...
 

2informaticos

Administrator
Staff member
Reballing why, like a masage at a wood foot???
Of course you can try; nothing to lose, if the board is trash anyway.
 

GoodguyFR

New member
haha exactly like a massage at a wood foot ! I'l try it and let you know at least if the short was coming from there. I would be very sad to remove the CPU and still have the short...
 

GoodguyFR

New member
Hi, so to let you know the end of this, I removed the CPU properly and the short is gone, so the problem was the CPU or the connexions to the CPU. I know it's a pain in the ass to reattach a CPU to a motherboard but I have time to spend on it and it worth the try as I have some junk 820-3437-B. Do you know where I could find a stencil to reball a CPU for this board ?
 

GoodguyFR

New member
Ok i find a stencil as the reference SR16Q. It will arrive here in two weeks or so, I'll let you know if the reparation is a succes or a disaster :)
 

2informaticos

Administrator
Staff member
Original CPU shorted, that is the disaster.
Even if repair will not be successfull, you will get some experience; soldering such type of BGA...
 
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