820-3476 pulsing. not turning on

[donnos]

Hello,

I have an 820-3476 that initially had short on ppvcc_s0_CPU. removed the damaged cap C1008 near the cpu without heat.
no more shorts but still does not boot.
i detected pulsing voltage when plugin in magsafe from s4 all the way to all_sys_pwgd. no other shorts found so far.
any suggestions on troubleshooting steps to continue ?

 

[2informaticos]

Post actual resistance to ground at L7310/20.
Any history fault, liquid signs?

Stable 3V3_S5/SUS?

 

[donnos]

resistace to gnd on L7310/20 is 42ohms

only liquid signs on c1801(removed) and Q1840(replaced)

and yes stable 3v3_s5/SUS.

 

[2informaticos]

Do you note any pulse at PPVCC_S0_CPU too?
Did you try to start in SMC bypass mode?

 

[donnos]

There is a pulse there and at VCORE as well..
same dead on SMC bypass

 

[2informaticos]

Any pulse at PLT_RST_L?
If not check pin 3 of U1950 too.

 

[donnos]

can't find PLT_RST_L, hopefully PLT_RST_BUF_L is the same.. and no pulse here
and nothing on pin 3 of u1950 either

 

[donnos]

***you can ignore this***
i'm measuring pp3v3_S0 @ 1.9V. is this right? i'm not even getting p3v3s0_EN always on how is there even voltage here @@?

 

[2informaticos]

Yeah, is PLT_RESET_L in fact.

Check for pulse at pins 7, 5, 6, 3 of U1950.
Inspect its area for corrosion.

 

[donnos]

no corrosion in the area, no pulse on pin 3,6. pulses on pin 5,7 on u1950

 

[2informaticos]

Tie together pins 5 & 6, then check pin 3.

 

[donnos]

now i get a pulse on pin 3, still not turning on. whats next?

 

[2informaticos]

PLT_RESET_L pulses now?
Try SMC bypass again.

 

[donnos]

Yes PLT_RESET_L pulses now but still no boot, Fan does not even attempt to spin with SMC bypass

 

[2informaticos]

Check all SPI Bus Termination resistors and traces.
Clean J6100; remove, if needed.

You should have something more corroded.
Also possible to have accumulated crap underneath SMC, or CPU.

 

[donnos]

all SPI termination resisters and traces are in order.. no other corrosion seen from the original c1801(removed) and Q1840(replaced).

J6100 is removed and SMC already reballed. which only left CPU. dead CPU or just reflow?

 

[2informaticos]

Try heating CPU with quality flux; just to burn eventually accumulated crap.