820-3437: Board Not Powering Up. PP5V_S0 missing with R8151 connected

Morning

Dealing with a board that doesn't power up. PP5V_S0 is missing, well it pulses from 0 to about 0.22V with R8151 in the circuit. Once i removed R8151, i now had a steady 5v for PP5V_S0. So i put R8151 back in the circuit, replaced C8159 with the same result, no 5v. I pulled Q8150 to eliminate the circuit being fed by it, and still wasn't getting 5v on PP5V_S0. All resistor measurements are good, and the only capacitor in the circuit wasn't shorted but i replaced it just as well. My confusion is why won't the 5v return when Q8150 is removed, since the only components on that line is 3 resistor, a capacitor and the bjt.
 
Remove Q8150, still getting the pulsing signal but no steady 5v.
R8151 - 50.8K
R8152 - 14.87K
R8153 - 0.99K

measurement from pin 5 to ground is 15.7K (this is with Q8150 still out of the circuit).
 

2informaticos

Administrator
Staff member
Forget that area.
Once Q8150 is removed, all these components are not usefull; keep it removed.

ALL_SYS_PWRGD pulse too?
Also check pins 7 & 3 of U1950.
 

2informaticos

Administrator
Staff member
Any pulse at VCORE coils?
Any history available?
Do you get one pulse only, or every 5s?
Try new BIOS, just in case...
 
Sorry for the late response.

Any pulse at VCORE coils? Yes
Any history available? It looks like it had corrosion but the customer had cleaned it all off before i got it without first taking pictures. I replaced all suspected components that had corrosion.
Do you get one pulse only, or every 5s? About every 5s
Try new BIOS, just in case... Reflaahed with new bios and also cleaned ME region
 
Correction:

i measures the wrong chip.
On U1950
pin 3 and 6 arent pulsing and 0V but pin 1, 2, 5 and 7 pulses.
Should I still change U1950?
 
Went ahead and replace U1950 and now it powers up, but doesn't stay powered up. Fan spins for a few seconds then shutoff, then repeats this process a few times before it stops, until i unplug and replug the MagSafe.
 
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