820-00138: Not powering up


I have this board that came in first with missing PPBUS_G3H. Replaced U7100 which resolved that. But then PP5V_S5 was then missing. Replaced SMC and i now have PP5V_S5. The board still isn't power up. I'm missing PP5V_S4, with P5VS4_EN also at zero. But after about 2min or so of the MagSafe being plugged in, the board finally powers up and chimes. Any suggestions


Staff member
Check power on signals before turns on.
Pay atention to PCH's RTC signals.

Any sign of liquid?
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PPVRTC_G3H - 3.3 the whole time
PP5V_S4- 0.02 during the 1min time before the board powers up
P5VS4_EN - 0.08 during the 1min time before the board powers up
all S5 are powered on during the 1min time before the board powers up.

I replaced the smc again with the same result.


Staff member
If the board doesn't even try to start, no reason to check S4 signal/voltages, or upper.
I was talking about signals required by SMC and PCH to start and complete power on sequence.
RTC signals are very important at this step.
And you didn't say anything about 3V3_SUS also.
Yep. Per you advice, start going through all the signals for the SMC and found PP3V42_G3H around 2.5v and slowly climbing. Resistance on that line was 90ohms. Isolated to U1900. Replaced and board now boots up.

Thanks for the assist.