A1278 820-2530-A Green Light, No power

[mon2]

Hi. Troubleshooting a macbook A1278 logic board that was working but owner was complaining about random turn off during use issues.

Now the logic board will not power on for the fan spin.

Magsafe power adapter is fine @ 16.x volts; green light on adapter is lit (always).

If the battery is docked, see the 3v42 rail ok. Remove battery, power adapter yields the same working 3v42 rail.

Have not checked all of the power rails but PPBUS_G3H is active @ 12v6.

The concerning rail (reviewing this one now) is that PPVCORE_S0_CPU_REG is @ 0 volts.

Could the SMC be defective?

Does the SMC actually contain firmware?

Using boardview, lit up the PPVCORE_S0_CPU rail and removed all related caps (tantalums and ceramics). Still no power on this rail.

The resistance (red on ground); black on PPVCORE_S0_CPU is @ 8.x ohms which sounds very low.

The PM_SLP_S4_L line is low. Believe this line is required to be HIGH to enable the mosfet @ Q7813 to enable the left half of the dual regulator U7200? The right half of the regulator is enabled and working fine.

Any suggestions on how to debug this logic board?

From our understanding, if all is well, the fan should spin and do so even without a power on button press? That is what we see on the youtube videos.

Thanks.

[piernov]

Re: A1278 820-2530-A Green Light, No power

Core 2 MacBooks with NVidia MCP need the power button to be triggered in order to turn on.
As always go through the list of power rails and measure voltages when triggering power button.

[mon2]

Re: A1278 820-2530-A Green Light, No power

Merci Piernov. After applying the keyboard flex cable again, the motherboard indeed works fine. Very embarrassed by this ghost chase as we removed SMD caps (ceramics and tantalums) in effort to remove the low resistance of the S0_CPU rail. In the end, there was no such issue to be repaired. Just watched too many youtube videos where none showed the use of a power switch. Weird.

Thanks again!