820-2879 Green light no start

[Cocobill]

Hello,

an 820-2879 came in with minor water damage around Q9807 area.
green light OK
ppbus_g3h 12.52v
PP3V3_S5 OK

Does not boot. What do I measure first?

Thank you

 

[dukefawks]

Shorted power pads? These do not auto power on. Checked resistance to GND on pp3V42?

 

[Cocobill]

Thank you Duke! Shorting pads does nothing. Resistance to GND on pp3V42 is 0.311v

 

[dukefawks]

That is not resistance that is a diode mode, need resistance.

 

[Cocobill]

Sorry... Resistance to GND on pp3V42 is 21.10KOhm

 

[dukefawks]

That may be a bit low but now 100% sure. Look carefully for corrosion around U6990, did anything get liquid around that? Battery charging?

 

[Cocobill]

Battery is charging. U6990 area is clean, minor corrosion on unconnected R5001 pin 2.

 

[dukefawks]

Checked all rails for shorts? Any corrosion around the TPS51125? PM_PWRBTN_L doing anything when you press power button?

 

[Cocobill]

Found short in ppvcore_s0_cpu line, diode mode is 0.014v. Also ppvcore_s0_mcp is 0.015v. No corrosion around TPS51125. PM_PWRBTN_L is going from 3.3v to almost 0 when shorting SMC_ONOFF pads.

 

[dukefawks]

Resistance to GND is the only way to check for real shorts. Diode mode measures a voltage drop, not quite the same.

 

[Cocobill]

ppvcore_s0_cpu and ppvcore_s0_mcp resistance to GND is 24 Ohm

 

[dukefawks]

and that is no short. I would say either crap under the SMC or the MCP is dead.