820-01521-a PPBUS_G3H Cycling and low?

[myiannakou]

I have an 820-01521-a board which had liquid damage. I was only getting 5v on my USB-C meter and 0v on ppbus_g3h. I went ahead and replaced isl9240 and now I get 20v on USB meter but ppbus_g3h only 12.58v for 30 seconds and then drops to 12.26v for 10 seconds. This process keeps repeating.

Any help appreciated.
Thnx

 

[2informaticos]

First of all, welcome to the forum!

Which board areas have been affected by liquid spill?
Any liquid sign on audio board, or its flex?

Try to see if machine is recognized in DFU mode.

 

[myiannakou]

Thank you.
I'm not sure which areas were affected as the board had been ultrasoniced before reaching me. There are no signs of liquid on the audio/power button board and the flex cable seems fine. There were some signs of burn marks on one of the pins of the LVDS connector.
How do I get it into DFU Mode? Is that the same as SMC by-pass? What should I be getting on PPBus? It says 13v on the schematic.

 

[2informaticos]

Puff, liquid on LCD connector, is the worst thing.
Very likely dead CPU now; it is directly connected to J8500.

 

[myiannakou]

When I hold my finger on the power button sometimes the fan spins about 4mm then stops

 

[2informaticos]

Which pins affected by liquid at J8500?
Check diode mode to ground for data lines.
Usually leakage between pins 1 & 43 fries at least CPU eDP out.

 

[myiannakou]

The pin that shows the burn is DP_INT_AUXCH_C_N

Here are some diode mode readings:-
PPVOUT_S0_LCDBKLT = 1.055
DP_INT_AUXCH_C_N = OL (Open Line)
DP_INT_AUXCH_C_P = 0.228

Do you need more readings or is that enough for you to come to a conclusion?

 

[2informaticos]

Is clear enough, dead CPU.

 

[myiannakou]

Educate me. How can you tell - I believe you I would just like to know which of the readings told you
Thanks

 

[2informaticos]

You should get similar reading on both DP_INT_AUXCH_C_P/N lines.

Pin 1 is placed next to pin 43.
In case of liquid damage, easy way to get overvoltage from backlight to data pins.

 

[myiannakou]

So DP_INT_AUXCH_C_N being OL (Open Line) was the biggest clue? Was DP_INT_AUXCH_C_P = 0.228 about right or was that even wrong?

 

[2informaticos]

OL is bad for sure.
0.228 in diode mode looks too low for me too.

 

[myiannakou]

Does a dead CPU also explain why ppbus was going from 12.58v for 30 seconds and then drops to 12.26v or would that be the SMC ? Also what should ppbus be for these boards? The schematic says 13v is that correct?