MacBook A1990 PPBUS_HS_CPU short

[Patrick501]

Hello everyone,

I have a A1990 which had a blown inductor next to the display connector. There was no short at this spot, so I replaced it.
I noticed that the big coils above the GPU/CPU (PPVCORE) had a pretty low resistance (2.2Ohms). I already talked about this with mon2.

PPBUS_G3H, PPBUS_HS_CPU showed 1.8ohm to gnd. So I removed the R5400 to seperate the two rails. PPBUS_G3H is fine again but the short is unfortunately on PPBUS_HS_CPU. Sounds like a no fix to me.
Plus I can't find any particular spot which his heats up when I inject 1V@max2A

Any suggestions how to move on?

[Patrick501]

I removed the cooling shields of the GPU/CPU and when I inject 1V into PPBUS_HS_CPU, the CPU heats up.
Doesn't look too good tho

[mon2]

Correct. Very likely the CPU is dead. Test the resistance from PPBUS_G3H (fuse) to PPBUS_HS_CPU (inductor). A low resistance means that the high side mosfet which is used to dial down the high voltage to a CPU safe one is defective and the high voltage has reached the CPU rail.

[Patrick501]

Thanks for your answer. Do you mean another rail instead of PPBUS_HS_CPU to measure?

PPBUS_G3H is conntected to PPBUS_HS_CPU by the R5400 (pic in my first post). So this should always be about 0 ohms. That's why I removed The R5400 yesterday to make figure out which rail causes the short.

Is the U7420 the high side mosfet you mean? The L7420 shows 8 ohms to PPBUS_HS_CPU. You're probably right that this Mosfet is defective or leaking.

[mon2]

With an 8 ohm resistance from L7420 to PPBUS_G3H / PPBUS_HS_CPU, the high side mosfet is dead. The CPU is very very likely dead.

[Patrick501]

Thank you for the confirmation.. sad, but it's kinda like what I expected