PPBUS Short 820-00840

[Mamdouh]

I have a 820-00840 board that has no power, originally customer said it was working and suddenly stopped after a restart, no liquid else than a bit on the ssd connector, put it through the ultra sonic and seems clean. pubs_g3h is shorted to ground with less than 1ohm to ground and shorted to PPVCCCPU_S0G with around 5ohms. would this be a dead cpu. Tried injecting voltage on ppbus and nothing got warm.

 

[2informaticos]

STOP to inject into PPBUS_G3H in this manner!!!

Check first if any high-side MOSFET is shorted.
If you have 1 ohm at PPBUS_G3H and 5 ohm at PPVCCCPU_S0G, then is not an high-side MOSFET from that power supply.
But you MUST compare with ALL big coils.

Only inject if you are sure no high-side MOSFET is shorted.
In such case, you can increase voltage, untill heat something...

 

[Mamdouh]

What is considered a high side mosfet? how do i check if they are shorted and what do i compare big coils with

 

[2informaticos]

High-side MOSFET is part of a buck converter (many on the board).
It is connected between PPBUS_G3H and the coil of correcponding secondary power supply.
Is paired with a low-side MOSFET, connected to ground.
Be aware, some buck converters use complex ICs, which include the both power MOSFETs.
If you will find the same ohm value to ground at PPBUS_G3H and a big coil, veru likely corresponding high-side MOSFET is shorted.

 

[Mamdouh]

L74(10,20,30) all had less tha 1ohm to ground removed corresponding u74** and no short on ppbus anymore. Ppvccgt_s0g is shorted which i assume was responsible for the 3 coils. That line just goes to the cpu and some caps could it be a bad cpu?

 

[2informaticos]

So you had shorted high-side MOSFET.
Injecting voltage on PPBUS_G3H could very easy killed CPU.

Low values on CPU coils can be normal, just compare with similar board.
But not too many chances to have a good CPU, after what you did on PPBUS_G3H.

 

[Mamdouh]

Only injected around .7v the board didnt take anything more than that even for the first time. Coils ended up going down to 0 ohms after waiting a while. if cpu isnt dead wat should i do next

 

[Mamdouh]

Correction board took around 0.1v at ppbus tried again on coil and took 0.1v and 3amps the only common rail that is shorted between those coils was ppvccgt_s0g

 

[2informaticos]

You should compare ohm values with similar board.
Less than 1 ohm is not good sign...

 

[Mamdouh]

I'm getting 3.5ohm on those 3 coils on another board. if i do leave the probes on this board, it goes down to 0 or 0.1 theres not much left on that line is there anything left to do?

 

[2informaticos]

Dead CPU, trash board...