CPU Hot when plugged in - Inpiron 15 3511 P112F LA-L241P

[willis6732]

Hey Everyone, This is my first post here. I opened a repair shop a few months ago and happy to have this as a resorce.


Ive had a laptop come in with a broken display. I was able to get some fan spin and funky colors on the display. I orderred a display and now that the display has arrived the laptop has another problem.

The issue happening causes symtoms similar to a shorted CPU but i am unsure that is the issue.

When i plug in the motherboard to DC (Nothing else connected) the two coils Feeding the CPU get hottest as well as the CPU.
https://photos.app.goo.gl/KTrViNjkwLM6sAdY6
https://photos.app.goo.gl/xJSKTSKY7nE7mTx28

When i take the Heatsink off the CPU, it seems to get hot evenly and no hot spots
https://photos.app.goo.gl/kcrKUXuNgS4jNLec9
https://photos.app.goo.gl/eVAxmR7nCRdhKxy39


The reason i am unsure that its a short CPU is because if i apply about 1v to the CPU coils there is no voltage drop and no Amps flowing. If i increase to 1.5V, I still get no voltage drop and 20ma now flows into the board.


THis makes it seem like it is not a short... Or am i wrong?

Thanks
Kevin

[mcplslg123]

Re: CPU Hot when plugged in - Inpiron 15 3511 P112F LA-L241P

Injecting more then 1V is a clear no no. Chances of you having shorting in 19V rail is very high due to perhaps high side fet shorting. Check resistance to GND on CLR.

[willis6732]

Re: CPU Hot when plugged in - Inpiron 15 3511 P112F LA-L241P

Injecting more then 1V is a clear no no. Chances of you having shorting in 19V rail is very high due to perhaps high side fet shorting. Check resistance to GND on CLR.

Thanks for the heads up, i will keep my test voltages lower.

There is no short in main rail. 30MegaOhm.

All the high side transistors seem fine. The low side transistor on CPU rail seem ok too.

CPU rail resistance to ground is about 60 Ohms.



WHat did you mean by "check resistance on CLR". Is that current load resistor on the main rail?

Could it be an issue with the IC chip circuit thats controlling the transistor on the CPU Circuit?


Confused if this is a shorted CPU or not

Any guidance on what to check next?


Hopefull i can figure this out soon, incase i need to return the replacement LCD ��

Thank you

[mcplslg123]

Re: CPU Hot when plugged in - Inpiron 15 3511 P112F LA-L241P

CLR-Current Limit Resistor-so you are right. A hot SOC(CPU+PCH) is a bad symptom. Usually SOC gets killed when one of the high side mosfet is shorted and thereby leaking 19V to cpu. It rarely survives after that.

Another reason is shorting in +3V LDO. This often leads to a shorted PCH.

Observe carefully which die of SOC is heating. Check resistance to GND on +3V LDO/Bios chip pin 8.