Dell Precision 7720 - Shorted main power rail

Hello,

On this laptop, the first mosfet (PQ5) after the DCIN connector was burnt, so I have removed it. I then discovered an exploded capacitor near PU1201. This capacitor has been removed, but I still have a 0.0Ω short to ground after mosfet PQ701. I have also removed PU1201, but still the short remains. Injecting voltage does not help, as only the grounds are heating up, so the short must be quite large.
I have removed coils PL1200, PL1201 and PL1202 and have a resistance to ground through the CPU (Intel i7-6920HQ) of 1.1Ω. This seems very low, but maybe OK for this processor, I'm not sure. If I inject 0.8v onto the coil pad, I have a current draw of 0.9A.

Before I investigate further to try and find the short to ground, can anyone confirm if the resistance I'm seeing for this CPU is OK?

Board marking is CAP10 LA-E321P Rev 1.0 (A00)

MendUsThis ha scritto:resistance to ground through the CPU (Intel i7-6920HQ) of 1.1Ω.

Hello!
In my experience the normal resistance for this generation of the CPU is around 2Ω.
Did you measure this resistance when the CPU is cooled to the room temperature?

MendUsThis ha scritto:Injecting voltage does not help, as only the grounds are heating up

At what electrical current did you perform the short circuit test?

matic ha scritto:Did you measure this resistance when the CPU is cooled to the room temperature?

Yes.

matic ha scritto:At what electrical current did you perform the short circuit test?

I started at 0.8v and 0.5A, then gradually increased in 0.5A increments until I got to 3.5A.

MendUsThis ha scritto:until I got to 3.5A.

You can go up to 5A without any problem. If the short circuit resistance is really low, sometimes it's necessary to reach up to 10A to find the shorted component.
Make sure to use thick enough test wires. If necessary, the voltage limit can be increased to 1V.

Unfortunately, my power supply has a maximum output of 5A, so I carried out the short circuit test at 1V and 5A. I've uploaded a couple of video files from my thermal camera. The first video shows me tapping the power on and off to try and see if anything flashes. The heat seems to radiate out from the 8 vias directly above chip PU1201. I've attached two photos of the area. You will notice that the blown capacitor had damaged the pad, so I've ground away the damaged area to make sure the layer underneath is not damaged. The opposite side of the board does not heat up at all.
In the second video, I have the power permanently connected, showing the general area that is heating up. The hottest part appears to be near to where the capacitor exploded. I'm beginning to wonder if there is some damage to the inner copper layers of the PCB that I can't see on the surface.

It's better to perform a short circuit test directly on the main power rail "+CHAGER_SRC". You are currently performing it on the power rail "+CPU_B+".
Is the coil PL700 present on your board? If so, solder the positive test wire directly on this coil, otherwise solder it on the jumper PJP701.

After carrying out the short circuit test on the correct power rail, the thermal camera showed one of the 10uF capacitors on the opposite side of the board to the one that had exploded was getting hot. After removing it, the main power rail was no longer shorted. The laptop now powers up and I can boot into Windows, so 1.1Ω resistance to ground for this CPU is OK.

Thank you once again matic.