Dell E5470 short in processor? LA-C831P

[seanc]

Hi all,

Reaching out for any thoughts as I'm stumped.

I have two identical failed laptop motherboards. Both sport an Intel i7-6820hq.

Both boards failed in the same way, just went 'dead' with a smell of burning, not shorting the charger, but the inductors get very hot when plugged in to the laptop power supply.

An obvious failure on both was PU603 (Vishay SIC530 Manual) appearing burnt at the Vin section and Vin showing short to ground.
Removing PU603 removes that short, but I'm left with a short to ground on the Vswh side which the datasheet calls "Switch node of the power stage". I think that connects to the processor?

I cannot find the short and I've pretty much resigned myself to it being the processor.

The first board I did a poor job at removing PU603 so I have essentially consigned it to scrap and dismantled that bit by bit looking for any changes.
On that board I have removed:
PU602, PU605, PU606 (all SIC530), no change.
A bank of capacitors next to PU604 and PU605 that appeared shorted.
PU601


No amount of checking/removing surface mount caps or other chips will lead me anywhere close - the problem is seems confined to any section with components labelled x6xx so I think that's all one related circuit. My last thoughts are the capacitors on the rear side under the processor.


Does anyone have any thoughts? Pictures of the second board with only PU603 removed.
If anyone wants clearer photos of a section, please let me know - I'm struggling with the phone camera.

[piernov]

Re: Dell E5470 short in processor? LA-C831P

What's the exact resistance to ground where you think there is a short to ground?

[seanc]

Re: Dell E5470 short in processor? LA-C831P

Between gnd and vswh 03.5ohms on the 200 ohms setting - admittedly I've done most of my testing on the 2000 ohms setting where it just shows 003. Happy to be proven wrong, especially if it's because I screwed up the first board.

[piernov]

Re: Dell E5470 short in processor? LA-C831P

It's a little bit on the low side as I usually see 6 ohms on Skylake, but it's most likely not a problem.
Doesn't mean the CPU is fine since newer CPU rarely survive when something goes wrong on CPU VCore, but it's not shorted.

[seanc]

Re: Dell E5470 short in processor? LA-C831P

I'll replace PU603 on the second board with another new one and see what happens... the first just turned on for a second and then off.

[seanc]

Re: Dell E5470 short in processor? LA-C831P

Replaced PU603 on the second board, same symptoms unfortunately, turns on, then off.

[seanc]

Re: Dell E5470 short in processor? LA-C831P

Replaced PU603 on the second board, same symptoms unfortunately, turns on, then off.

I went over the board again today and noticed that the replacement SIC530 had a ground short on a pin that should have no connectivity.
I replaced it and a capacitor next to it (it looked tested open circuit, it was) with one from the scrap board, but sadly no change the power light comes on, then almost immediately off, sometimes it stays on a bit longer.

I measured all of the caps under the processor on the reverse of the board and all show ~2.6ohms or higher depending on which side of the processor I measure.

What has changed, is that there is no longer a short across the inductors after p602, 603, 605, 606 - 3.4ohms now.

I suppose the next steps are to look for voltages - what should we expect at PU603?

[pepegotera454]

Re: Dell E5470 short in processor? LA-C831P

You have checked if it is a direct line to the CPU

[seanc]

Re: Dell E5470 short in processor? LA-C831P

You have checked if it is a direct line to the CPU

I'm unsure of which points to check - any suggestions?

[chris1379]

Re: Dell E5470 short in processor? LA-C831P

Did you ever find the problem? I have a very similar board with the same problem. Mine is an LA-C841P with PU602 burnt.

[seanc]

Re: Dell E5470 short in processor? LA-C831P

Nope, still got the 2 dead boards and an number of slightly different donor boards. Hope to try and investigate again when time allows.

[mantza]

Re: Dell E5470 short in processor? LA-C831P

Nope, still got the 2 dead boards and an number of slightly different donor boards. Hope to try and investigate again when time allows.

did you find any solution? do you think the cpu is shorted?

[mazenkataya]

Re: Dell E5470 short in processor? LA-C831P

any news regarding this dell baord issue with pu 602 And pu 603

i changed ics after short in charging line but still have power issue only showing white light on power button

[mauri1990]

got the same board and same problem in those ics pu603 in my case. board try to start then power goes off ( aparently no shorts to ground), i was checking voltages and 3valw and 5valw are present, 1,1v , something is pulling off the board. already replaced pu602 and 603, no changes

[mon2]

Post #2.

[mauri1990]

Post #2.

1.5 ohms on both coils, looking with thermal camera when you press the power button cpu try to start , current goes up to 300mA after less than a second then it shutdown but the light (next to power button) keeps on

[mon2]

Power may be going off for other reasons including a defective current loop circuit.

Remove all power. Meter in resistance mode. Measure the resistance between pins # 1 & # 2 on the charger IC @ PU801. We are wanting to check the total current sense path resistance and compare to the values showing in the schematics. This measurement should be ~100 ohms since the series mosfets in the path will be off.

Confirm you have ~0 ohms if tracing from the actual pin on the charger IC to the mosfet on the same trace. Check both pins. Need to be sure there is no disconnect in this region of the board.