Electronics > Repair

Dell XPS 13 (9343) power fault on motherboard

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xyberlink:
Hi,

I have a Dell XPS 13 (9343) with a power fault on the motherboard.

The laptop works fine from a charged battery, so the motherboard is mostly ok. However, it will not run, or charge the battery from the AC adapter.

So...
- It works fine from a charged battery.
- It detects the presence of an AC power supply, i.e. screen gets brighter and the BIOS reports "45 watt" against the power supply line in the BIOS, as opposed to "unknown" (when using some other non-dell supply) or "none" when it's removed.
- It doesn't draw any current from the power supply either to power the laptop or to charge the battery.
- I've tested the power supply with a separate load and it appears fine.

Closer inspection of the motherboard reveals a burnt out capacitor, just near an MDU1515 mosfet. It appears there are 3 capacitors in parallel (not 100% as I can't see the tracks, but the sides appear shorted, hence assuming they are in parallel).

The MDU1515 mosfet isn't shorted, though I guess that doesn't necessarily means it's ok either.

I'm thinking that the blown capacitor is symptom rather than cause, in part because I believe the 3 are in parallel.

The other 2 capacitors appear to behave like capacitors (in circuit, according to multimeter), hence assume (for now) that they are good. No easy way to determine the value that I know of other than to remove one and attempt to measure it in a tester.

I know the odds are rather long on repairing motherboards, especially with no schematic, but figured it's worth a punt as it feels like this should be fixable.

Photos attached (burnt capacitor, now missing, is the middle of the 3 to the left of the MDU12515).

Any thoughts welcome and appreciated.

PKTKS:
Based solely and loose by your notes...

Notebooks have 2 input MOSFETS which act like keys.
One from outside to inside rail
Other from inside rail (BAT) to outside rail (so charger port
will not be BAT biased)

I would definitely check both and the source gate 
control signals.

Worst case scenario common to short those keys
with some jumper for testing

Paul

deadlylover:
The charge controller looks to be the bq2477x here.

Schematic attached to the post for the laptop, let me know if you don't see it I'll ping you a link. It's a fairly popular model laptop so I'm sure the boardviews are somewhere out there.  :P

It's late here and my eyes are going, if the cap that shorted were the decoupling caps for PQ203, then you need to look at the components before the cap;

PQ201 (near wifi card slot, check for shorts)
PR202 (shunt resistors are beefy probably fine)
PL200 (above the MDU1515, is it open?)

xyberlink:
Thanks for the input thus far and also for the schematics.

PL200 - appears ok - reading as shorted
PR202 - appears ok - reading as shorted (due to its tiny value).

PQ201 - I've got a near short (reading 0.012) between D1 and G.

Prior to these checks I'd also checked voltages on PQ203 (MDU1515) and PQ205 (7716A). Can't do this easily with the laptop on, so the tests are with battery only connected, and then with battery and AC adapter connected (where it should be charging)....

With battery only....
PQ203
- S and G show ~6.5v
- D shows ~6v

PQ205
- S and G show 0v
- D shows ~6.5v

With battery and AC connected....
PQ203
- S and G show ~7v
- D shows ~19.5v

PQ205
- S and G show 0v
- D shows ~7v

deadlylover:
Looks like you have a lead, just make sure it isn't PC200 causing the short but I doubt it. Any ol' dual mosfet will work given similar ratings and footprint but it looks like the original is still readily available.

Make sure you cover that wifi card slot with kapton tape/foil because things can get dicey when trying to rework a mosfet that close to a connector.

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