Dynabook Satellite Pro L50-G-1C2 - No boot / no charge

[nosfay]

Hi,
I've got a Dynabook Satellite Pro L50-G-1C2 that doesn't charge or power on.
Checked AC adapter first, it's good.
I've found some corrosion near a mosfet, and it's shorted from source to drain, identical mosfet close is shorted the same way.
I cannot find the datasheet or spare, reference is 6794 - GPOH11.
I need help to find what it is please

[mon2]

The insignia appears to be Alpha Omega from a distance.

AON6794.pdf (aosmd.com)

Jellybean mosfet. N-channel type. Can be replaced with many others from other donor boards that will have the BQ charger IC onboard so they too will be driving N-channel mosfets like this one.

[nosfay]

You're right, thank you !

[mcplslg123]

Are you sure the mosfets are shorted? They are likely to show very lows impedance in circuit as part og cpu/gpu rails. better to de solder the mosfets and then test them out of circuit. Verly likely not shorted.

[nosfay]

Indeed, both are good out of board. I've found one component close to them that heat a lot when AC charger is plugged.



I can't read the reference, it's something like APBM 861.

[mon2]

MPS # NB680AGD

(from the above sticky thread on IC part markings = APB*)

monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/NB680/document_id/4031/

(5piece)100% New NB680AGD-Z NB680AGD NB680A (APBF APBE APBG APB*) QFN-12 NB680 NB680GD NB680GD-Z ALV.. - AliExpress 502

Parts Markings thread:

IC Marking Codes and Datasheets - Badcaps

[nosfay]

Thank you

[nosfay]

I've replace the part, and it's still heat a lot. Do you know what can I check next? I dont have schematic.
I've found 19v and 5v on the board.
Replaced part is a 3.3v buck converter.

[mon2]

Remove all power. Meter in resistance mode.

Measure the resistance from the output of this switching regulator (see the red arrow) to ground (see other red arrow). What is the resistance in ohms?

[nosfay]

This is what I got:

[nosfay]

Are these measurement OK ?

[nosfay]

I've found a similar issue here :https://www.badcaps.net/forum/troubl...-gets-very-hot

They talk about a possible short on RTC circuit, but I don't know how to check it and what it does, if someone may help me

[nosfay]

Iv'e done some voltage measurement, here is what I've got:

Vin = 19v
Vout = 0v
LDO = 0.9v
EN = 0.8v
VCC = 3.66v
BST = 3.5v
SW = 0v


I've joined aswell motherboard photo.

[mon2]

The EN voltage is too low to enable the switching regulator inside this device. The signal needs to be a logic ‘1’ = ~ 3 volts to enable.

[nosfay]

If it's too low, it can heat the buck converter?

[mon2]

With such a low enable voltage, the switching regulator will not enable. The resistance to ground at the output is not low enough to cause the immense heating of this regulator.

The other regulator is the LDO rail which appears to be a fixed ON low current regulator. Such regulators literally burn the high input voltage of 19v down to the target LDO voltage so it will heat up the part in the process. The output of the LDO rail does not appear to be shorted either yet the output LDO voltage is very low. The other thread mentions a short in the RTC (real time clock) but the resistance to ground of this LDO does not show a short condition.

The posted pic looks clean with respect to soldering. Do confirm again the resistance to ground of the LDO and the SW rails - with no power to the board.

[nosfay]

Hi,
Resistance are the same.
I've removed the buck converter et do voltage measurements:
- EN = 0.15v
- LDO = 0.18v
- Vout = 0.15v

Then I've injected 1.5v / 0.5A to LDO, and saw a hot component near the battery connector.




It's a Schotkky diode, model BAT54C, I'll try to replace it.

[nosfay]

Hi,
I've replaced the diode, but the converter is still heating a lot (130° with thermal camera).
But now I've found 5.1v, 3.4v and 1.05v on some board test point, then I lost 3.4v and 1.05v because I guess the buck converter goes in thermal protection (140°c in datasheet)

Tension :

Vin = 19v
Vout = 3.4v before thermal shutdown, 0v after
LDO = 2.1v
EN = 1.95v
VCC = 3.66v
BST = 3.5v
SW = 0v

Resistance (no power) :
Vin = 75kΩ
Vout = 27kΩ
LDO = 1.7kΩ
EN = 475kΩ
VCC = 0.85MΩ
BST = OL
SW = 27kΩ

1.05v test point : 45Ω

[nosfay]

Any idea on what I can try next ?

[mon2]

Which diode was used for the replacement? Was it with the same suffix? It is important.