Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

[chrisphl]

Hi folks,

I got a Asus VivoBook S13, Model: S330U. It is absolutely dead after the power cord was bit by cats...

I disassembles the device and did some measurements already. I attached two photos of its motherboard.
What I've done so far:
* I tried to check voltages but couldn't find any other than 19V and some <1mV at some Mosfet gates (The single pin at the side where there three further source source pins).
* I searched for all caps and checked if they *beep to ground* for more than a sec. on both sides. There are a lot of them especially near the CPU which is normal, isn't it? But no one else
* I watched out for 0-Ohm resistors (written '0' on it) and fuses (green parts) and checked their resistance. They should have about 0 ohms...
* I inspected the mainboard under the microscope in the hope to find some obviously burnt parts or the like. Nothing.

If I'm not wrong I need to figure out, why the Modfets aren't switched to power-on further circuits.

I need to mention that I'm just an repair enthusiast with an electro-technical/IT background. This is my first dead laptop repair attempt.

Would you please guide me throw the process?
Don't hesitate to ask me questions on how I did what, to get every information you need to help out of my head.
Every help appreciated.

kind regards
Christian

[mon2]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

In order of importance:

1) are the cats ok ?

2) review the DCIN adapter connector -> there will most likely be 2 x mosfets (8 pins each) -> locate these and take clear pics of each.

Nearby the same mosfets will be the charger IC with probably 'BQ' markings on the topside. Which charger IC is onboard ?

Take pics and post them here.

[chrisphl]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

Hey mon2,

thanks for your fast reply!
I'll try to answer as detailed as possible:

Yes, the cats are absolutely fine.

I cannot locate the nearest Mosfets to the power connector in terms of shortest trace to the Mosfets. However I was able to take some interesting photos and I found the BQ...-chip.
I made copies of the overview images and marked the spots where I took the microscope shots.

Can we get further with the help of this material?

kind regards
Christian

[Edit] I was even able to find some voltages around the BQ. Attached another photo. [/Edit]

[mon2]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

Yes, very helpful.

1) Datasheet for the charger IC is attached.

2) your logic board will have a very similar design if not identical to figure 16 of this attached datasheet.

3) Review the schematic and the pinout of this device. Respectively, interested to confirm the voltages on each of the pins of this IC.

Proceed with great care when taking the measurements.

We need to check if this IC is being powered (fed from the DC adapter).

Also, if this charger IC is able to detect the presence of the adapter -> create the boost voltage to enable the mosfets, etc.

Post your readings for this charger IC.


Update:

See attached.

The DCIN mosfets are the ones you have shared with the green box.

Proceed to attach the power adapter and confirm the voltages on each pin of these mosfets.

Often the 2nd mosfet is defective on logic boards but check the voltages on each pin.

[chrisphl]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

Hi again,

thanks you for your explanation. This helps me a lot to understand how the motherboard should work.

I did some measurement on the Mosfets Q1 & Q2 and also an the BQ-chip according to the schematic.
Drain and Source of both Mosfets are 19,54 V, Both Gates show up 24,22 V, which seem very OK from my point of view.
Find the attached image to get the readings on the BQ-chip.
My understanding is that it is working fine according to the switching of the Mosfets using ACDRV output. (It set's 6,0 V on ACOK.)
Also !TB_STAT is pulled to 0,0 V, which means "IC is operating in boost mode" according to the datasheet.

I'm not sure how I should understand the description of BTST ("High-side power MOSFET driver power supply.") which is read as 5,89 V. Is this sufficient or is it a sign of "short behind that line"?

Also the battery detection does not seem to work properly, !BATPRES shows 0 V even if battery is connected to the board.

What do you think about the current information? Do you agree with my assumptions? Is there something else you can "see"? What are the next steps?

kind regards

[mon2]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

BATPRES# (pin 15) is an active LOW signal.

If logic low, the battery is present.

If logic high, the battery is absent.

Voltages so far look good but the mosfet can be defective. Remove all power. Meter in resistance mode.

Check the resistance across the mosfet pins.

Measure

Source and drain
Source and gate
Gate and drain

Source pins are 1-2-3
Gate pin is 4
Drain pins are 5-6-7-8


Post each measurement. Repeat for the 2nd mosfet.

No power in resistance mode checking.

[chrisphl]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

BATPRES show 0,0 V not matter if battery is connected or not :-/

(Q1: M3056M, the larger one; Q2 M3006M, the smaller one)

          | Q1,M3056M       | Q2,M3006M
Source and drain  | 0,846 / 0,435 MOhm  | 1,22 MOhms / 57 kOhms |
Source and gate  | 1,55 / 0,504 MOhms  | 1,55 / 0,504 MOhms   |
Gate and drain   | 1,18 MOhms      | 1,77 / 0,435 MOhms   |

Note: If there are two/values it depends on changing the probes.

Why does the probes direction matter? Because of the "Diodes" inside a Mosfet?

[mon2]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

Review this webpage:

http://electronicsbeliever.com/how-t...0good%20MOSFET.

Check the voltage at each large coil on the logic board.

[chrisphl]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

Thank you for the educational link! Very helpful... :-)

Coil voltages are 0,0 mV for each but the one nearby the BQ-chip and the battery connector.

Update: After reviewing the datasheet of the BQ I think the 239 mV Coil must be (the only) L (3.3uH) in the schematic since I found the same voltage on SRP

[mon2]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

post the voltages on each pin of the BQ charger IC.

[chrisphl]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

Here we go: I wrote every measurement result into the BQ-chip overview image including their names to make it easier for you. Nevertheless please check that I named the pins correct.
I am very excited to see what we have to do next. :-)

kind regards
Christian

P.S.: I was able to determine D1 and D2 of schematic on page 36 in bq24780s.pdf. So at least the upper left quarter can now be matched to the broken motherboard. :-)

[mon2]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

You are missing a pull-up voltage required for:

ILIM - see the ref design for the charger IC. This signal line cannot be 0 volts else battery charging will not work.

SDA and SCL should be ~3v3. This means the local resistors on this line are floating.

Remove all power. Meter in diode mode. Connect the meter probes together and you should hear a beep tone. Confirm it.

Now connect one probe to the sda or SCL resistor but to the side that is used for the pull up voltage. Other meter probe to the next sda or SCL resistor on the side that is for the pull up voltage. Meter should beep.

Now, one meter probe to this common voltage side of the resistor, other meter probe to any local grey coil. Do you hear a tone?

If not, move the meter probe to another coil.

We know that this ~3v is missing but do not know yet, where is the power supply for this required voltage rail.

This method should locate the power supply circuit.

[chrisphl]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

Hi mon2,

thanks again for your reply!

You are missing a pull-up voltage required for:

ILIM - see the ref design for the charger IC. This signal line cannot be 0 volts else battery charging will not work.

...which I fully understand. :-)

SDA and SCL should be ~3v3. This means the local resistors on this line are floating.

...and means 3v3 line is not up and running, does it? :-)

Remove all power. Meter in diode mode. Connect the meter probes together and you should hear a beep tone. Confirm it.

That was easy so far...

Now connect one probe to the sda or SCL resistor but to the side that is used for the pull up voltage. Other meter probe to the next sda or SCL resistor on the side that is for the pull up voltage. Meter should beep.

...which I understand but that's the not so easy part for me: As one can see on the last image I posted, the connection from BQ to {SDA, SCL, ILIM} go straight to a via each. Unfortunately I cannot any tracks on the other side of the board so it is difficult to find the I2C-pull up resistor or the ILIM resistors...
Do you have any smart ideas on how to resolve this issue easily?

We know that this ~3v is missing but do not know yet, where is the power supply for this required voltage rail.

This method should locate the power supply circuit.

understand... :-)

kind regards
Christian

[chrisphl]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

I need to post further news on this case:
While looking and measuring around the board I found a chip that gets very hot for the first few seconds after connecting the power cord. I marked it on the attached photo. It has "2N=5F" written on it, must be a RT8249C which is most probably the 3V3 source of the board.

Unfortunately I was not careful enough when measuring the pins and shorted VIN with LDO5 (?). The Chip was fired up immediately. :-/

What I can say is that VIN is 19.54 V and according to the datasheet (p.3) it should always output 3V3 in that case:

3.3V Linear Regulator Output. It is always on when VIN is higher than VINPOR
threshold.

So I think I need to replace the RT8249C anyway. Hope I can get it asap...

As always, your thoughts on this are greatly appreciated.

kind regards
Christian

[mon2]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

Yes, it should be replaced.

US $1.02 15%OFF | (1-2piece)100% New RT8249CGQW RT8249C (2N=2J 2N=EG 2N=DE 2N=...) QFN-20 Chipset
https://a.aliexpress.com/_m039zLe

Do check and confirm the estimated time of arrival to your address. Install their phone app to order.

We use this vendor often and is a reliable shop.

[chrisphl]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

Thanks for the link. Very good price but ETA is 22. Oct... phew... I'll check my options... :-)

[piernov]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

Need to check resistance to ground on LDOs. After removing the IC since it's burnt, however never apply power with the IC removed or the board will definitely be beyond repair.
Short on 3.3V LDO is never a good sign anyway.

[chrisphl]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

Need to check resistance to ground on LDOs. After removing the IC since it's burnt, however never apply power with the IC removed or the board will definitely be beyond repair.
Short on 3.3V LDO is never a good sign anyway.

Thanks for the advice! :-)

What do you think, is it a 3V3 short to GND for sure?

[mon2]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

If you pay a bit more for shipping, the ETA will be September. You can view the options inside their app or on your PC after you login.

[chrisphl]

Re: Dead Asus VivoBook S13 (Model: S330U); Help on repair wanted.

If you pay a bit more for shipping, the ETA will be September. You can view the options inside their app or on your PC after you login.

I just ordered two at aliexpress and two more pcs an ebay which shall arrive between 6. and 8. Sept.
That's OK for me.

To speed things up, would it be OK to desolder the destroyed chip and inject 3V3 on LDO3 to determine some hardware that's getting hot which should be "short to GND"?


Kind regards
Christian