Asus Vivobook x421a

[DJbapt59]

Hello everyone,

I recently picked up a laptop that was about to be thrown away to try to repair it. I've tested quite a few things, but I don't know where to look anymore. Could you please help me?

Brand: Asus Vivobook
Motherboard reference: x421da, Rev 2

Context:
The person who gave it to me mentioned it might have been exposed to water, but they weren't sure. After opening it up, I couldn't find any signs of water damage or oxidation on the motherboard.

The first thing I did was to connect the PC to a power supply: 19V stable and 2.5mA of current draw.

Voltage measurements:

• 19V at the shunt resistor.
• I then tested the voltages at all the coils, and they were all at 0V.

I tried to locate the 3V/5V chip (boardview and schematics were impossible to find) but couldn't identify any chip with a matching reference and location.

I did find two chips with the exact same reference (TPS51396A), located close to each other near the BIOS and the SIO. I thought they might handle 3V and 5V separately.

I measured the voltages on these chips, and they have identical readings (see image TPS51396A.png). The chip is activated, so there should be voltage at the coil, but there isn't.

Next steps:
I disconnected the power supply and measured resistance between ground and various points (see board_picture.jpg):

• Main power rail: O.L
• 1 - Battery coil: O.L
• 2 - Fan: O.L
• 3 - Unknown: 0 Ohms
• 4, 5 - 3V/5V?: 0 Ohms
• 6, 7 - CPU: 2.5 Ohms
• 8 - Unknown: 0 Ohms
• 9 - RAM: 0 Ohms

After that, I injected current step by step at coils 3 and 9 since their traces are the largest. At most, I used the following parameters for short durations (5 seconds): 0.05V - 3.5A (CC protection). I didn't notice any hot spots while using isopropyl alcohol (that's the only thing I have available).

After checking the entire board and finding nothing, I tested the voltage at the main rail again, but there was no longer any voltage. I then tested the Vgate of the main rail MOSFET and observed a 12V spike repeating roughly every second on the oscilloscope; I believe a short circuit is being detected, and the MOSFET shuts off immediately.

Resistance measurements remain the same as before the current injection, so there's no short on the main rail.

Feel free to ask for more information. Thanks in advance to those who respond kindly—if I made any mistakes, I'm here to learn, and the PC was going to be thrown out anyway.

[rogfanther]

Please re-check those resistance measurements in the coils. One probe to ground, one probe at the coils.

With those 0-ohm measurements, you should have some pretty nice shorts on the board.

[DJbapt59]

Please re-check those resistance measurements in the coils. One probe to ground, one probe at the coils.

With those 0-ohm measurements, you should have some pretty nice shorts on the board.

I check again; my probe are 0.8 Ohms... and those coils are 0.8 Ohms ... If only I had a thermal camera, it might be a very small component that I can't see with isopropyl alcohol

[Sephir0th]

This is for sure a fried APU. And with such straight shorts you should feel something (after a while), even without thermal camera

[DJbapt59]

This is for sure a fried APU. And with such straight shorts you should feel something (after a while), even without thermal camera

Thanks for your answer
If APU is fried, coils 6 and 7 would be 0 Ohms isn't it ?

[Sephir0th]

That's a way to optimistic view. What I suspect being a shorted 3.3V, 0.9VS, 0.75V and a shorted RAM supply are enough indications for my taste.

Move on, inject and observe will be the only way to figure