Asus X401A Dead

[orientalsniper]

Hello all, I'm working on this Asus, first thing I did was measure EEPROM for the 3V, which was missing and the capacitors around the power IC. So I desoldered it and programmed it with a downloaded dump. When I soldered the EEPROM back I was reading the 3V, normal voltages (non 0V) in the capacitors around the power IC and 3V in the power button.

But as soon I pressed the power button, I lost all the voltages reading. I reprogrammed BIOS but now I'm one step back.

And I can't seem to find the schematic.

[alihomee]

Re: Asus X401A Dead

try to find any short on motherboard

[orientalsniper]

Re: Asus X401A Dead

I couldn't find any heated component with a lab power supply. I'll see what else I can do tomorrow.

[orientalsniper]

Re: Asus X401A Dead

Does anyone know which control the power LED? The KBC or the BIOS? I'm starting to think the KBC might be the cause since programming software verifies successfully the eeprom and BIOS buffer. And nothing gets heated in the board.

[dellxps15]

Re: Asus X401A Dead

do you have also 3.3 volt on the pwr botton pin ?

[orientalsniper]

Re: Asus X401A Dead

Came with no 3.3v, reprogrammed BIOS and it had 3.3v. Then when I pressed PWRBTN, I lost 3.3v, so I no longer have 3.3v as of now.

[dellxps15]

Re: Asus X401A Dead

that 3.3 volt should coem from SIO chip. then after pressing pwbtn it should go to northbridge.

[orientalsniper]

Re: Asus X401A Dead

Yes, but I don't have any readings now, not even the capacitors near the power IC, right now I'm swapping different KBC chips from other dead boards.

[orientalsniper]

Re: Asus X401A Dead

Also to note is, I can measure 19V in the capacitors near the DC power jack, however I get 0V in the rest of the board.

[spleenharvester]

Re: Asus X401A Dead

Maybe there's a shorted cap somewhere? Haven't seen the schematic but perhaps bringing 3.3V on the power button low opens a transistor somewhere, which then involves the shorted cap in the circuit.

[orientalsniper]

Re: Asus X401A Dead

Any idea on how to locate this shorted component? I have a lab power supply, but haven't had much luck with it.

[3d0]

Re: Asus X401A Dead

Any idea on how to locate this shorted component? I have a lab power supply, but haven't had much luck with it.

I have 2 dell motherboards with the same problem, though I was able to narrow down where the short might be by checking the coils first.
You might already know this, I will put it in the attachment just in case someone is interested.

[orientalsniper]

Re: Asus X401A Dead

I have 2 dell motherboards with the same problem, though I was able to narrow down where the short might be by checking the coils first.
You might already know this, I will put it in the attachment just in case someone is interested.

Wow, I didn't, thank you a lot. I found a short in a coil against ground, so I desoldered it and one side was shorting while the other was not, so I applied current there, but the only thing heating was the SJTNV BGA chipset. Is that a false positive?

[3d0]

Re: Asus X401A Dead

Possibly, but what I would do next is lift the bga chipset and check for the short you had. Most likely it's not the problem, and there's a big chance it's some small shorted capacitors.
Anyway it's up to you, it does add a lot of work, but you never know until you do it.
Also check all the mosfet's before you try it.

[orientalsniper]

Re: Asus X401A Dead

I don't think it's the BGA either, the laptop is very new, also I don't have infrared machine, I can't resolder it back if I take it out with my hot air machine.

Here are 4 pictures I took: http://imgur.com/a/qPKi9

The caps in the processor socket are shorting against ground. Do you still think taking out the BGA is the absolute next option?

[3d0]

Re: Asus X401A Dead

Don't do it unless you have done it before, you might damage the pads on the mb.
Leave the chipset for now I don't think it's the problem, there are other things to rule out first. Do you have the 3.3V and 5V on the regulator IC.
Searching for the short is a pain in the ass isn't it, having no schematic makes it even worse.

Maybe someone has some ideas what could be the problem?

[khaahk]

Re: Asus X401A Dead

"I found a short in a coil against ground"
Define some terms:
what do you call a short, how many ohms?
If a cpu socket cap is shorted (you measure across the cap ~ 0 ohms), then the same resistance should also be in cpu power coils (without the cpu), is it so? If it is, then use a 1V from lab supply to find the short...
The desoldered coil does not look like it is from cpu power rail.

[orientalsniper]

Re: Asus X401A Dead

Don't do it unless you have done it before, you might damage the pads on the mb.
Leave the chipset for now I don't think it's the problem, there are other things to rule out first. Do you have the 3.3V and 5V on the regulator IC.
Searching for the short is a pain in the ass isn't it, having no schematic makes it even worse.

Maybe someone has some ideas what could be the problem?

I have practiced before taking out BGA chips, but I haven't had much success putting them back since I lack a stencil tool. I was starting to measure RT8206L, had 3.5V at pin 14 and 1V at pin 13V, couldn't measure the rest cuz I had to do something, when I got back they were all 0V.

"I found a short in a coil against ground"
Define some terms:
what do you call a short, how many ohms?
If a cpu socket cap is shorted (you measure across the cap ~ 0 ohms), then the same resistance should also be in cpu power coils (without the cpu), is it so? If it is, then use a 1V from lab supply to find the short...
The desoldered coil does not look like it is from cpu power rail.

I was using the continuity test in the Fluke DMM. If it beeped I called it a short. And for some reason now, only 3 out of the 8 capacitors are showing shorts.
With the coil removed, the shorted pad has 15.4 ohms and the other pad has 180k ohms.

I attached picture for better visualization.

[khaahk]

Re: Asus X401A Dead

Forget the continuity test when looking for shorts on laptop mainboard. 15 ohms is normal resistance for low voltage rail which supplies northbridge, 6 ohms is normal resistance for graphics chip.
I modded the picture to show the probable schematics of the mainboard in the cpu socket. One side of the caps is connected to the ground so 0.3 ohms is ok, the other side is connecting to the cpu voltage rail, this must show high resistance when cpu is not installed, as it does. So you do not have short in these rails. Look for short elsewhere or some other fault.

[orientalsniper]

Re: Asus X401A Dead

Forget the continuity test when looking for shorts on laptop mainboard. 15 ohms is normal resistance for low voltage rail which supplies northbridge, 6 ohms is normal resistance for graphics chip.
I modded the picture to show the probable schematics of the mainboard in the cpu socket. One side of the caps is connected to the ground so 0.3 ohms is ok, the other side is connecting to the cpu voltage rail, this must show high resistance when cpu is not installed, as it does. So you do not have short in these rails. Look for short elsewhere or some other fault.

Thank you, at least I can start looking somewhere else.