Asus X555U dead (seeking help)

[Urnacz]

Hi,
I have an Asus X555UB-DM037T (out of warranty) that died on me. No status lights, not charging, nothing. i7 6500U,4+8GB ram, GTX940M 2GB.
I have two identical laptops one just stopped working. Both used in the same office, both mostly at wall power with little to none travel out of the office. No water or fall damage, user claims it has not been used with an incorrect power adapter (95% trust factor ) The dead board draws around 7W from the wall, while good board draws close to 0W (both without battery).
Component (in attached photo) outlined in red heats up (not like 200°C but i can feel it on my finger, and alcohol evaporates only on this chip).
I have not found any schematics for this board (exept a googled post about mosfets closest to the dc-in jack - they seem to work fine). I have "some" practical repair experience, but without a guide or schematics i am kind of lost. - Where shall i start?

Tested in working machine:
adapter - OK
battery - OK (works/charges)
RAM (the detacheable part) - OK
HDD board + ribbon extension with usb - OK

Fun part at the end, the good board has a light water damage(around on-board ram and touchpad port) when the dead one is spotless.

Any help would be appreciated.

[BlueMidnight]

Re: Asus X555U dead (seeking help)

It's hard to see but I think the markings on that chip begin with "2N=", which would make it a Richtek RT8249C 3V/5V regulator. It is responsible for controlling 4 power rails:

- a 3V and a 5V linear regulator
- a 3V and a 5V switching/buck regulator

The chip should not be noticeably hot during normal operation. The chip controls 4 nearby MOSFETs to generating the switching outputs. However, the linear outputs come from within the chip. Therefore, when the chip is getting hot, it is usually because there is a short (or otherwise high current draw) on one of those linear outputs. Alternatively, it is also possible for the chip to have an internal short of some kind.

I also do not have the schematics for this board. However, this one has a wonderful feature in the form of solder jumpers connecting the various sections.

PJP8702 and PJP8704 on the top side of the board appear to be the solder jumpers for the 3V/5V switching outputs. You can look at the bottom of the board and see if any of those solder jumpers in that area have continuity to the 3V or 5V linear outputs (pins 3 and 13 on the Richtek chip). Be aware that sometimes only one of the linear outputs has a jumper.

If you desolder a jumper and the chip no longer gets hot, you know you've found the power rail with the problem. From there, it is just a process of elimination to find the actual problem. Of course, this is much slower process without a schematic..

It is also good practice to unplug the board and measure each of those 4 rails with your multimeter in diode mode. Connect the red lead to ground and the black lead to each of the 4 outputs (pins 3 and 13 for the linear outputs, and each of the 2 big grey coils next to the Richtek chip for the switching outputs).

[Urnacz]

Re: Asus X555U dead (seeking help)

Thanks fot the post,
I also suspect a short somewhere - that is why i started the "Find the heat" quest. I found this older post ( https://www.badcaps.net/forum/showthread.php?t=63668 ) but that alone did not help me.
My part is labeled :
2N=2K
T1X

I already measured (some) pins.

1 0v
3 ??
5 2,010v
6 3,253v
8 3,287v
9 4,59v
11 0v
13 5,00v (LDO5) 5V OK
17 4,61v
20 3,247v

I will fill in the rest tomorrow ( or in like 8 hours when my work day starts). I'll also check the jumper connections.

Thanks

[Urnacz]

Re: Asus X555U dead (seeking help)

OK, since i am unable to edit/delete my own posts here we go >

3 3,288v
13 5,00v
(measured before removing following components)

The PJ8704 had a short to ground, so after i cleared the jumper connection, and followed the sort to ground... now after removing a few components, red indicates ground at pads of removed components. In white are highlited two jumper connections that are no longer jumpable.

Getting closer and closer to CPU/GPU and everything is grounded

[BlueMidnight]

Re: Asus X555U dead (seeking help)

Okay, pins 8 and 18 of the Richtek chip are connected to the outputs of the switching regulators. You measured 3.3V on pin 8 and I don't see pin 18 listed. So I will assume that the 5V switching output is the one with the short for now, unless you have info pointing in another direction.

It's necessary to be a little more methodical about finding the short. Many/all of the components you removed are not being fed by the 5V switching rail. The CPU and GPU voltage regulators are powered directly from the AC Adapter.

You must also be careful about checking for shorts around the CPU/GPU because they are very low voltage, high current chips. This means that they typically have very low resistances to ground (even less than an ohm). Ohm's Law: resistance = voltage / current. And because they have a low resistance to ground, that means they will also have a low resistance to anything else that has a short to ground. So, if you are checking continuity to find where the 5V output goes, this can get confusing.

Typically, the 5V switching rail is used to directly power other ICs on the board, or connected to data/clock lines through pull-up resistors, etc. The best way to find a short like this without a schematic/boardview would be to inject some voltage with a current-limited bench power supply and see what gets hot. Do you have a bench power supply?

But first, to be clear, which side of the solder jumper PJP8704 is the short on? Is it the side closest to the grey coil or the other side?

[alpha325]

Re: Asus X555U dead (seeking help)

It,s good