Asus N751JK no power, no sign of life

[Francisc Records]

Hello,

First of all im glad i've found this forum and i'm happy to start learning new things and sharing my knowledge with others!

Long story short : I have a laptop from work that has no power and no signs of life. My first question is: A 90W charger(19V 4.5A) is enough for this laptop? Because the original charger was 19V 6,4A(120W), so all the testing was done with the 90W charger because i don't have a 120W charger.

This being said, i opened up the laptop and i've inspected the MB and the components, everything seems fine, no sings of explosion, burn, or ripping. Then i proceed in measuring the input voltage and output voltage of first and second mosfets, everything seems ok. After a while i noticed that the laptop is taking 0.16 amps all the time, so it must be a shorted ground. Finally i've found that the tps51285a ICs pin 13 (+5VAO) is shorted to ground. Could this be the issue?

I measured resistances relative to the ground of the IC tps51285a:

1 CS1 = 39k
2 VFB1 = 6k
3 VREG3 = 35k
4 VFB2 = 9.4k
5 CS2 = 15k
6 EN2 = 350k
7 PGOOD = 17.7k
8 SW2 = 15k
9 VBST2 = 45M
10 DRVH2 = 126k
11 DRVL2 = 45k
12 VIN = 2M
13 VREG5 = 3.2 !!!!!!!!!!
14 VO1 = 15k
15 DRVL1 = 140M on pin but 4,7 Ohm on PQ8103
16 DRVH1 = 66M and rising -> but 130k on PQ8102
17 VBST1 = goes over 200M
18 SW1 = 15k
19 VCLK = 66k
20 EN1 = 300k

The output voltage of the 3.3V pin is correct.

The output voltage of the 5V pin shows 0.5V

The battery pins voltage on the MB shows 2.6V

At this point i have no clue where exactly is the short to ground, i know it's on the 5V rail but can it be the IC's fault?

Thanks in advance.

Link to boardview https://www.badcaps.net/forum/docume...ptop-boardview

[mon2]

Hi. Welcome to the forum. Good work on the review.

yes, this rail is shorted. Often the 5v rail is shorted due to the usb port which are linked to failed usb load switches. These are current limit switches.

Continue your review to inspect each vbus pin on each usb connector. Also shorted? Test the resistance to ground. Next, take a pic of the local parts of the usb connector before reworking. Flux and remove the load switch. Measure the resistance again. If ok, power up but the 120w adapter should be used.

[Francisc Records]

Resistance from gnd to Vusb for each usb is 1.6-1.8M

Resistance from the +5VAO pin of the chip to ground is 4.5 ohm (yesterday was 3.2 ohm)

+5VAO is going to a single capacitor (MLCC 4.7UF/6.3V), i thought that capacitor is shorted so i desoldered it, i still have the short. According to the schematics, the +5VAO is not going anywhere else.

I injected 1V on the +5VAO pin, and the only thing heating up was still the IC, so at this point i'm 90% sure the IC is shorted to ground. I won't mind for any suggestions though .

[mon2]

You may be right. Suggest to take pics of this regulator area and use low air pressure but high enough heat to melt the solder and remove without disturbing other parts in this region. Watch the pin #1 location for the future replacement. Then be sure the PCB pads are not shorted and test (without power) the resistance to the same rail. Was the short removed? If not, keep injecting a low voltage.

You are getting close...

[Francisc Records]

You may be right. Suggest to take pics of this regulator area and use low air pressure but high enough heat to melt the solder and remove without disturbing other parts in this region. Watch the pin #1 location for the future replacement. Then be sure the PCB pads are not shorted and test (without power) the resistance to the same rail. Was the short removed? If not, keep injecting a low voltage.

You are getting close...

Indeed. Thanks for the tips and suggestions.

I ordered the IC. Now i hope it's shorted, because I've spent way too much time diagnosing this laptop. I'll keep this thread up to date.

[Francisc Records]

After removing IC short to ground disappeared. I'm waiting for the new IC to arrive and probably the laptop is fixed.

[mcplslg123]

Reminder-dont power up up the pc without this 3/5V regulator soldered. Else the mb may become unrepairable.

Lot of guys commit this mistake.

[Francisc Records]

Replaced IC, now I have a short on the main power rail. A P2003BEA mosfet is heating up and it shuts down the power every few seconds (main power tries to rise, gets cut at 1.5V. At this point I don't think its feasible to continue working on this laptop.