Asus K55VM no power

[senso]

Good morning, I'm trying to revive a dead Asus K55VM motherboard.
I have measured some voltages and already tried changing a couple components in an identical case without success.
The problem with both of them is that right after the DC-IN there are two mosfets for reverse polarity protection, those mosfets never turn on, they have about 600-800mV at the gate.
The BQ25A has 2.6v in the AC-DET pin and 2.8v in the AC-OK output, the Vcc for the BQ25A is 18.8v, all this when powering the board using a Rigol DP832A power supply set to 19v, current consumption measures 0.0001A.

In the other board that has the same problem I tried exchanging the two mosfets and the BQ25A but with no success, but I used a BQ25A from another dead laptop, maybe the BQ25A chips are fragile and I should buy some new ones?

Any ideas of what the problem might be?
Saddly I can't find schematics for this board.

Best regards, and thanks in advance.

[atsio]

Re: Asus K55VM no power

Post photos of the area around BQ25A and fets.

[senso]

Re: Asus K55VM no power

I have attached two images, one is of the BQ25A and the mosfets area, the other is from the other side of the board, were the dc-in cable connects into the board.

[atsio]

Re: Asus K55VM no power

What is the voltage on all pins of those two fets as well on the fet on the other side next to pjp9002?

[senso]

Re: Asus K55VM no power

The first mosfet in the first picture measures the following:
Pin 1: 800mV
Pin 2: 800mV
Pin 3: 800mV
Pin 4: 800mV
Pin 5: 19v
Pin 6: 19v
Pin 7: 19v
Pin 8: 19v

The second mosfet measures 800mV on all pins.

The mosfet next to the PJP9002 text:
Pin 1: 1V
Pin 2: 1V
Pin 3: 1V
Pin 4: 800mV sometimes oscilates to 1V, might be related to the probe contact
Pin 5: 800mV
Pin 6: 800mV
Pin 7: 800mV
Pin 8: 800mV

[atsio]

Re: Asus K55VM no power

What is the voltage on pins 1,2,4 and 19 of BQ25A? What is printed on those three fets (make, model)?

[meritosari]

Re: Asus K55VM no power

link photo HD quality for reference
http://rusfolder.com/37705413

[atsio]

Re: Asus K55VM no power

Thanks for the photos. It would be nice if somebody could post and the boardview files.

[senso]

Re: Asus K55VM no power

The mosfet near the PJP9002 text is marked:
A5 GNA
802809

The two in the other side of the pcb are both marked:
A5 GNB
100H09

Voltage on BQ25A pins:
Pin 1: 800mV
Pin 2: 800mV
Pin 3: 800mV
Pin 4: 800mV
Pin 19: 1V

[senso]

Re: Asus K55VM no power

I have found a boardview file for this board in a website that I can make downloads:

https://drive.google.com/file/d/0B-M...ew?usp=sharing

[atsio]

Re: Asus K55VM no power

I suspect if you supply 19v at pins 5 to 8 of the second fet (the one that reads 800mV on all pins) the board will start. My personal experience is that BQ25A rarely fails and usually the problem lies with a problematic fet. You mention that you changed the two fets (I do not know if you have checked the replacement ones before replacing). Try to replace and the third.

[fddizon]

Re: Asus K55VM no power

check gate voltage on second fet. if same with source, check 2 resistors coming from gate to ground and going to source.

also, check resistance of 2nd fet's drain pins to ground if shorted.
if the above are all good then feed 2v 1.5amps from dc bench and check warm or hot components, if none increase amps gradually while looking for warm components.

hope this helps

[kolumbo]

Re: Asus K55VM no power

hi. i have the same problem with one asus ux32v this part A5 GNA it´s burn when i try to connect the power. how much voltage must be have this integrate ? and were i can find to buy this part?
thanks .