LA7401P Short to ground

[alfasys]

Hi there.
I have a LG laptop that was completely off. I open and check on the board, there was 2 burnt mosfets (PQ401 and PQ402). I removed them from the board and tried to plug the laptop, the laptop switches on and goes off after a few seconds. Now I replaced both mosfets, the laptop don't switch on and PQ401 is overheating(very hot). Where can I test and how can I resolve this issue? Any help is much appreciated. I have attached the schematics and a pic of the affected area.

[caspian]

Re: LA7401P Short to ground

measure resistance to GND on all pins of PQ402.

[alfasys]

Re: LA7401P Short to ground

Must I put back PQ402 on the board? because I removed both pq401 and pq402

[caspian]

Re: LA7401P Short to ground

now that you removed them, just measure resistance on the empty pads of the two transistors on the board.

[alfasys]

Re: LA7401P Short to ground

ok.
pq401
pin 1 to 3 = 391 ohm
pin 4 = displays 1700 ohm then meter goes to 1
pin6 to 8 = displays 1700 ohm then meter goes to 1

pq402
pin 1 to 3 = 008ohm
pin 4 = 1415 ohm
pin6 to 8 =225 ohm

ps: I put the negative prove on the ground and the positive probe on pins to test. is that okay?

[caspian]

Re: LA7401P Short to ground

it is ok.
now measure voltage on pin4 of pu401 before pressing the power button.
then measure voltage on pin1 of pu401 after pressing the power button.

[alfasys]

Re: LA7401P Short to ground

pin4 of pu401 = 4.38V before pressing the button and pressing the button it stays the same, 4.38V.

[caspian]

Re: LA7401P Short to ground

what about pin1 of pu401?

[alfasys]

Re: LA7401P Short to ground

Hi there
Pq401 Voltage:

pin 1 to 3 = 0.02v
pin 4 = 4.39 v
pin 5 to 8= 18.99v

Pq402 Voltage:

pin 1 to 3 = 0.0
pin 4 = 0v
pin 5 to 8 = 0.02

ps: both Mosfets are off the board. I am only testing the pads.

thank you for your assistance

[caspian]

Re: LA7401P Short to ground

pin1 of pu401 is important.

[alfasys]

Re: LA7401P Short to ground

what do you suggest?

[alfasys]

Re: LA7401P Short to ground

PU401 pin 1 = 0.1v

[caspian]

Re: LA7401P Short to ground

you must measure this pin less than 1 second after pressing the power button.

[alfasys]

Re: LA7401P Short to ground

Cool, PU401, pin 1
before pressing the button= 0.01
After pressing the button = 0.03

what does this mean?

[caspian]

Re: LA7401P Short to ground

it means the laptop has also got another problem on some power rail.

[alfasys]

Re: LA7401P Short to ground

5V alw and 3V alw are present before pressing the button, what other power rail can I check?

[alfasys]

Re: LA7401P Short to ground

Could you please quickly explain to me how this 1.5 circuit works (Pq401, Pq402 and the PU401 that controls the mosfet?

[caspian]

Re: LA7401P Short to ground

I think the problem is somewhere else on the board.
now you have to check voltage on the following pins of KB930 after turning on:
26, 65, 76, 95, 100-to-117, 121, 125.

[alfasys]

Re: LA7401P Short to ground

Hi Caspian. I tried to remove the inductor, PL401 these are the measurement I got:

PQ401
pin 1 to 3 = 1.70v (before pressing the button) 3.56v (after pressing the button)
pin 4 = 4.26v (Before and after pressing the buttonn)
pin 5 to 8= 18.7v

PQ402
pin 1 to 3 = 0.01v (before pressing the button) 0.03v (after pressing the button)
pin 4 = 0.03v (Before and after pressing the buttonn)
pin 5 to 8 = 1.70v (before pressing the button) 3.56v (after pressing the button)

[alfasys]

Re: LA7401P Short to ground

So I assume that there is a problem in the 1.5V power rail on the board side. what is your thought about this? Thank you for your responses