Second opinion on g750js motherboard power circuit.

[bjhawfam]

Working on a g750 and found the common power problem around the plug in.. I removed the cap (out of circuit) pce6000 and the meter showed it was bad. Replaced with a new 15uf (tested) and also replaced pc8805 22uf and pc8812 1uf. I believe the pq8801 is bad, I get 19volts everywhere in the picture with the yellow dots.. pq8801 pins 1,2 and 3 are not 19volts nor pc8812 and pq8801 pins 1,2 and 3 no voltage... My question is if it's bad would anyone know of a replacement mosfet (markings)...

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

Went ahead and just ordered both mosfets.

[BlueMidnight]

Re: Second opinion on g750js motherboard power circuit.

They are pretty standard FETs, so no harm in having a couple on hand. But are they bad? It's unclear with the information you've given so far. It's not impossible. They aren't really related to the capacitor failure in any direct way, but if there was a voltage spike on the input it could have damaged multiple components at the same time.

What is the voltage on pin 4 of the FETs? In other words, is PQ8801 being told to turn on but isn't? If so, then it's bad. Otherwise, you'd need to investigate why (e.g. is there a short on AC_BAT_SYS, etc).

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

Good to hear because I ordered a handful each.

My assumption was they were bad I get nothing from pin 1,2,3,4 ..

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

Good to hear because I ordered a handful each.

My assumption was they were bad I get nothing from pin 1,2,3,4 ..

Got nothing at pin4 (gate) not being told to turn on?

[piernov]

Re: Second opinion on g750js motherboard power circuit.

It's quite common for DC in MOSFETs to go bad in those Asus laptops, but it's not always the case. You would have to check if they are shorted.

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

I will pull them and check. You are absolutely correct fixed a few Asus laptops from bad fets..

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

It's quite common for DC in MOSFETs to go bad in those Asus laptops, but it's not always the case. You would have to check if they are shorted.

The mosfet is actually good, pulled it out of circuit and the fet isn't shorted.

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

Okay finally had time to remove pq8802,,, that mosfet is bad (short)...pq8801 is fine.. When the new chips get here... I will report back...

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

Also noticed that the boardview shows 8801 and 8802 as the same mosfets,however, 8802 was a 30v 48a mosfet not the 30v 106a on the boardview..

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

Okay got the mosfets today and replaced them both. Now the charger pulses (shorts) and I noticed the capacitor between the two mosfet heats up really fast (hot).. I replaced the cap, however, the only info I have to go off is the boardview. Boardview shows a .1uf_25v cap, that's what I used and the result is the same. Somethings still not right. I did try two of the 4983nf mosfets as the boardview shows and still the same result..

EDIT: It's the resistor that heats up,,,, I put flux on each chip and applied power to the board and the resistor melted the flux fastest.. I then put my finger there and sure enough the resistor gets hot and not the cap.. Will replace the resistor and check in (boardview shows a 4.2kohm)....

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

No luck, the cap heats up now and the charger shorts.

[myth77]

Re: Second opinion on g750js motherboard power circuit.

Remove PR8806, check then is it shorting when you plugg the charger? also when you removce PR8806 check resistence to ground on pin 1 and 2 ,p_chg_path_19v shape and ac_bat_sys? this is just to eliminate some things.
Also what is resistence between ac/dock in to the ground?

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

Remove PR8806, check then is it shorting when you plugg the charger? also when you removce PR8806 check resistence to ground on pin 1 and 2 ,p_chg_path_19v shape and ac_bat_sys? this is just to eliminate some things.
Also what is resistence between ac/dock in to the ground?

Removed pr8806 and no short to the charger... pin1 is nothing OFL and pin 2 is 0.00....

[myth77]

Re: Second opinion on g750js motherboard power circuit.

So, pin 2 ac_bat_sys shows 0.00 resistence to the ground, it means ac_bat_sys is shorted to the ground. The 19v mosfets were not your only problem...
If you look at the boardview and select net ac_bat_sys you will see that it is connected with many other nets through motherboard...and short to the ground can come from any of those nets through bad capacitor, mosfet, chips or joints...My suggestion is to start with the joints (PJP8001....) eliminate it one by one...

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

So, pin 2 ac_bat_sys shows 0.00 resistence to the ground, it means ac_bat_sys is shorted to the ground. The 19v mosfets were not your only problem...
If you look at the boardview and select net ac_bat_sys you will see that it is connected with many other nets through motherboard...and short to the ground can come from any of those nets through bad capacitor, mosfet, chips or joints...My suggestion is to start with the joints (PJP8001....) eliminate it one by one...

Sounds fun, thanks and I'll get started on it later today and check back in when I found the culprit. I figured a short when I checked ac in..

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

Unfortunately, the boadview isn't the exact one I need. Good for reference, however, my ASUS is the g750js board. I did check all that was listed on the BV but no changes and the charger still shorts...

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

Just for shi*ts and tickles, I removed the the joints pjp8001 and pjp8002 and I do get lights on the power button and the charger stays on... Will dig further.. We're close team don't give up on me...

I'm guessing at this point the short would be after that circuit which has 3 mosfets and some caps, reistors, etc... Not many, so I'll remove each one until the short is found.. There is in fact a short after the joints..

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

FOUND a short in the pq8002 mosfet,, removed the mosfet and replaced with a new one.. Before turning on the laptop I insured there was no short to ground ( all was good ).... When I gave the laptop power the mosfet pq8002 was blown instantly.. Removed the mosfet and checked gate, source, drain with multi-meter and it too is now blown??????

I BELIEVE THE PROBLEM IS FOUND !!!

[bjhawfam]

Re: Second opinion on g750js motherboard power circuit.

The board is shot.. The problem was bad mosfets from ali-express... The 4983nf mosfets they sent were the problem.. Tried it twice and it literally rendered the motherboard useless... The mosfet whined and burnt the pad off.. Tried a second and the board is now toast... I'll stick to mouser.. 4955n mosfet actually was protecting the board by blowing first..