MS-16J91 help in identifying a component

[mon2]

Your ACDET voltage is good. Most likely the ACOK signal is attempting to output a logic high but unable to do so due to the missing +3VALW ldo rail.

At this time, do not believe the 2nd mosfet should be on yet it is passing the +19v onto the logic board. However, even with this leakage voltage, the +3VALW is missing and is required.

From where do you plan to purchase the parts for the repair? For sure you need to replace this regulator. Suggest also the dcin mosfets. Each are inexpensive so buy spares.

do you have hot air tools? Flux? Ipa alcohol? Tweezers? Practice on a donor or dead board to remove parts. Then flux and replace the 2nd dcin mosfet.

power up again to check if the new mosfet remains off since gate voltage is too high to be on.

then review location of pin #1 on this square regulator (qfn package = quad flat pack = pins on each of the 4 sides). Flux it well + hot air with low pressure to not lose the other parts. Then lift the part with a gentle nudge of the tweezers. If you rush it or too much heat then the board will burn or the pcb tracks will lift.

[Fronczek]

Alright, got it! I have all of these tools available. I purchased the buck controller from a shop closer to my location, apparently they get their supply from Mouser. Should I also purchase the exact mosfet or will one from a donor board suffice?

[mon2]

A donor mosfet can work but criteria is as :

1) same type so p-channel
2) same size- check the dimensions in the datasheet of each part
3) same or higher source / drain voltage of 30 volts.
4) same or higher rated current as the original

[Fronczek]

All of it will be in the datasheet right? The regulator arrives on Tuesday so see you then!

[piernov]

Mosfet PQ8:
19.4V on source
19.9V on drain
19V on gate

If the gate is at the same voltage as the source / drain pins then the mosfet should be OFF. Perhaps this mosfet is defective and needs to be replaced. PQ8 = P-channel mosfet so for this mosfet to enable, the GATE voltage must be LOWER than the source / drain voltages yet there is voltage on each side of the mosfet.

Current is flowing through body diode of MOSFET, as seen by the diode voltage drop from source to drain. Gate not pulled low because ACOK not pulled up because +3.3VALW is missing since 3V/5V regulator has been damaged.
If MOSFET wasn't letting current through without being turned on, there would be no way to power the 3.3V/5V circuit so no way to turn on the MOSFET.

There was no need to work on any of these circuits if the voltages were good, and as always the first thing in a "no display" situation is to clarify if it's "no backlight", "no image" or "no POST" or even "not turning on".

[Fronczek]

Alright, I replaced the regulator and my ALW lines are back.

Piernov, I'm not trying to just get an answer to my issue and be gone, I want to actually learn how to check circuits.
The issue is that it's not giving image nor backlight. It's not detected by bios nor OS, and we already established that it's an issue with the motherboard not the LCD screen.