Asus VG278 mosfet identification ?

[momaka]

Well no luck !! I solder a AP2334GN-HF in place of Q308, put a jumper between Drain and Source of Q9, plug everything back together and no Asus logo ...

Before I made any changes, the screen was always white and now when I turn it on, it goes white a second, then black a second, and white again before stand by (amber light)

I think AUO P303-19 is bad ??

I wouldn't jump to that conclusion yet.

With the more powerful transistor in place of Q308, it looks like your monitor's power supply is now cycling On and Off due to the overload

Again, there are three boosted rails on every T-con board:
* AVDD (typically 12-20 Volts)
* VGH (about 25-35V, very low current for driving the TFT on)
* VGL (about -5 to -10V, very low current for driving the TFT Off)

If any of these rails are shorted to ground, that can make the main switching MOSFET to overheat / die, blow a fuse on the T-con, or simply make the PSU power-cycle.

Now, your T-con is a bit more strange from what I'm used to seeing. Normally there is only one switching MOSFET and one inductor. But here, I see three MOSFETs / transistors: Q308, Q301, and Q302... and likewise four inductors.
One of these MOSFETs is for AVDD generation. Not sure if that was Q308 that you just replaced. Normally, VGH and VGL rails are derived via charge-pump circuit (a combination of ceramic capacitors and diodes taking the pulsed current from the inductor for AVDD.) But here, it's possible that VGH and VGL have their own boost circuits separate from AVDD.
In any case, since Q308 was overheating, let's follow that lead.
First, was the old Q308 transistor bad? (Shorted between any of the pins?)... or just overheating due to being over-loaded?
Either way, I would suggest to check the following resistances relative to ground (i.e. keep black multimeter probe on ground and measure resistance with the red probe):
- Anode side of diode D301.
- Cathode side of diode D301... I suspect D301 is used for AVDD generation, since it's the largest diode in that area.
- Anode and Cathode side of diode D350
- all pins of Q308, Q301, and Q302

Again, do all of these resistance tests relative to ground... and obviously with no power to the board. Let's see what values you get so we could hopefully make more sense of this T-con board's circuit.
Alternatively, if you like reading datasheets, see if you can find a datasheet for AUO P303-19 and see if it has a sample / application circuit. Usually the application circuit looks pretty close to what may be on the board.

As for the Q302 marked "W1".... I'm getting BFT92 (PNP transistor) as the most likely result. However, I will confess that I'm not that good at finding datasheets for SMD parts, so I'll leave that for the more experienced members here to confirm (or disprove.)

[guizmo1967]

First, was the old Q308 transistor bad? (Shorted between any of the pins?)... or just overheating due to being over-loaded?

I wanted to check if it was shorted between pins but it slip through my fingers and I lost it !! It was getting really hot when I injected 2V, 1A but I don't think that's over-load ?

Since P303-19 is a DC-DC converter IC, I'm pretty sure that Q301, Q302 and Q308 are all mosfets !

Resistance relative to ground:

Q301
Gate 26MOhms
Source 55KOhms
Drain 75KOhms

Q302
Gate 75KOhms
Source 58KOhms
Drain 0 Ohms

Q308
Gate 3.7 Ohms
Source 0 Ohms
Drain 58KOhms

D301 (tested good in diode mode, both ways)
A 0 Ohms
K 54KOhms


D350 (tested good in diode mode, both ways)
A 880 Ohms
K 0 Ohms

I check again every capacitors in that board and non are shorted. The +12V that comes out of the connector (From the main board) is not shorted either !

[momaka]

Q308
Gate 3.7 Ohms
Source 0 Ohms
Drain 58KOhms

Yeah... that points to a bad AUO P303-19 IC... unless your new MOSFET shorted again (probably will have to check to remove.)

Looking for a P303-19 on the internet, look what I found:
https://www.eevblog.com/forum/repair...g-a-component/

Appears to be a different monitor but with the same T-con and very similar problem.
You can see what the OP did to get the monitor fixed.

Q302 looks to be OK in your case, so probably can leave it be. It's -NOT- a PNP transistor as my searches suggested. More likely, it's a P-ch MOSFET. 0 Ohms resistance to ground on Drain pin is normal, since Drain is connected to ground through an inductor. It's a negative voltage boost converter... and from that thread on EEVlog, looks like it's outputting -7V, which is pretty much inline with what one should get for VGL voltage. Anode side of D350 is the output of this negative voltage (VGL.)

I check again every capacitors in that board and non are shorted. The +12V that comes out of the connector (From the main board) is not shorted either !

Yup, probably AUO P303-19 is indeed bad. :\

[guizmo1967]

Yeah... that points to a bad AUO P303-19 IC... unless your new MOSFET shorted again (probably will have to check to remove.)

When I removed Q308, I check with my multimeter just for the heck of it the pads and to my surprise the Gate and Source had continuity even with no mosfet, was this normal ??

I have a question, what should be a good resistance value for the Gate of Q308, that goes to the AUO P303-19

Oh well ! Guess I'll have to order a new AUO P303-19 ...

[momaka]

When I removed Q308, I check with my multimeter just for the heck of it the pads and to my surprise the Gate and Source had continuity even with no mosfet, was this normal ??

I have a question, what should be a good resistance value for the Gate of Q308, that goes to the AUO P303-19

No, not normal.

Gate resistance of the IC should be in the high KOhm range to possibly a few MOhms, like Q301 and Q302.

[guizmo1967]

I ordered five AUO P303-19 but it's gonna take 3-4 weeks for delivery .... Hope they are not fake !!

I will update this thread when I receive it.

[momaka]

Sounds good, indeed let us know how it goes. Seeing that there other failures of this same exact T-con with the same exact IC, it would be nice to get to a resolution with this.
My experience in the past with cheap ICs is that they are usually not fake (counterfeit? -yes, probably, but that's a different story) and have a good chance of working OK, so long as they don't have some kind of a large power transistor inside (e.g. the STK audio modules that are infamous for burning out at much lower output power than genuine ones.)