Originally posted by guizmo1967
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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.)
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