Viewsonic N2750W dead

As mentioned in my introdution post, I recently acquired 6 dead monitors and a dead LCD tv for free.
One monitor has a broken panel, but otherwise works, so I stripped it for parts,
including 4 good CCFLs which should come in very handy for testing!

So here's what happened with the first repair, the Viewsonic TV. If this looks familiar, I did post it on another site a week or 2 ago...

Comments/criticisms on my diagnosis, and troubleshooting methods would be most welcome.

The Patient: Viewsonic N2750W 27" LCD TV
Symptom: dead. No led, push the button on the front, and nothing happens.
The screen is scratched up, but it would make a good gaming TV for a friend's kids who happen to need a new one.

Here's how I went about troubleshooting:

I did manage to find a service manual on-line. No schematic for the power supply or inverter,
but schematics and wiring diagrams for the rest.

The obvious place to start is the power suppky. I fully expected a blown fuse, a shorted component or bad caps in the power supply, but it looks good.
12V out measures 11.98v. I don't see a 24v out for the inverter circuit, but it probably won't exist until the set actually powers on.

The main board has the rest of the DC power regulation on it, and the 5v, 3.3v and 2.5v regulator outputs are all within spec.
I traced the power button, and the signal on the connector to the main board goes from 5v to 0v when pressed.

Rather than shotgun a bunch of new capacitors in, I scoped the power at the 12v out, and at the output of 3 regulators, no ripple.
They are known bad quality caps (Capxon brand), but no point replacing them yet....

Next I scoped the microcontroller, I see a nice sine wave at the crystal pins, and a square wave at one of the output pins, I don't recall which one.

Ok, still stumped.
The MCU definitely has power.
When turning on the master power switch, the reset line goes high for a few hundred ms,
then goes back low. I'm a bit rusty, but if it's labelled RESET, it's active high, and RESET with a line over it is active low, right?

The reset for this chip is a simple RC circuit 10uf cap + to +v - to reset then thru a 10k resistor to gnd.

The power button signal is getting all the way to the MCU.
LED output signal is high on the MCU, but doesn't make it to the connector that leads to the LED.
I go to check transistors that switch the led (q126 and q127) and it appears that
I may have been barking up the wrong tree, or the wrong branch of the right tree....

I thought that because the MCU had the LED outputs high, that the LEDs should be on, so something was blocking that signal to the LEDs, perhaps the transistors.
But it turns out that they're PNP transistors. I'm a bit rusty on my theory, and I have always used NPN transistors when designing something that needed transistors to switch.
The LED cathodes are tied to ground, and the PNP emitters go thru a 100ohm resistor to +3.3
So that would mean the MCU would have to have that output go low to turn on the LEDs.
So perhaps the MCU isn't happy about someting.

While checking inputs to the MCU I notice one button on the top panel was inoperative. Actually stuck low.
Suspecting the switch might be shorted, I pulled the wire for that pin out of the connector. Still stuck low.
Checking further, that line, and the other inputs from the switch panel get pulled up by 10k pullup resistors to +3.3v
This particular input's pullup resistor is OPEN! Woohoo, could that be it? I wish...

Whether that would prevent a power up, is hard to say, maybe, but unlikely.
Well bad news. With a new resistor in there, that input is still held low.
The switch is fine, the line is low even with it disconnected.

So can I assume that because that input can't be pulled up, the MCU is toast?

As a last resort I pulled the chip, and the line gets pulled high no problem, and goes low on button press,
so nothing else is taking that line to ground.

I'm assuming:
A) it's a faulty MCU and
B) it's likely difficult, and not worth it, to obtain a replacement.

So one marked down as uneconomical or even impossible to repair.

thanks for sharing smason. Your troubleshooting skills/techniques are great and I really appreciate you taking the time to post them, you may feel rusty but it's obvious you retained a great deal of your electronics knowledge and yes reset with a line over it is reset on low. Sounds like you got a dog and you are correct finding an MCU is usually not an option and when it is it is because you pull it off another main board from the same model TV, but sometimes you can find a decent price on a main board pulled from a broken TV from places like discount merchant and shop jimmy. Have a great day smason and keep posting please.