Viewsonic N2750W dead

[smason]

Hi all. I recently acquired 6 dead LCD montors and a TV, plan to fix 'em all ('cept the one with the broken display) over the next little while.

The first one is the Viewsonic TV. I thought the hard part would be taking it apart, but now that I have it apart, it's not a simple fix.

Symptom: dead. No led, push the button on the front, and nothing happens.

Here's what I've found so far:

Power supply looks good, 12V out measures 11.98v. (1 in photo) I don't see a 24v out for the inverter circuit, but it might not exist until the set actually powers on.

Main board has the rest of the DC power regulation on it, and the 5v, 3.3v and 2.5v (2 and 3 in photo) all within spec. Traced the power button, and the signal on the connector to the main board goes from 5v to 0v when pressed.

Scoped the power at the 12v out, and at the 3 regulators, no ripple.
Scoped the mcu, I see a nice sine wave at the crystal (4 in photo), and a square wave at one of the pins.

I plan to trace and measure the ON signal tomorrow when I have less caffein shakes

Am I on the right track? Anything else I should be looking at?

Better photos tomorrow when there's some light.

Thanks,

Steve

[smason]

Re: Viewsonic N2750W dead

I should mention, the power board has Crapxon caps, they're visibly ok, haven't measured them yet. I'm assuming (could be a mistake) that because there's no ripple, and accurate output, that the caps aren't a problem yet.

[NxB]

Re: Viewsonic N2750W dead

Sounds like something is amiss with one of the micro controlers. All get power? All output? The power section seems fine and you have a scope.

[smason]

Re: Viewsonic N2750W dead

Thanks. I found some schematics, so tonight I plan to trace the LED on signals, power switch signal, and I'll watch the reset line on the mcu when attempting power on.

[smason]

Re: Viewsonic N2750W dead

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 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?
Forgive my bad ascii art...
The reset for this chip is an RC circuit +
|
|
--+
10uf
--
|
|------reset
10k
|
gnd

I *think* that's behaving correctly, but I'm a bit foggy today.

The power button signal is getting all the way to the MCU, but the LED signal is high on the MCU, but doesn't make it to the connector that leads to the LED. I'm looking for the transistors that switch the led now q126 and q127 as soon as I find my glasses. Durn newfangled smt components......

[smason]

Re: Viewsonic N2750W dead

Well 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 switched by PNP transistors. The LED cathodes are tied to ground, and the PNP emitters 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.
There is another chip (not listed in the parts list, this service manual is far from complete) I'll check its clock, and reset lines next.
This is a far cry from the fuse/caps/diodes I was expecting. Guess I should have started with the BenQ monitors that probably just need some 2SC5707s...

[smason]

Re: Viewsonic N2750W dead

I feel like I'm blogging here
Ok, the other chip isn't on the parts list, because it's not on the board. (options like card reader, USB ports etc.)

I may be on to something though.
While checking inputs to the MCU 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. Looks like the 10k pullup resistor on that line to +3.3v is OPEN.
Now whether that would prevent a power up, is hard to say, maybe unlikely.
I'll put in a new resistor tomorrow, and try it. Stay tuned......

[smason]

Re: Viewsonic N2750W dead

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.

[PlainBill]

Re: Viewsonic N2750W dead

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.

That sounds like you have identified the problem.

PlainBill

[mbates14]

Re: Viewsonic N2750W dead

MCUs are known suspect in this model. unfortunately i havent found a source. I tossed away 3 of these TVs on different occasions because of CPU faults.

A-Stars are the SAME way on certain models. the CPUs will start to "glitch-out" before going bad completely. reguardless of capacitor condition.