ViewSonic N3251w Doesn't Turn On

[Paul678]

Ok, got this one for $5, and the PS didn't even have the always on
+5V. So after searching the web, I found this solution:

http://www.fixya.com/support/t354648...c_n3250w_power

So I checked the Q7 and the zener ZD4, and sure enough, they were bad. Replaced them, and I was overjoyed to see the standby, always on +5V present!

But.....I celebrated too early!! Depressing the front "ON" button
didn't turn the TV on, and didn't change the color of the front red LED.

So I checked that the +3.8V at the main board was being shorted to 0 volts
when I depressed the switch, and it was.

Then, using this diagnostic chart:

http://www.eserviceinfo.com/download...01%2032E2.html

I checked for the 5V=On, 0V=Off enable signal on line 10 of the main
cable, and all I found was +0.06V, which doesn't move at all when I
depress the Power button.

So it looks like the micro-controller on the main board is faulty. The
board # is JC328AA71UA.

I doubt I'll find a schematic for this one.

Do people sometimes get desperate, and just jumper the always-on
+5V to the enable line, in bad situations like this, so that they at least have a working TV, that has to be turned on and off with an external AC switch?

I'm starting to get depressed with this one, if I can't even Ebay the main board!!!!!!

[tom66]

Re: ViewSonic N3251w Doesn't Turn On

It won't usually work. The backlight may fire up and stuff may happen, but the TV will still be in a zombie state and will not fully boot up. I have done it before with a Samsung out of desperation - just saw a bit of noise on the screen because it wasn't initialising the video processor.

Check all the voltage regulators and switching supplies.

[Paul678]

Re: ViewSonic N3251w Doesn't Turn On

It won't usually work. The backlight may fire up and stuff may happen, but the TV will still be in a zombie state and will not fully boot up. I have done it before with a Samsung out of desperation - just saw a bit of noise on the screen because it wasn't initialising the video processor.

Check all the voltage regulators and switching supplies.

You mean the ones on the main board, right? Like perhaps the
micro-controller is not being turned on, because it's +3.3 volt
regulator is bad?

I couldn't find a schematic....

[tom66]

Re: ViewSonic N3251w Doesn't Turn On

All regulators. Check them all.

[Paul678]

Re: ViewSonic N3251w Doesn't Turn On

All regulators. Check them all.

Ok, I found the schematic for the main board here:

http://www.scribd.com/doc/72795409/N3251w-1M-SM


First thing I notice is that the +5V enable line is kept to 0 Volts
by the micro-controller energizing the base of Q103 on page 68
via trace UP3_4, so it's clear the problem is not due to the controller not getting proper voltage. The +3.3V and +1.9V regulators close to the
controller were working fine.

I also verified this by tracing UP3_4 to test point 1, which was +0.87 Volts
(bottom of page 70, close to the controller).

Then I traced the power button to the keypad interface (pg. 91), to trace
UP3_0, leading to pad A7 of the BGA on page 70. There was a
test point here, very close to the chip, where I was able toggle
the +3.8V to 0 Volts using the power button.

You can see on page 91 that the ON/OFF signal is energized from +5V
through a 10k resistor, so it's possible the BGA solder has
failed on pad A7, so that the controller never sees the power button
toggling. And perhaps it has an internal pull-up resistor, so it always
remains off.

What else can I do in this case, besides trying to bake the main
board in the oven? There's a reset circuit on page 93; is it possible the system can be reset to
factory default values, and maybe start working?

I guess I could sell the now-working power supply on Ebay,
but it doesn't fetch as much as a working main board!!! And the panel would be
a pain in the ass to ship without cracking!!

AAAAAAAAARRRRRRRRRRRGGGGG!!!!!!!

[Paul678]

Re: ViewSonic N3251w Doesn't Turn On

Update: I just tried using my frequency counter, and
I was NOT able to see a stable 27 MHz around crystal
X100, close to the microcontroller.

Maybe the crystal, and therefore the clock, has died?

[tom66]

Re: ViewSonic N3251w Doesn't Turn On

Or, a more likely alternative: Whatever powers the microcontroller has failed, or there is a RESET line for the micro which has failed (common problem with JVC, Sony and Toshiba sets.) Usually a damaged crystal will still oscillate, just at the wrong frequency. I wouldn't rule out a complete failure though, I would just recommend checking other things first.

[Paul678]

Re: ViewSonic N3251w Doesn't Turn On

Update:

I got a used main board off Ebay for $32 total, came from a unit with a
cracked screen. The TV turns on now!

The VGA input works, as does a DVD into the component
inputs. I don't have an HDMI device to test that port with.

But..... When I tried putting my old UHF loop antenna into the
DTV port, and put it into DTV mode, I was not able to see any
channels (neither picture nor channel #), and also, the menu wouldn't allow me to get into the DTV section, so I couldn't do an automated channel scan.

When I put the loop onto the analog TV input, I was able to get
one station, channel 65, but all others were "no signal".

I suppose I could still sell this as-is for cheaper, but if
someone has any ideas, I'm all ears......

[Paul678]

Re: ViewSonic N3251w Doesn't Turn On

Update:

Ok, I was able to do an auto channel scan with the
analog TV tuner, and I was able to get 4 channels over the
air, so the analog section definitely works fine.

But, the menu is still not letting me get into the
DTV section. When I try to enter that section, the menu
mysteriously turns off, and I just have the blank blue screen,
and I can't get back into the menu mode for like at least a
couple of minutes. Very weird!

I should still be able to sell this to some gamer as-is,
because I'm pretty sure the HDMI input will work.....

[Paul678]

Re: ViewSonic N3251w Doesn't Turn On

Update:

There is a procedure in the service manual to re-initialize the EEPROM, which I performed, but still the same problem.

Oh well, I think I can still get like $100 for this as a gaming/computer/dvd monitor......