Vizio VO420E - No Video But Has Backlight,Sound.

[dodadew]

Picked up a broken Vizio VO420E dirt cheap as I have the exact same model working now...figured worst case I have some spare parts, best case I have another TV which I also needed. Now the problem:
Mainboard #0171-2271-3060
Powersupply #0500-0407-0750

Tv Turns on...Back-light lights up...nothing displaying on the screen whatsoever. Sound from HDMI works, yet there is no picture. It seems everything works, except displaying an image. The screen is just pitch black. Remote Works, Volume Buttons Work, Input button works (took a million presses to figure out where I was at)

Power / Inverter board *was just replaced* by the previous owner as he thought it was a back-light problem...but had same problem with the new board. (Did not get an extra power supply to prove it, but someone was definitely inside the back of the TV).

I took it apart and did some quick voltage checks @ the power supply. I have 5v, 12v. (don't remember the exact voltages, but I know they were less than .5v over, but none under. The 12v is taken while turning on. Doesn't fluctuate at all it seems. I visually checked the caps...none are bulging, burnt, or leaking. Checked the 2 fuses on mainbaord (F1,F2) and both are passing voltage (12v and 5v). I really have no idea where to go. Any insight would be greatly appreciative. I have been searching for hours, and can not find much that can help with my exact model.

[audiofreak19]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

I would say to check the t con.

[budm]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

http://www.shopjimmy.com/media/catal...5542t06c17.jpg

Check to see if you do have 12V on both ends of the SMD fuse F1, TV must be on.

[dodadew]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

Well I rechecked a bunch of things. First off, I have
F1: 5.168vdc/5.151vdc (ON)
F2: 12.29vdc/12.29vdc (ON)

Power @ the Power Supply:

+5_1: 5.212vdc (OFF) - 5.189vdc (ON)
+5_2: 5.212vdc (OFF) - 5.189vdc (ON)
+5_3: 5.212vdc (OFF) - 5.189vdc (ON)
SGND_1: 0vdc (OFF) / 0.4ohms - -0.007vdc (ON)
SGND_2: 0vdc (OFF) / 0.4ohms - -0.007vdc (ON)
SGND_3: 0vdc (OFF) / 0.4ohms- -0.007vdc (ON)
SGND_4: 0vdc (OFF) / 0.4ohms - -0.007vdc (ON)
SGND_5: 0vdc (OFF) / 0.4ohms - -0.007vdc (ON)
+12_1: 0vdc (OFF) - 12.29vdc (ON) 
+12_2: 0vdc (OFF) - 12.29vdc (ON)
+12_3: 0vdc (OFF) - 12.29vdc (ON)
PS_On: 2.264vdc (ON)

I also checked some other things: ( I was not thinking and forgot to check the tabs as well)

U2 (1117)  - In 5.111 / Out 3.292 / ADJ 2.046
U5 (1117)  - GND 1.365 / OUT 2.612 / IN 5.113
U7 (G1084) - 5.115 / 2.069
U8 (1117)  - GND 0.001 / OUT 1.250 / IN 3.303
U9 (1117)  - GND 1.248 / OUT 2.495 / IN 5.115
U11(7805)  - In: 12.28 Ground: 0 Out: 5.012
TU31 (EH13A) - 0.000 / OUT 0.426 / IN 0.021

The voltages at TU31 seem off to me. Guess try to trace back and see where it gets its supply from as there is virtually nothing there?

Also, do you think it would be feasible to take apart my known working one and swap mainboard (if its the same version) to see if the problem is actually in the LCD? (Without knowledge of this TVs past, im worried that could be it) I don't see any marks at all in it, but trying to make sure I am barking up the correct tree.

[budm]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

Well, you still did not check the voltage on the SMD fuse on the T-CON board as I suggested in post #3.

[dodadew]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

OK, do you have some insight into where that would be located? Because I was assuming it was integrated into my mainboard as I only have a Power supply, and a Mainboard in the back of there. Power goes into Mainboard...2 ribbons come from the Mainboard and connect to the Driver Boards on the top of the LCD. I am sorry if I am not noticing something here, but I can seem to see where this would be? I guess I assumed that the top section of my mainbaord was an integrated T-Con as it has most its components prefixed by TC. (Pic in 1st post) I will tear it apart again in the morning and see what I am missing. I just looked at the service manual and don't see it listed...Do I have the one VO420 without an external T-Con Board?

[dodadew]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

I did some searching of pictures...and see a fuse I did not check..Labeled TF and circled in the pic. I think this is the equivalent of the one on the actual T-Con Board...Do you? I will give it a test tomorrow. Or maybe I am blind and there is an external T-Con board in here somewhere that I cant see. Either way Input still accepted and I will post back update tomorrow morning.

[budm]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

"Do I have the one VO420 without an external T-Con Board?" It looks like it after I did more search on your model. Your version does not use the AUOPTO LCD panel.
Can you see the label for this LCD panel you have? We can try locating the spec sheet for this panel.
Yes, that is another SMD fuse.
The empty J8 on the board looks like an option for installing the LVDS connector for the external T-CON board version.

[dodadew]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

I will pop it open and have an answer in about 10 minutes. Thanks for the help thus far too.

[dodadew]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

LCD is LC420WUG (SB) (R1) (Pic Attached)

I Also checked that fuse...all was Good, but does not seem to be getting the 12v it wants. 12.29 is passing thru FU to U21 (4953GM) out of there to L57 (In at 12.29, Out @ .8-1.1V Fluctuating) then into TF. Now...what is L57. No markings does not help either.

[dodadew]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

After some quick research i think L57 is a ferrite bead. Does anyone think that a jump across it would work for testing? I'm not going to try it until I do some more research though.

[budm]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

"U21 (4953GM)" seems to be the switched MOSFET for providing the voltage to the circuit, it is turn on by the processor on the main board. L57 should show <1 Ohm. You can try disconnect those two white ribbon going to the panel to see if the 12V will come up.

[dodadew]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

The 12 v does not come up after L57. Shows 12 in... And 0 from turn on until Backlight lights... Then climbs from 6v to 8.4v. This is with white ribbons unhooked. L57 has a reading of 8.7kohm. From output of U21 to out of TF1 same reading. Starts out at 10kohm and settles down at 8.7kohm. Same with ribbons connected.

[budm]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

"L57 has a reading of 8.7kohm" That does not look right at all, I expect <1 Ohm. You can see L16 which is the smaller FB, check that. It looks to me you do have bad FB. Look around the board to see if you can locate anther FB withe same physical size.

[dodadew]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

L16 comes in at 0.4ohm...various other ones also come in under 1 ohm. Still looking for one about the same physical size.

[budm]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

OK, let find out if the L57 goes high resistance due to short in the circuits or not, check the resistance reading between one end of the fuse (the one with N) and the chassis ground to see what you read.

[dodadew]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

Testing Fuse (N) to Ground...Starts up high and settles down to 7.7kohm. Testing from Fuse(N) to opposite end (U21 Side) of Ferrite L57 I get 0.992Kohm.

[budm]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

OK, so there is no shorts circuit after the fuse (but can be shorts after the active circuits), and since the fuse is not blown, you can put your meter in DC A mode and put each probe on each end of the L57, you may want to solder wires to each end of the L57 so they can easily be connected to your meter and not worrying about probes slippage and short things out. We will wan to the current draw on the meter and see if you will get the pictures back on the screen. I do not expect the current reading to be more than 2~3A. You know how to use the meter in DC current testing mode, right?

[dodadew]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

I sure do. Ill solder on some leads and get a reading right quick. Thanks for the expertise again...would not be this far without your help.

[dodadew]

Re: Vizio VO420E - No Video But Has Backlight,Sound.

Progress. I get 0.52amps immediatley after the backlight turns on ...displaying the vizio logo!!!...and I when it goes to the cherished blue screen, I get 0.78a. When I remove the probe from L57 with the set on...I lose video, and I assume the processor resets as the backlight turns off then back on. Should I get a reading with a video source / white screen as well?