VIZIO VT420M back light/no picture

[mpbarber]

Tried to do this yesterday but didn't seem to work so this may be a duplicate...

looked at a lot of posts regarding this problem but am having trouble understanding the suggested repairs.

I'm going to put some pictures on here hoping that someone can help me identify the names of the various cicuirt boards and what things I can try to fix this TV.

Thanks in advance for your help.

[CapLeaker]

Re: VIZIO VT420M back light/no picture

Either Tcon (pic #2 under that metal shield) or main (pic #4) board. Pic #3 is the power supply.

Remove that metal shield of the Tcon and post a high resolution picture. There may be a a little smd fuse on that Tcon board, so check it.

[attainteddragon]

Re: VIZIO VT420M back light/no picture

what is the actual issue with the tv, no power, stby only, no picture with back lights working, distorted picture?

a good start would to be to get the voltage readings from the ps to the main. that is the harness with multi coloured wires in your pics.

list it as
pin no, pin desc, v in stby, v when power on is applied.

[mpbarber]

Re: VIZIO VT420M back light/no picture

Thanks to both.... I will work on this and get back to you. I thought I'd get an email when someone responded but I did not. Is there something I can do to get emails?

[mpbarber]

Re: VIZIO VT420M back light/no picture

what is the actual issue with the tv, no power, stby only, no picture with back lights working, distorted picture?

a good start would to be to get the voltage readings from the ps to the main. that is the harness with multi coloured wires in your pics.

list it as
pin no, pin desc, v in stby, v when power on is applied.

Sorry. There is no picture or sound but I have the back light. It started out as fuzzy picture that kind of jumped... It would go away after about 5 minutes but then after about a month of that the picture and sound quit.

I'll need some help identifying exactly where to get the readings. Would I be measuring voltage to ground from various wires on the ps to main, or voltages from the ps wires to the main wires? I have a multi-meter.

[budm]

Re: VIZIO VT420M back light/no picture

You need to check the Voltages on that T-CON board that is pointed out to earlier post.

[mpbarber]

Re: VIZIO VT420M back light/no picture

Either Tcon (pic #2 under that metal shield) or main (pic #4) board. Pic #3 is the power supply.

Remove that metal shield of the Tcon and post a high resolution picture. There may be a a little smd fuse on that Tcon board, so check it.

OK... I took the cover off took pictures but.....

I can't add the pics.... there is a dropdown arrowhead just to the left of "Undo Redo" and to the right of the dropdown that shows "Smilies" which shows "Attachments" when I put the cursor on it but when I click on it nothing happens.

When I click on the Insert Image icon it displays.... Please enter the URL of your image....

[mpbarber]

Re: VIZIO VT420M back light/no picture

You need to check the Voltages on that T-CON board that is pointed out to earlier post.

Thanks. I would but I'm not sure where to check or what the values should be.

[mpbarber]

Re: VIZIO VT420M back light/no picture

OK... I took the cover off took pictures but.....

I can't add the pics.... there is a dropdown arrowhead just to the left of "Undo Redo" and to the right of the dropdown that shows "Smilies" which shows "Attachments" when I put the cursor on it but when I click on it nothing happens.

When I click on the Insert Image icon it displays.... Please enter the URL of your image....

OK Got it to work... Is that a fuse in pic 2 (F1 125V/3.0A) and if it is how do determine if it's bad?

[budm]

Re: VIZIO VT420M back light/no picture

F1 is the fuse.
TV turned on, black probe of the meter on the chassis, red probe on each end of the fuse one at time to see if you do have 12VDC or not.

[mpbarber]

Re: VIZIO VT420M back light/no picture

OK, so I finally got back to this problem.... it's my bedroom TV so no rush.

I plugged in the TV and measured the voltage of the multi-collored connection on the power board, see attached pics.

I listed the voltage readings per pin, again, see pics. That seems OK.

But when I measured both sides of the fuse, I got zero volts. I tried to take a close up picture of that fuse. Not sure this is my problem, but "no volts" is probably not a good thing....

thanks again,
mike

[budm]

Re: VIZIO VT420M back light/no picture

https://www.badcaps.net/forum/attach...0&d=1512148644
That must be errors:
O/F (bl-on) = 0.47V? Typical value <1V = Backlights off, >2V~3V = Backlights ON
DIM = 0.47V?
PS-ON = 0.5V? Typical value <1V = OFF
But the bottom line is that you do have 12V and backlights so those reading must have been an errors, especially you have the exact value for O/F and DIM.

'But when I measured both sides of the fuse, I got zero volts." Is that measurement with ref to chassis GND? If it is then you have problem with main board.

[mpbarber]

Re: VIZIO VT420M back light/no picture

THANKS! Yes, all measurements were done with with black probe to a chassis screw. So can I test the main board or should I just replace it?

I made all measurements using the same chassis screw for ground.... I have the unit face down on a blanket on a table so I just plugged in the power cord and did not actually "turn on" the TV.

Could that explain the reading errors and would it help if I turned the TV on and then make the measurements?

[budm]

Re: VIZIO VT420M back light/no picture

You need to turn the TV ON and at that point you should see those command Voltages goes up if the main board is working.
At this point you do have backlights but the main board is not sending the switched 12V to run the T-CON board.

[mpbarber]

Re: VIZIO VT420M back light/no picture

You need to turn the TV ON and at that point you should see those command Voltages goes up if the main board is working.
At this point you do have backlights but the main board is not sending the switched 12V to run the T-CON board.

Thank you... as my kids used to say..... DUH!

[mpbarber]

Re: VIZIO VT420M back light/no picture

New measurements.............. O/F 4.98, PDIM 3.30, 12VDC 11.41, 12VDC 11.41, GND 0, GND 0, GND 0, 24VDC 23.8, 24 VDC 23.8 PS-ON 4.97, 5VDC 5.14, 5VDC 5.14 F1 125V/3.0A .02 - .03 to ground both ends of the fuse

thanks in advance for any suggestions....+

[dick_barton]

Re: VIZIO VT420M back light/no picture

To check the fuse turn the power off to the tv and set your meter to ohms and connect each of the meter leads to the ends of the fuse. Should read close to 0 ohms. However you have the various voltages at the connector so your fuse is OK.

[mpbarber]

Re: VIZIO VT420M back light/no picture

Hi Dick,
With power to the TV and the power switch turned on, the back light and a blue light on the board that has the fuse, cycles twice. I measure .045 to .051 @2K ohms setting on the meter. I also noticed that the screen flashes a blue/white horizontal stripe pattern across the screen which eventually stops.

Thank you for trying to help. I'm hoping that I can perhaps find that one of the boards is bad and it is a simple replacement, if I can find the board.

thanks again,
mike

[mpbarber]

Re: VIZIO VT420M back light/no picture

F1 is the fuse.
TV turned on, black probe of the meter on the chassis, red probe on each end of the fuse one at time to see if you do have 12VDC or not.

I must have missed this. I plugged in the power cord, turned on the TV, got a temporary blue light on the board with the fuse. Measured each end of the fuse to ground and got 0.91 - 1.01 on both ends.

That would indicate that I'm not getting 12 volts to the fuse?

thanks

[dick_barton]

Re: VIZIO VT420M back light/no picture

To measure the voltage on the fuse, power up the TV and set your meter to DC volts with the black meter probe connected to chassis and red meter probe to the fuse. Check the voltage reading at both ends of the fuse.