Vizio VT420M No backlight but seems to turn on

[budm]

Re: Vizio VT420M No backlight but seems to turn on

BTW, if you are going to throw away the old board, I would like to keep it for future troubleshooting so I will have the real board in my hands to look at instead of the pictures.

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

budm
You are going to hate me for this, but turns out I actually didn't solder the rectifier diodes back on before taking the tests, and I was so tired the night I did it that I didn't even notice.
After soldering the rectifier diodes back on, I was able to get the 24 volt line and the 12 volt line back up and running at
11.97v for the 12vdc
24.50v for the 24 volt line.
As for the last 2 voltages, the
PDIM is at 0 when off and 3.3 when on
On/OFF is at 0 when off and 110 Millivolts when on.

All the other info given prior should still be correct.
As for the board, if we do conclude in the end that the Power supply is bad, I can definitely send it to you since they are really cheap to replace. But if the main board is the problem, and its expensive I might want to try a component level repair. Not sure. we will see when we cross that bridge.

[budm]

Re: Vizio VT420M No backlight but seems to turn on

"On/OFF is at 0 when off and 110 Millivolts when on." That means the main board is not sending the BL-ON (ON/OFF) to turn on the backlights, it should be >2.50VDC.
You can force on that pin using any resistor between 330~1K Ohms between the ON/OF pin and the 5V STANDBY PIN, BUT you MUST remove that wire from the connector first because we do not want to back feed the signal back to the main board.
I am glad you caught that mistake.
If the backlights comes on and the pictures and sound and everything else works then we need to find out why the main board is not sending the signal.

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

In the past I have shorted the PS_ON and the 5 volt line to force the tv on, and that has been working for 90 percent of LCD Vizio power supply boards. So I had done that as well prior with no response, everything aside the 5 volt line and PS_ON were showing strict 0vdc.

I just tried yours and shorted the 5vdc line to the on/off line, and nothing happened. Still 0 volt everywhere else. In the past I have never used a resistor, I only have used copper wire with the legs of a component soldered at the tip of the wire to insert into the connector part. I made sure to disconnect the connector at the main board however.

Am I doing something wrong? Do I absolutely need to use a resistor to short out the ON/OFF with the 5 volt since its a different voltage? If so why is it ok to use resistor in such a large array of ohms?

Thanks

[budm]

Re: Vizio VT420M No backlight but seems to turn on

Did you disconnect the PS-ON wire going back to the main boar first? if the PS-ON transistor on the main board is stuck on and you try jumping the PS-ON to the STANDBY 5V without using resistor and did not disconnect the wire first, you can damage the transistor on the main board.

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

Yes every time I short anything on the power board I disconnect everything from it. Then if I can get the 24 and 12 volt lines to appear, then I connect the backlight, but I didn't even get that far.

[budm]

Re: Vizio VT420M No backlight but seems to turn on

OK, thanks for the clarification. I always use a resistor (330~1K) to limit the current for forcing on.
So when you jump it with resistor what DCV do you get at each end of the resistor?
Right now you do have ps-on ok because you are getting the 12v/24v.
If you are going to run the power supply board without the main board, then you need two resistors, one for the PS-ON, and one for the ON-OFF (bl-on).
The resistor is for limiting the current, it does not need that Base much current to turn transistor fully on (Saturated), it is also for safety in case the wire accidentally connected to the chassis or connected to the wrong place.

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

Ok, I have not tried connecting the PS_ON and the ON/OFF to the 5 volt at the same time, so I will go ahead and do that. I will also make sure to use resistors this time and let you know if the 24 volt and or 12 volts appear at all.
I will report back in a couple hours.

[budm]

Re: Vizio VT420M No backlight but seems to turn on

What you need to do is to apply the PS-ON first to turn on the power supplies and let it settle down first for a couple seconds then apply the ON/OFF to turn on the inverter circuit.

[stj]

Re: Vizio VT420M No backlight but seems to turn on

this feels like a bad or missing pullup resistor!

[budm]

Re: Vizio VT420M No backlight but seems to turn on

"budm
You are going to hate me for this, but turns out I actually didn't solder the rectifier diodes back on before taking the tests, and I was so tired the night I did it that I didn't even notice.
After soldering the rectifier diodes back on, I was able to get the 24 volt line and the 12 volt line back up and running at
11.97v for the 12vdc
24.50v for the 24 volt line.
As for the last 2 voltages, the
PDIM is at 0 when off and 3.3 when on
On/OFF is at 0 when off and 110 Millivolts when on. "

Since the only signal that is missing is the ON/OFF (BL-ON), then all you need to do is to have everything hook back up, remove the ON/OFF wire going back to the main board and then put the resistor between the ON/OFF pin of the power supply board and the 5VSTBY pin to force on the backlights on AFTER the TV is turned ON.

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

I have tried that, and I still don't get the tv to turn on.

[budm]

Re: Vizio VT420M No backlight but seems to turn on

I have tried that, and I still don't get the tv to turn on.

Do you mean you do not get the backlights but everything else are on, it must be turning on because you do have 12V and 24V hwich is turned on by the main board when power switch is activated and then the main board sends out the PS-ON, but right now it is not sending the BL-ON. So if you do the BL-ON force on with BL-ON wire going back to the main board disconnected, then yo have two problems, one is the main board is not bad and not sending the BL-ON, the second problem is the Inverter circuit that you force on with the resistor and it still does not have backlights.

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

Ok, so I didn't understand I had to have the connector still connected to the main board but just without the ON/OFF wire connected to it. That's why I didn't get it to work the first time.
So I just did that, let it turn on, waited 2 to 3 seconds, then made the connection between the resistor and the on/off pin, and nothing happened. So I did it again but this time I had the resistor touch the on/off pin from the start, and I got the backlight to finally turn on. So I am not sure if its because I didn't have proper contact, or if I screwed something else up, but having the resistor connected at the on/off pin immediately, still turned the backlight on.
So now that we have done that, we know that the power board is working, and that we probably just have a main board issue right??
If that is the case, can I just replace the EEPROM? Is there a way to test it to see if it works?

[budm]

Re: Vizio VT420M No backlight but seems to turn on

So now you do have backlights and IF everything else are still OK then you should see the pictures on the screen and sound and all other functions, if not then you have to replace the main board. You can contact shopjimmy to see if they have EEPROM to fix the symptom you are having.

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

Yeah I am only getting backlight, I am not getting anything else like picture and what not. All black with nothing else.
So that's it then I guess just EEPROM and hope for the best.
Well thank you for all of your help and so on, I do appreciate it, I will make sure to right down everything in my notes so that If I ever come across another one of these, I know what to do.

Thanks again

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

EEPROM is not available anywhere, and neither are repair kits.
I definitely don't want to spend $150 on a new board.
Anyone know how to fix the main board on a component level at this point??
cbpf9d1kz4 is the part number.
I already posted pictures in previous replies, but if someone wants me to post more or of more specific areas of the board, please let me know.

Thanks

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

I was able to get a repair service on the main board for just about $100 from moduslink and now the tv turns on fine without any issues.
I inspected the board very closely to try and find what they might have replaced, and honestly I think they had a machine do the solder work since I could not find any signs of tampering on any of the IC's or components.
Anyway tv works just fine now.