Vizio M240NV two seconds to black

[tibimakai]

I have just picked up this TV, haven't really to well what is going on with it.
When I start it up, flickers for like two seconds and the screen turns off, TV stays on and can be turned off by the remote.
The ex owner said that the TV had sound, but no picture. I haven't try that out yet.
What I'm trying to find out for now, how to diagnose this LED TV?
It's a first for me.
I have a feeling that some LEDs flicker and that's why is turning off itself.
What kind of voltage is at the connectors which lead to the LED bars?
There is a way to test them out with a different power source?
Or should I take the screen apart to see the LEDs what actually they do in those two seconds?
The output(secondary) of those little transformers are all the same resistance, all those FET transistors are the same.
Couldn't check the primary, they seem like are under the bottom FET transistors and I can't figure out which is the second joint.
Any input about LED TVs are welcomed, trying to learn.

[budm]

Re: Vizio M240NV two seconds to black

I can see 12 feedback lines (6 feedback lines per connector), so it has 12 LED string, each string is connected to sensing resistors so it can maintain constant current (by monitoring the voltage drop on the sensing resistors, the more current flows, the brighter the LED will get) that flow through each LED strings. But we do not know how many LED's it uses per string. I have one 55" SONY LED TV with broken panel, I will be playing with it so I can learn more on how these LED backlights work.
You can Google 'driving LED STRINGS IN LED TV' for example:
I think VIZIO uses this one (you can see one MOSFET and one inductor per string, 12 total)

http://www.atmel.com/applications/li...t/default.aspx
https://cdn.badcaps-static.com/pdfs/...060393cf1f.pdf

If you have variable power supply with adjustable current limiter set to about 20mA, then you may able to drive each string and find out which string is bad or if they are OK. Check those 12 MOSFET's for now also.

https://cdn.badcaps-static.com/pdfs/...0a18b8dcde.pdf
https://cdn.badcaps-static.com/pdfs/...36a4a3a713.pdf

[tibimakai]

Re: Vizio M420NV two seconds to black

I was about to take the screen apart to see if one of the LED strips is out and to my amazement I have found some kind of white and gray powder like oxidation on the thin panel board. Mostly around the connectors, but it reached a couple of SMD components too.
I have cleaned as best as I could with rubbing alcohol and electronic cleaner.
I have found some damaged contacts on the ribbon cables.
I have managed to cut down from the length of the ribbon cable and sand down the plastic layer to expose fresh contacts.
The TV it's working great now, but there is a pixel thickness blue line exactly at the center of the screen(vertical line).
What could be that?

[budm]

Re: Vizio M240NV two seconds to black

I hope the corrosion did not reach the tab bonding. Does it look like it got damaged from liquid?

[tibimakai]

Re: Vizio M420NV two seconds to black

You maybe right, it could be liquid. Not sure if there is any opening at the top of the TV. That area it was well protected with the panel's metal case and some tape.
Do you think that liquid would look like powder?
I will have to pay more attention to the damaged area.

[tibimakai]

Re: Vizio M240NV two seconds to black

I have cut shorter the second ribbon cable too. since two contacts were pretty black all across.
By cutting this cables down, I'm losing the locating tiny tabs from the side edges, but no matter how I reinstall them, it seems that it won't affect that thin light blue line from the center of the screen. It most be something with the screen itself.
Before I opened up the TV, for those two seconds while it was on, it didn't show this line. When I'm going home I will try reinstalling the main board side too, since I have removed those too.

[budm]

Re: Vizio M240NV two seconds to black

When you get that thin line on the screen, it means that the TFT or the driver for that column is stuck on, it can be the blue/red/green column or combo.

[tibimakai]

Re: Vizio M240NV two seconds to black

Do you think a reset would help? Course I don't know how to do that either.

[budm]

Re: Vizio M240NV two seconds to black

I do not think so.

[tibimakai]

Re: Vizio M240NV two seconds to black

There is a band also, which I only noticed later on.

[budm]

Re: Vizio M240NV two seconds to black

That is bad panel, you can try pushing on the tab bonding to see if it will get worst or better.

[tibimakai]

Re: Vizio M240NV two seconds to black

That's the thing, doesn't change at all.
What would you do with this TV? Sell it very cheap as is, or take it apart for parts?

[budm]

Re: Vizio M240NV two seconds to black

I would see how much they sell each board on EBAY first before selling it as a broken set.