Need some advice for TV repair

[Skyfox]

Today's project is attempting to diagnose and hopefully repair a Vizio LCD TV, a small one that's maybe 22" diagonal.  The thing powers up with the little lights on the invisible buttons turning on (I'd like to bitch slap whoever is designing TVs with invisible touch buttons so that you have no idea where they are) and they respond to being touched, but the backlight and the LCD don't turn on.  With a flashlight on the screen I can see the LCD isn't doing anything at all, unlike a screen where just the backlight went out.

Nothing looks popped and I don't smell any burning, so visually everything looks fine.  I checked all the voltages on the power supply board where it leads to the main board, and everything checks out except for the LED ON/OFF line (there's also a Dim line, sitting at 0V until shutdown).  On standby that line sits at about 0.1V.  On powerup it jiggles just a hair above that level.  On shutdown, after several seconds it slowly ramps up to about 0.5V and then slowly ramps back down.  The flat flex lines to the LED backlight come directly out of the power supply board instead of the main board (and they are seated tight).  The Dim line stays right at 0V until shutdown, when after several seconds, about the time the LED On/Off line ramps up and back down, it has a split second spike up to about 1.25V and then right back to 0V.

Should the voltage signal for the LED On/Off line come from the power supply to the main board, or from the main board back to the power supply to signal the supply to turn on the backlight?  And would the Dim line come into play in all this?  Also, any idea why the LCD wouldn't turn on even if the backlight is inoperative?  I just need some pointers for what to look into because my past success has always been more with CRT TVs.

[metrologist]

I have a monitor with invisible buttons. I'll stand in line behind you for the slapping...

[JoeO]

Did you Google the model number to look for hints?

[Skyfox]

Yes, and the search didn't turn up anything helpful.

[4thDoctorWhoFan]

Are any of the LED's on the invisible buttons flashing?
When there is an error, the LEDS will flash an error code on a large number of sets which will help to narrow down the troubleshooting search.

[Skyfox]

No.  Just as when it was working normally, on first powerup all the invisible button LEDs turn on, and then all but the far left one go out, and then after several seconds that one goes out too.  All that remains lit is the power I/O button (always dimly lit) and the big VIZIO logo.  When I touch the invisible buttons, all the LEDs turn on again to indicate where the buttons are, and after several seconds of not touching anything they go out as usual.  And as I described before, when doing a flashlight test I can't see any of the popup menus for the buttons or anything.

[lordvader88]

So does the STBY DC-DC  converter keep running after shutdown ? If it's ok, check the PWM controller of the main switcher to see it's in fault protection power-wise, or if it's being turned off some other IC or problem, that would make the power turn off.

[Skyfox]

When it's shut down, the power supply continues to put out the ~5.3VDC as normal.  All the other lines show nothing.  When it's powered up, the 12V lines come to life, and the 5.3V Var lines (variable?) all come up to 5.3VDC.  Besides the grounds, the only other lines are the Dim and LED On/Off lines which act like I described above.

I don't know what you mean by the main switcher, and I don't know which part is the PWM controller or how to check if it's in a protection mode.

Quote from: lordvader88 on Today at 05:17:16 am

So does the STBY DC-DC  converter keep running after shutdown ? If it's ok, check the PWM controller of the main switcher to see it's in fault protection power-wise, or if it's being turned off some other IC or problem, that would make the power turn off.

[bob225]

what is the model number ? usually its a t-con board/cable issue

[Skyfox]

It's a Vizio E220MV.

I forgot to mention, on the power header between the power supply and the main board there's a PWR SAVE line.  When it's off, that line shows 0V.  When the TV is turned on, that line jumps up to about 3.3V.  When it's turned back off, after a few seconds that line drops back to 0V, at which time the LED On/Off line goes through that ramp up/ramp down cycle.

[bob225]

Silly question is the psu brick putting out the right voltage under load ? peak of 12v at around 4 amp

[james_s]

I had to use one of those Vizeo TVs for a while, it had a great picture but those invisible touch buttons were horrid! The worst part is changing room light, reflections and who knows what would often trigger them causing the buttons to repeatedly pop up on the screen. Absolutely stupid design, form over function to a shocking degree. I would fire whoever came up with that idea if I had the chance.

[lordvader88]

So the low voltage stuff probably works fine if STBY 5V stays on and the 12V type works while other stuff tries starting. (but maybe not, depends on the PSU. How long does 12V say on for? A few seconds? That's probably a good sign, a chip doesn't usually wait seconds to decide if a 12Vrail is good/bad)

Yeah the STBY power is like a mini-version of the main PSU. But I guess there's also a high voltage supply for CCFL ? I never really worked on these yet.

By main switcher I mean main SMPS (switch mode power supply), if u don't know.


Here's a book I really need to read inside out, I'm pretty new myself
Troubleshooting & Repairing Switch Mode Power Supplies
https://readerspage.files.wordpress.com/2016/07/smpsrepairebook.pdf


That doesn't cover the HV supply for CCFL tho. But they are also a type of SMPS.


What in the world all the big computer chips on LCDs do tho...no idea.


There's lots of saftety circuits in there, such as, if the CCFL lamp is not working right after a few milliseconds, or some other thing is using too much or not enough power, it all shuts down.

[james_s]

If it's the Vizeo model I'm thinking of then my recollection is that it uses an LED backlight, I could be wrong though, it's been a couple years since I dealt with the thing. I would certainly start by checking the voltage and ripple.

[bob225]

the psu board is also backlight driver

psu wise its a 12v 4 amp wallwart

single board construction - I have asked a few contacts re. a service manual for this model but nothing has come up yet

The processor does all the decoding/encoding and switching as well as all the osd

[bob225]

794791300601R




Psu board 793881400600R

[totalmedtech]

have you checked the condensers?replacing them could solve your issue

[lordvader88]

Oh so it's powered by a walwart. Then onboard I think they do some 5V and 3V regulation for the big IC chips. Like a on a broken display I have...an inductor fried on it in it's voltage regulation section. And it's main processot chip get's so hot I'm sure it fried. But I still have it, 1 of these days I look at it again, as I learn more.