RCA RLVD3282a No Led Backlight

[mmartell]

I can see the picture is there when flashing light on it. Main cap has 169v on it but never rises above that when the power button is pressed although like I said the pic comes up and I have 5v and 12v rails and blon is 3.8v.

I have 35v at led connector which rises to 90v then immediately starts to go back down to around 40v.

Is there something wrong with my PFC circuit ? Please advise.

[budm]

Re: RCA RLVD3282a No Led Backlight

No PFC Voltage booster used in this power supply due to low power.
It only has one string LED and it sounds like the LED sees open circuit so the Voltage rise up to 90V and then it goes into protection mode due to over Voltage.

[mmartell]

Re: RCA RLVD3282a No Led Backlight

Ok before I open it up the diode to the right of the led connector (in the fourth pic) D18 reads shorted. Would it be normal depending on design to put one or more low ohm resistors across a diode because I can't imagine why you would use a diode in the circuit if that's the case.

[budm]

Re: RCA RLVD3282a No Led Backlight

I would lift one leg up and test the diode again, I believe it is connected in parallel with a low Ohm resistor so the V drops across the resistor will never be higher than the Vf of the diode, what for I do not know.

[mmartell]

Re: RCA RLVD3282a No Led Backlight

Yes I did lift it yesterday and the diode is good so I guess this is by design and not shorted. Will pop open the panel. Thanks bud.

[mmartell]

Re: RCA RLVD3282a No Led Backlight

Here's what the light box looks like. They appear to be wired in series, twenty-one of them. There are pads on either side of each led though despite one would assume they are for +/-for each led I cannot for the life of me get ANY to light. I refuse to believe they are all dead - that surely must be an impossibility. Will keep trying...

[Hamie]

Re: RCA RLVD3282a No Led Backlight

Unsure how you are testing the leds.

The way I test them is to use the + and - located down at the connection wires.
I use a boost converter, you'd need about 30v steady output to test each string.

[mmartell]

Re: RCA RLVD3282a No Led Backlight

The last leds I needed to test I simply put my meter across each one in diode mode to see if they would light. I don't have the facility to test a strip at a time but I can surely rig something for 3-4v to test individually. Problem is that hasn't worked yet so either those pads are not the led terminals or I'm not outputting enough current.

[mmartell]

Re: RCA RLVD3282a No Led Backlight

I was able to get the top and the bottom strips to light with 2x9v batteries at the strip connector. I thought these were wired in series but now I'm not so sure - still can't see how to tap into each led for testing individually.

[budm]

Re: RCA RLVD3282a No Led Backlight

But the middle strip does not light up?
Can you draw out how the red wire and the black wire are connected the strips and to each others?
May be closed up pictures of each LED strip's connector.

[mmartell]

Re: RCA RLVD3282a No Led Backlight

Ya the strips were tested individually with all wires unplugged.

Each strip has only a + and a - pin. The led connector on the board is the same two pin header, red/black wires.

In the pic of the strips above if you imagine the top strip as Strip 1, followed by Strip 2 in the middle and 3 on the bottom then...

Red wire from led driver board goes to Strip 3+, Strip 3- to Strip 2+, Strip 2- to Strip 1+ and Strip 1- is the black wire back to led driver.

Hope that explanation does more good than harm lol

[budm]

Re: RCA RLVD3282a No Led Backlight

So look real close at each LED of the dead strip, look for blackened LED/s.

[budm]

Re: RCA RLVD3282a No Led Backlight

If the LED strips are in parallel then the middle strip should have 3 wires on each pin of the female connector.

[mmartell]

Re: RCA RLVD3282a No Led Backlight

Here is a very crude drawing of the wiring. They are definitely in series but I can't tell how the individual strips are wired.

[budm]

Re: RCA RLVD3282a No Led Backlight

OK, that makes more sense that they are in series, so you do have one bad string. Just have to find the replacement or try to find the bad LED on that dead strip some how.
Just another sad story for LED back-lit TV.

[mmartell]

Re: RCA RLVD3282a No Led Backlight

Yes trying to find out which led is open but can't find how to isolate them.

And this model is obscure, there must be a close cousin which would make it easier to match parts with.. any ideas ?

[infringer]

Re: RCA RLVD3282a No Led Backlight

Just curious in this case would it hurt anything if a guy just bought a cheap roll of bright white stick on strip light and tied into the 12v on the SMPS seeing as how he still gets a picture I don't see why it wouldn't work... Or heck just tap a power brick into it and splice that in and splice it in to the AC supply line one of those 75 watt ones. If you got picture and sound but backlight is gone it would be a cheap fix I think them rolls are fairly cheap on amazon or ebay and they are fairly bright we use them for back lighting on marquees and such so you get enough of them in there it would be a cheap and effective solution. Only issue being is you would not be able to dim the backlight big whoop I suppose would be interesting to see.

[mmartell]

Re: RCA RLVD3282a No Led Backlight

Ya I half considered that but for a tv of this size and quality it sadly isn't worth the time/effort and outlay imo. Now if this were a larger, more valuable television...

[mmartell]

Re: RCA RLVD3282a No Led Backlight

Update it turns out after reading a little more and inspecting the strips closely that each led is wired in series on each strip. In fact the whole thing is wired in series so an open led anywhere kills the whole backlight. I was able to scrape off a little paint (as per another thread) and test each led on the bad strip. The third from the left was open.

I'm putting a different led from a different tv in its place to see what that looks like. If it doesn't match in the least I will just jumper it and box it back up since it's all I have.

Btw the caps or the "domes" pop off but seem to be glued on so there will be no popping them back on I will have to use a little glue to put it back.

[budm]

Re: RCA RLVD3282a No Led Backlight

Can you show me the pictures of what you scraped off the board to get to the pads for testing the LED?
21 LEDs in series x 3.3V = 69.3V total Vf.