Samsung UE46D5520 backlight problem

[pelago]

Hi, I have a Samsung UE46D5520 TV with a backlight problem, for which I need some advice. It uses edge-mounted LEDs for the backlight, along the top of the screen only.

The first two attached pics demonstrate the symptoms before opening up the TV. The first pic is a testcard, the second is a pure white still picture. Note that the horizontal lines on the white picture are an artifact of the camera and aren't there in real life.

You'll see that the left-most roughly quarter of the top of the display is in shadow. The next quarter looks like it is too bright. The right half is fine.

The third pic is after removing the bezel, and with the set upside down, so you can see the backlight leaking out. The fourth pic is supposed to show that the top-left quarter of the LEDs are completely unlit.

I see that the backlight strip is actually made of two strips, a left and a right, each of which has 72 LEDs (144 LEDs in total). The left-most 36 of the LEDs in the left strip don't seem to light at all.

The numbers intrigue me, as 36 is obviously exactly half of 72. Is that just a coincidence? I understand that the LEDs are wired in series, so if one fails, all the others after that will not light, so I guess the first one that isn't lit after the lit ones is dead. Is that right?

The fifth pic shows the LED strip after taken out of the TV. The connector on the left is in the top-left corner of the TV. The 36 leftmost LEDs do not light (the 36 LEDs closest to the connector). The sixth pic shows the LED that is the first one that doesn't light to the left of the 36 that do. It is marked with a pencil mark just under the LED. I can't see anything wrong with it with the naked eye but I guess that is the problem.

Interestingly, I notice that one LED (seventh pic) is completely missing close to the left edge (this isn't the case on the right strip). The panel doesn't look like it's ever been opened, and I can't see an LED rattling around loose inside, so my guess is that is how it has been since it was first assembled.

Is it easy to fix or replace individual LEDs? I guess I would need two. Any idea what ones to buy?

[comptek]

Re: Samsung UE46D5520 backlight problem

image 6 of 7 is not working

[ReeceyBurger123]

Re: Samsung UE46D5520 backlight problem

You cant actually buy the Leds but you could try the following:

Substitue the failed Led(s) with a zener diode in reverse bias.
Buy a smashed screen edge lit Led which isnt economically fixable and take the good Leds off it and replace them on to yours, only the size is different on Leds.

[budm]

Re: Samsung UE46D5520 backlight problem

Pictures of the power supply/LED driver board?
BTW, those LEDs may be dual-LED in one package.

[pelago]

Re: Samsung UE46D5520 backlight problem

Thanks for replies so far.

image 6 of 7 is not working

All images seems to be working for me, are they still not for you?

You cant actually buy the Leds but you could try the following:

Substitue the failed Led(s) with a zener diode in reverse bias.
Buy a smashed screen edge lit Led which isnt economically fixable and take the good Leds off it and replace them on to yours, only the size is different on Leds.

Just to check, if I replaced the LED with a (non-light-emitting) diode, that would mean a small dark patch, but maybe not too noticeable? I wonder if it's possible to get a diode small enough to fit, as the LEDs are placed right against a piece of glass.

It's a shame the individual LEDs (or even the strips) don't seem to be easily buyable. I could try transplanting one or two LEDs from another strip, and see https://www.youtube.com/watch?v=_poWTEihIJU and https://www.youtube.com/watch?v=LqaeqCSDWFQ show how to solder/unsolder from the underside. Are all LEDs on edge-lit TVs the same, then?

Pictures of the power supply/LED driver board?
BTW, those LEDs may be dual-LED in one package.

I attach two pictures I took earlier of the power board, which drives the LEDs. I can take better pics if you need them. There are four wires coming out of the board going to the LEDs, two to the left strip, two to the right. What are you looking for, out of interest?

I'm intrigued by you saying they could be two LEDs in one package. I noticed there seem to be four contacts on each LED. Some better pics of the strip (not mine, but identical part number) are at http://www.tvparcasi.com/ltj460hn01-...jvg4-460sma-r1 especially http://www.tvparcasi.com/ltj460hn01-...-5126-43-B.jpg if that helps.

[budm]

Re: Samsung UE46D5520 backlight problem

'I attach two pictures I took earlier of the power board, which drives the LEDs. I can take better pics if you need them. What are you looking for, out of interest?' I am looking to see how the LED strings are arranged. I see two blue wires and 3 black wires on the LED connector. What is the P/N of the board, BN44-??????

[pelago]

Re: Samsung UE46D5520 backlight problem

'I attach two pictures I took earlier of the power board, which drives the LEDs. I can take better pics if you need them. What are you looking for, out of interest?' I am looking to see how the LED strings are arranged. I see two blue wires and 3 black wires on the LED connector. What is the P/N of the board, BN44-??????

I agree it looks like three black wires on that earlier pic, but one of them must be a shadow. I've attached a better pic of the (disconnected) connector at the power board end. The four wires feed through the back of the panel (pictured) then two of them feed the left strip, and two the right, also pictured.

The pics on the link I posted earlier http://www.tvparcasi.com/ltj460hn01-...jvg4-460sma-r1 link also show the overall setup quite well.

The individual strip I have a problem with is labelled 2011SVS46-FHD-5K6K-RIGHT (despite the fact that it is mounted on the left of the screen as the viewer looks at it), which is mounted, along with 2011SVS46-FHD-5K6K-LEFT, on a bracket which is labelled BN64-01644A.

Also see a pic of what I assume is the power board part number, which is BN44-00422A.

[budm]

Re: Samsung UE46D5520 backlight problem

OK, SJ has good pictures of the board, so the LEDs are connected as one big string (not 100 % because 144 LEDs in series will require 144 x 3V = 432V, the power supply is rated at 255V for the LED), so some of the bad LEDS must have shorted out to allow the current to keep flowing, it is constant current source power supply. More likely the LED string may be connected in parallel but I cannot verify due to balk paint at the LED connector bottom side of the board, 72 LEDs, x 3V =216V makes more sense.
Pictures provided by shopjimmy.
You will have to probe the 4 pins of the LED to find out if they are dual-LED or not.
Strange to have one LED missing.

[pelago]

Re: Samsung UE46D5520 backlight problem

Thanks for the board pics. I see what you mean about them being just one long strip, with two of the pins on the power board connector joined together like that. Seems a strange way to wire it to me, but I guess it works.

I have a DMM but I'm not sure exactly what I would need to do to determine whether they are dual LEDs or not - maybe you can say? I'm not sure what difference it makes really, as would have to replace like with like anyway, e.g. from a donor strip.

Yes, it is strange about the missing LED. I notice the two to the right of it are also looking a bit ropy, like they've been scraped, which I don't think was something I did.

[budm]

Re: Samsung UE46D5520 backlight problem

This is what I see based on the pictures. Looks like one big string.
So it has 72 LEDs per strip?

[pelago]

Re: Samsung UE46D5520 backlight problem

Yes, there are two strips, one along the left half of the top edge of the screen, the other along the right half of the top of the screen. Each has 72 LEDs. The connectors on the strips are in the corners of the screen. In the middle of the top of the screen the two strips meet but are not connected.

[pelago]

Re: Samsung UE46D5520 backlight problem

I think I'm working out what is going on.

I'm pretty sure the two strips are in series. The middle two pins of the connector at the power board end are definitely connected together, according to my DMM, and these two pins correspond to a black lead going to one strip connecting to the blue lead going to the other strip. Measuring the voltage at the power board end between the two end pins (the blue lead going to one strip and the black lead going to the other) it is two hundred and something volts (it varies, depending on backlight brightness that the software can control).

Also, if I disconnect just one strip, the other strip goes unlit too, which would also indicate they are powered in series.

But looking carefully at the PCB traces on each strips, I think each strip is divided into two subsets of 36 LEDs (the 36 LEDs wired in series), and these two subsets are wired in parallel. This is why exactly half of the dodgy strip is lit. The other half isn't lit due to the missing LED.

Furthermore, with my DMM in the Diode position, I found that if I connected the black probe to one end of each LED and the red to the other, each LED would light, so all the LEDs on the strip work individually.

So I'm guessing the missing LED must have somehow dropped off inside the panel one day (as the screen used to work fine), although I didn't see it when I opened up the panel. In any case, if I can replace that LED (and maybe the two next to it which have the top yellow surface partially scraped off), it should be fixed.

Does this sound likely?

[pelago]

Re: Samsung UE46D5520 backlight problem

I looked again inside the panel and found the missing diode after all! Holding it in its proper position with tweezers causes the whole strip to light up, although it doesn't light up itself (I guess it's shorted inside). Anyway, at least I know what to do to fix it. In hindsight, it was obvious that the missing LED was the problem.

Of course, one question is why did it drop off in the first place?

[budm]

Re: Samsung UE46D5520 backlight problem

Check the resistance of the LED that fell off the board.

[pelago]

Re: Samsung UE46D5520 backlight problem

Backlight fix success!

I managed to get a donor LED strip, from the 40" version of my 46" panel, and desolder/resolder an LED to my strip, using the techniques shown at https://www.youtube.com/watch?v=_poWTEihIJU and https://www.youtube.com/watch?v=LqaeqCSDWFQ , although I had to buy a better, temperature controlled, soldering iron to do it.

Unfortunately, right at the last minute of putting the panel back together, I think I must have pulled on one of the tabs, and have caused myself a tab bonding issue, where a few vertical lines on one side are appearing. They go away if I push on the tab. How annoying! I have ordered myself some silicone thermal pad from DealExtreme (http://www.dx.com/p/diy-silicone-the...9#.VngN3VKmThU), and already have some Kapton tab, so hopefully can bodge this back working.

[ReeceyBurger123]

Re: Samsung UE46D5520 backlight problem

Nice one

[TechGeek]

Re: Samsung UE46D5520 backlight problem

Annnd just in time for Christmas!

[pelago]

Re: Samsung UE46D5520 backlight problem

Thanks. I was pleased with myself, although very annoyed I damaged the tab bond, so I haven't put the screen back in use for Christmas. Hopefully I can fix that new issue soon.