Samsung UA32F5000AMXXY TV failed backlight. Second fix.

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Problem found - now looking for parts i(in Sydney?)  Bodge #1 failed (not unexpected)  Bodge #2 in play.

I have a Samsung TV with a failed backlight that belongs to a relative in a nursing home.  I felt obliged to 'have a look at it', but since this is my first hands on with a flat panel TV, I've been approaching it a little more cautiously than I might with a familiar piece of gear.

I know it's the backlight as the sound and remote work, but the screen is dark and by shining a bright torch onto the panel, enough light gets in that you can see the image.

The image has the back off and I have identified the input/signal processing board at left (green), the power supply board at right (brown) and the LCD driver board and the centre bottom (blue) with the speakers each side of that.

I can only guess that the backlight is powered from the 16 pin header marked 'SMPS' at the top right of the power supply board - but without a bit of help, the next step is unclear.  Some schematics with voltages would be good right now.

On cursory examination, it would seem the next step in delving deeper into the heart of the beast will involve getting into the display layers ... and that's not something I'm keen on doing without some guidance.

Google is not particularly helpful.  There's lots of stuff out there, but I can't find anything that is specific enough.

Any pointers would be greatly appreciated.

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I finally found a video showing the disassembly of a UE32F5500, which is virtually identical for all the things I needed to know.  Along the way, I also came across some other useful bits of information ... as you do.

Accessing the LED strips allowed me to locate one dead LED - and since all the LEDs are in series, it only takes one miscreant to spoil the party.

Now to find a replacement...  hopefully somewhere in Sydney (where I don't have to mortgage anything).

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The annotation above should read 'found problem no. 2'.  The question I should have asked was 'why did an LED fail in the first place'?

I did a bodge to check for operation and there was an initial response from all 45 LEDs, so I put everything back together.  However when powering up, there was no backlight once again.

Having been inside the LED strips, I was able to work out the connections, so I could now take an intelligent voltage measurement.  The LED supply voltage started at 220v DC and dropped down to about 205v over 2 or 3 seconds, then jumped back up to 220v.  Aside from this curious behavior, 220v across 45 LEDs works out at nearly 4.9v per LED.  No wonder I had an LED failure.

It seems I've fried another LED after the bodge - but now I know how they are wired, I've been able to test the strips without having to pull the LCD and diffuser panels off again. (That will come again - but later.)  The good news is that it seems the LED strip that has failed again is the same one I did the bodge on.

Now the question is - do I go chasing the overvoltage problem on the power supply board - or just go for a replacement board?

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Scratch that last idea.

I was sitting down at dinner late last night and a casual thought just floated through my mind.  It caused me to drop my head in embarrassment and I nearly faceplanted in my salad.   

The LED string was open circuit when I measured the supply voltage, so of course the voltage would be higher.  But what was the voltage meant to be?  After some digging, I finally came across a photo of the exact board that some generous person (who uses the Cyrillic alphabet) had put up, showing two voltages - one of which was the one I was seeking.  150 volts.  70v less than O/C?

With a clearer idea, my subsequent test proved the PSU was fine.  Now all I need is one LED strip.

[amyk]

What type of LEDs are they? I don't think they would be all that difficult to find one of appropriate rating and solder it in.

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Aside from compatible electrical ratings, there is colour temperature, relative brightness and, to a lesser extent, beam angle considerations.

They look like a 5050 - but not having any white ones on hand (I only have a few RGB ones), I couldn't check.


Might get some...

[wraper]

You cannot just put any led there. Nor it will fit there, nor it will emit a light with proper wavelength distribution.

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I've already checked the pad setup - and a 5050 will physically fit well enough ... but the electrical and optical parameters are, indeed, somewhat important.


Does anyone know what LEDs they use?

[Rasz]

best case scenario would be to find same model with cracked screen sold for few $

[donkey77]

If your 'bodge' involved linking the faulty LED out would not the voltage have then been higher on that string possible taking out the second LED?!

Without trying to sound harsh but I doubt your relative has 20/20 vision if in a nursing home, if you can get a couple of 'close enough' LED with respect to cost, output, colour etc, could you not stick the new ones in the corners and move the originals into the faulty positions? I'm sure your relative would be more than thankful of your help.

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My 'bodge' was mainly intended for testing - but consisted of substituting a white LED I had on hand with almost identical electrical characteristics.  Though compared to the failed emitter, it's package was a lot more bulky and couldn't use the strip as a heatsink, the colour was a bit off, the beam angle was too narrow and it was far too bright.  I did play around with some ideas to see if I could make it work - and I did come up with one or two that might do in a pinch - but they are butt ugly.  While this ugliness is hidden, there are still some optical imperfections that I can't help but notice, especially on a screen with large areas of a single colour.  However, when viewing normal programming, it's much more difficult to see.

While I have no idea how long this substitute LED will last, I reassembled the TV and demonstrated the result to this person's sister - who tended to agree with the 'they won't notice' idea.  Can't say I'm overly comfortable with the result - but it does work ... for the moment.

best case scenario would be to find same model with cracked screen sold for few $

Agreed.  Even one from the same chassis would probably be OK - but finding one is the problem.

[Brumby]

... if you can get a couple of 'close enough' LED with respect to cost, output, colour etc, could you not stick the new ones in the corners and move the originals into the faulty positions?

Actually, I would NOT put the 'slightly off' LEDs in the corner.  I believe it would be better to have them with one original LED on at least 3 sides of the substitute, with the substitute set up such that it's brightness was a little less than the rest.  With this, the diffuser can spread the correct colour across most of the screen with the substitute merely helping lessen the problem of a less bright region on the screen.  My logic is that putting the substitute off in a corner increases the effective impact of the colour difference, since the neighbouring light on two sides will not be from original LEDs, but reflected from itself.

The other thing is that I would prefer to not desolder/resolder SMT LEDs


Still, this is a bodge any way you cut it and it doesn't sit well with me.  I might hand it back for the time being, but I'll be looking for the right part ... and now that I know my way into this panel, I could replace the LED bar in the field fairly quickly.

[poot36]

I fixed a bigger model of this TV by just shorting out the 2 bad LEDs in it.  They were in the top right and left corners of the panel and you could not tell the difference in the picture.  This was a few months ago and the TV is still working.

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That thought did cross my mind....

The diffusers do a really good job.

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My bodge fix required an alternative heatsinking solution - which isn't easy considering the geometries involved.  I had hoped my crusty attempt might be adequate but, alas, it folded a couple of days ago.

Previously, I had great trepidation opening up my first flat panel TV - but I dropped in to the nursing home today and just went straight into it.  Off the wall, disassembled, patched up the LED strip, reassembled and up on the wall in just over half an hour.  No rushing involved - I just knew what to do from my previous effort.

The solution was to simply short out the LED position.  The string now has 44 LEDs instead of 45 and considering the power supply sags 70V (from 220V to 150V) between no load and loaded in normal operation, I think it will cope with an extra couple of volts.

I haven't chased sourcing a replacement strip for a while - but it will be on my 'round tuit' list.  I did find some for $5-$10 (plus shipping) that looked promising ... on Taobao.  Might be an interesting purchase experience....