Samsung ue55nu7096u - no backlight - faulty LEDs?

[ohren]

Hello,

I'm looking at a Samsung model ue55nu7096u that I've only ever seen in a non-working condition, i.e. I don't know if the failure was progressive or instant.

Symptoms

No backlight at all — LCD and logic board are working normally. Resetting all settings and installing system updates has no effect.

I tried reseating the connectors between PS and logic board, and PS and panel. No difference there.

I could not find a service manual.



Measurements when powered on

Bulk cap: 388 V. Stable.

PS panel/LED connector:
Pins [1,2,3,4]+ are all connected with each other on the PS board.
Pins 1-, 2- are likewise connected on the board.
Pins 3-, 4- are connected on the board.

Positive pins measure ~237 V, relative the negative pins. Stable.

Total power draw about 23 W, compared with 12 W when unplugging the panel connector from PS, and 3 W in standby.



Goal

Determining with high probability whether the cause is a faulty LED strip before attempting the dreaded task of lifting the panel off. I imagine first having to acquire a pair of suction cups to do that safely anyway, not to mention replacement LEDs if there are any to be found.


Questions

1. Is there anything weird with the measurements? On the PS board I can read '55": 109 V ↑' which I'm not sure about... It's the 55" model.

2a. Are all the LEDs in series? If so, I assume it's probable enough that they're at fault, so that I can confidently order replacements.

2b. If not all in series, is it still likely that multiple strips have died?

3. Having a bit of trouble finding listings for a replacement — only finding LEDs for similarly numbered models like UE55NU7300, UE55NU7100, UE55NU7105 — but I'll cross that bridge when I get to it I guess.



Background

I do repairs at a hobbyist level for fun. So far, every repair I've received guidance for on this forum has been successful (1 x PSU, 1 x CCFL monitor, 1 x amplifier). Thanks!

[dskall]

Unplug t-con and see if lights come on when powered.

[lotas]

Originally Posted by dskall

Unplug t-con and see if lights come on when powered.

T-CON cannot be disabled on this platform, it is built into the mainboard.

[ducky29]

Originally Posted by lotas

T-CON cannot be disabled on this platform, it is built into the mainboard.

He meant unplug the flex cables that come from the screen and go to the mainboard.

[ohren]

Thanks for the suggestions.

Originally Posted by lotas

T-CON cannot be disabled on this platform, it is built into the mainboard.

That was the conclusion I came to as well after looking in the sticky tv troubleshooting guide for the definition of T-Con board. As a side note I found the description a bit hard to understand, since the description could, at my level, also have fit the long thin board on the LCD (name? Addressing board?), except that the picture mostly ruled that out.

I disconnected the two ribbon cables going to the LCD from the logic board, and powered it on — there was no visible change even in a dark room. Power draw reduced to 19 W, everything else behaving normally (I.e. bootup sound). If you don't mind me asking, what was the theory behind this particular test?

[dskall]

Some tv's will turn on backlights with cable unplugged.

[R_J]

The voltage for the led's should be around 126 volts, you measured 237v, this indicates the led circuit is open and not drawing any current.

[ohren]

Got it.

I suppose that now is a good time to conclude the LEDs need replacements?

Let's say there is more than one LED strip — are they in series?

I'm wondering a bit why the connector has one positive pole and two negative poles. Anything in particular I should read up on to understand that, or is there a simple answer?

[lotas]

https://www.youtube.com/watch?v=oSb7i6AeLHk

[R_J]

Originally Posted by ohren

Got it.

I suppose that now is a good time to conclude the LEDs need replacements?

Let's say there is more than one LED strip — are they in series?

I'm wondering a bit why the connector has one positive pole and two negative poles. Anything in particular I should read up on to understand that, or is there a simple answer?

There are likely two strings of led's The positive is common to both strings then each strings negative controls the current through the string.
Some smaller panels use one continuous series string, lager panels can use four or more separate strings.

[alfatv]

Unplug connector between power and main board, then power in on. Backlight should turn on if good.
Another way to test LED's is LED tester.

[ohren]

Unplugging the main board results in 5 W power draw and ~197 V LED output. No visible light.

[neilc6]

NU series are edge lit from the bottom and 55 in has two strips. LED failing open circuit would have acted like a fuse and might prevent the plastic panel from melting. https://techhometravel.com/samsung-n...ight-failures/

[ohren]

Wow, what appalling product quality that article demonstrates.

Do you think I have enough information to conclude the LEDs are faulty now? If so I have to figure out how to find the right part before removing the panel. I found no exact hits on eBay or aliexpress, so I'm thinking that perhaps I should search for something other than the model number...

What I don't understand completely is, if there are two strips, why would both fail? Surely you don't keep using half a TV until the other strip goes. If each strip has its current regulated individually (the explanation for separate negative poles) then I don't see one failure causing the other strip to receive more current and failing more quickly.

[neilc6]

Though there are two strips, they are likely wired as a single string so one open circuit would create no backlight at all.

[R_J]

There is only one controller ic that monitors the current as well as over voltage, when there is a fault in either circuit the controller shuts down the backlights.
It could be that just one led in one string failed, you could locate the bad one and just replace it. then keep doing it as others fail over time, risking screen damage every time you open the panel. Or replace them all once.

[ohren]

I see. I just can't consolidate those explanations with the pictures in the linked article showing half lit sets. I feel like I'm missing something (unless this NU model simply behaves more uniquely) but I guess I'll drop it.

If I don't get any direct search hits for replacement strips when searching for the TV model number, are there any alternative strategies for identifying replacements except for lifting off the panel and inspecting the LEDs directly?

[neilc6]

You likely have an open circuit in each strip. With bad backlights, just replace them all with new. They can have different wiring configurations so there isn't a point in trying to figure out how the strips are wired. Search UE55NU7105 on Aliexpress and there are lots of sellers. Edge lits are extremely finicky and any imperfections can cause bright spots or an uneven backlights. Easiest way to confirm an open circuit in the string is with a LED tester at the backlight connector.

[ohren]

Thanks. Yes, my intent is to replace all LEDs — I really don't enjoy opening stuff up many times because I was lazy the first time.

I try to wring as many lessons as I can from each repair attempt, so that's part of why I try to figure out what I can even if it's not strictly required for the repair. In the same vein, how did you know to search for UE55NU7105? How could I have found that number myself? Granted, when it comes to shopping for spare parts I'm not very experienced and know no tricks...

[lotas]

AOT_55_NU7300_NU7100 BN96-45913A BN61-15485A

https://aliexpress.ru/item/400114220..._source=aerkol

https://russian.alibaba.com/product-...188010675.html