Samsung 49KU6400U backlight

[russianbar]

Hi everyone thanks for accepting me to your forum. I have a problem with the above set, functions perfectly apart from backlights. I've tested them with LED tester and both sets light up ok in fact with the tester connected the TV is perfectly watchable. When connected to the BN44-0876A supply they don't illuminate, I've removed the connection to the main board which I believe should have brought them on as well but nothing. Is it possible for the fault sensing in the power supply to pick something up in the LEDs which the tester isn't showing or is it more likely to be a fault in the PSU?

[alfatv]

Re: Samsung 49KU6400U backlight

You need to get a multimeter and do some measurements.
Also, with LED tester, are all LED's on? No dark spots? What are voltage readings with LED tester?

[russianbar]

Re: Samsung 49KU6400U backlight

You need to get a multimeter and do some measurements.
Also, with LED tester, are all LED's on? No dark spots? What are voltage readings with LED tester?

Light spread looks ok no dark spots (it's edge lit) I'm showing 72 volts on each array (there's 2). Refering to the picture I'm measuring with LEDs connected:
Pin 3 1+ 365v
Pin 7 1- 296v
Pin 4 2+ 296v
Pin 8 2- 227v
All measurements to case.

[alfatv]

Re: Samsung 49KU6400U backlight

Did you measure it with multimeter or LED tester?
If with LED tester, you should unplug connector and measure at connector pins.

[russianbar]

Re: Samsung 49KU6400U backlight

Did you measure it with multimeter or LED tester?
If with LED tester, you should unplug connector and measure at connector pins.

The 72 volt was measured by the tester when connected to the LEDs which were disconnected. The others were measured using a Fluke 789/E multimeter at the LED connection to the the power supply with the LEDs connected.

[dick_barton]

Re: Samsung 49KU6400U backlight

The BN44-0876A appears to be a power supply for a 55" set with led voltage when in 2D is given as 200V @520mA and when in 3D given as 231V @ 1040mA

Is this the correct power supply for your set?

https://www.shopjimmy.com/samsung-bn...w-112345157940

[russianbar]

Re: Samsung 49KU6400U backlight

Yes mate that's the one. I believe it's fitted to quite a few different sets

[alfatv]

Re: Samsung 49KU6400U backlight

Supply voltage to LEDs looks OK.
There is a mosfet that controls current through LEDs.
It's drain should be connected to pin 8 of connector.
Check the voltages on that mosfet pins.
And obviously inspect PCB for solder joint cracks, check transistors and diodes with multimeter.

[russianbar]

Re: Samsung 49KU6400U backlight

Supply voltage to LEDs looks OK.
There is a mosfet that controls current through LEDs.
It's drain should be connected to pin 8 of connector.
Check the voltages on that mosfet pins.
And obviously inspect PCB for solder joint cracks, check transistors and diodes with multimeter.

I'll check it this morning and get back

[russianbar]

Re: Samsung 49KU6400U backlight

All looks ok apart from what looks like a choke/coil inductor which is open circuit L9101CS?

[dick_barton]

Re: Samsung 49KU6400U backlight

Run some fresh solder on the inductor pads and test again. L is generally a prefix for an inductor.

[russianbar]

Re: Samsung 49KU6400U backlight

Took it out still o/c but big crack in the base looks like we may have the culprit, anyone know of an equivalent because the number doesn't bring up many results

[alfatv]

Re: Samsung 49KU6400U backlight

I found this part reported bad in other forums, so it seems to be typical failure.
And I found datasheet too:
http://www.mu-sem.com/bbs/download.p...&wr_id=14&no=0
3mH is quite high value for inductor.
You can search replacement by inductance and other specs in the datasheet.
Or if you have a meter with inductance measurement, you can try some parts from scrap boards like transformers, common mode chokes. And you can connect windings in series to get the right inductance. It doesn't need to be precise, can be 20-30% off, usually slightly more is better.
Good luck!

[russianbar]

Re: Samsung 49KU6400U backlight

I found this part reported bad in other forums, so it seems to be typical failure.
And I found datasheet too:
http://www.mu-sem.com/bbs/download.p...&wr_id=14&no=0
3mH is quite high value for inductor.
You can search replacement by inductance and other specs in the datasheet.
Or if you have a meter with inductance measurement, you can try some parts from scrap boards like transformers, common mode chokes. And you can connect windings in series to get the right inductance. It doesn't need to be precise, can be 20-30% off, usually slightly more is better.
Good luck!

Thanks alpha, can you link to any threads on this?

[alfatv]

Re: Samsung 49KU6400U backlight

In your language, but not much useful
https://kenotrontv.ru/topic/7464-sam...ont-podsvetki/
https://kenotrontv.ru/topic/7455-isc...p-bn44-00876a/
http://monitor.espec.ws/section48/topic320937.html

[russianbar]

Re: Samsung 49KU6400U backlight

In your language, but not much useful
https://kenotrontv.ru/topic/7464-sam...ont-podsvetki/
https://kenotrontv.ru/topic/7455-isc...p-bn44-00876a/
http://monitor.espec.ws/section48/topic320937.html

Thanks again alfa (I'm not Russian just once owned a bar in Greece called Russian Bar )

[russianbar]

Re: Samsung 49KU6400U backlight

New power supply board from Ebay £10 and sorted. Not impressed with picture on the set though (given to me by daughter fix and keep) it'll be ok for a bedroom I suppose. Menus and guides are razor sharp but the definition even UHD below par. My main set is Sony KD-55X8509C am I expecting to much from the Samsung in comparison. Anyone know if there is any adjustments needed to match supply with set? Might swap the good inductor out of the new board and put it in the original?

[Storyteller12]

Re: Samsung 49KU6400U backlight

This is a entry level tv and not one of Samsung’s best. I believe it’s only 60hz refresh rate. The ku9500 is probably closer to your Sony.

[russianbar]

Re: Samsung 49KU6400U backlight

Massive thanks to all contributors