UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

[triplefour]

this tv is throwing me for a loop. do samsung tvs no longer have the backlight self test feature built into the power supply?
i tested the backlight with my LED tester and it lights up both strips. one of them reads 100v and the other reads 63.7v.
unplugged mainboard from power supply but left backlight plugged in, and then plugged psu wall power in. expected to see backlights light up but see nothing.
according to how ive diagnosed in the past, this would almost always be a bad power supply. replacement power supply was cheap like 20$ from ebay. but still i get not even a flash of backlight.
seller had a bunch of them listed as open box. not a used pull from a broken tv so unlikely a bad bord.
but how can my backlight tester make light but the power supply cannot? this is a very new tv so i start to fear/suspect that samsung has changed things up and now no longer builds the self
test feature into the new power supplies?


so now i have to dig deeper.
on the board i see that there are only 4 pins going to the LED backlight and 2 of them are bridged together (2 and 3)
to recap i was seeing 100v when i applied positive probe of LED tester to pin 1 and negative probe to pin2
and i was seeign 64v when i applied positive probe of LED tester to pin 3 and negative probe to pin 4
when i bridge the gap between pin 2 and 3 of the LED connector with 1k resistor (its all i had on me) and power both of them at the same time i got 190v on the LED tester.
100+64 does not equal 190v so i dont see why there is such a difference.
with the psu plugged into wall power:
with my voltmeter i see 280v at the LED output of the PSU but not even a flash of backlight. this voltage does not vary whether the power supply is plugged into the backlight or not.
and it doesnt vary when i inject 5v to any of the pins ANA_DIM, PWR, or BLU_PWM
i dont understand how i can have this high voltage at the LED and have no backlight lighting up at all.
high voltage but no current is flowing...??
its not throwing any blink codes...just acting totally normal with no backlight.

this power supply only has 13v rails. no 5vsb.
pinout of mb connector and voltages:
pin label reading
-----------------------------------
2 ANA_DIM 0.08V
4 NC 0V
6 BLU_PWM 3.25V
8 PWR 3.93V
10 GND 0V
12 GND 0V
------------------------------------
1 GND 0V
3 A13V 13V
5 A13V 13V
7 A13V 13V
9 A13V 13V
11 GND 0V

i tried injecting 5v to BLU_PWM, AND_DIM, and PWR both by themselves and together but was not able to get lights to come on at all.
what is the proper way to "trick" these new power supplies into turning on and turning on the backlight?

i tried also with the mainboard connected, with and without the panel connected, and even tried disconnecting the panel board bridge connectors... i get a tv that turns on and has picture and sound but no backlight!
are these new tvs different? am i doing something wrong?
i havent seen any post about this TV in the forum yet. i am seeing a lot of tvs that are only a couple of years old dying already! why samsung, why!?

someone please pardon my pun and shed some light on my situation here!
thank you in advance!

[Storyteller12]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

You got a bad power supply it seems like. If backlights are good and it doesn’t light up when main board disconnected from power supply. It means the power supply is bad

[nomoresonys]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

Seems like the problem is in the backlight circuit, the test is if the backlights come on, then probably backlights and powerboard are ok, when they don't light then problem is powerboard or backlight circuit, can be bad led/s, a bad connection in backlight circuit etc.

[triplefour]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

how can i have a problem in my backlight circuit when the backlight tester lights up the lights? i know my tester doesnt output the full current these lights actually run at but if i had a problem only when full power goes to backlights, shouldnt i at least see a flash as it powers up? shouldnt they at least be able to get as bright as my tester can make them before they fail?


... and are we SURE that samsung has not taken away our handy backlight+PSU auto test?



the odds of being sent a bad psu seem really low in this case. look where i bought from. he has many open box like these were brand new psu never installed before. https://www.ebay.com/itm/373394390286


besides disassembling the panel to look at the LED's or buying another PSU (which i bet will do the same thing) what other test can i do to be sure of anything?

[Storyteller12]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

First all there is no such thing as open box new power supplies. They all come from broken TVs, whether it be broken screens or what not but they’ve all been installed in a tv.

Unplug power supply from mainboard and remove backlight socket from power supply and plug tv in. Measure voltage output on backlight circuit.

If voltage is good and steady for all outputs then your backlights are bad. If no power output or if it fluctuates its the power supply

[Davi.p]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

I always sustain that new boards build only for spare parts does not exists except maybe few units only for authorized centers, the others are took out from working tv like your listing says.. this board maybe has different way to turn on backlight, but have you ever asked yourself why power boards have a different command to turn fully on main b. suplies and bl supply? Are them both needed? Since this is the first time i see label PWR instead of other ones already used i think that it turns on both MB and BL, you can also follow back the circuitry to the BL ic if you have time, if you want to make things short i usually suggest my test.. but i have lost my time many times explaining this, no one wants to make this stupid test.. i have never done it , if at least you want to try it... take a very small flat screw driver and extract from the holder the return pin of the leds wire, take a good leds bar, if that has no test pads create some, now solder a wire from after 1 led of the row and connect it to the place you slip off the return wire, place in some way (ad. tape?) the return wire to the input pin of the led bar, in other ways put 1/2/3 leds in series with the tv leds and try.. if BL comes on you have some bad leds..

[R_J]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

with my voltmeter i see 280v at the LED output of the PSU but not even a flash of backlight

You have a failing backlight led, The normal led supply is 192v@700ma When one of the led's fails the voltage is high due to no current so no voltage drop. Your led tester likely supplies about 10ma, those testers are ok for testing one led at a time.
With full current the bad led will go open.

[EazyBone]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

I'm working on this exact TV right now. I have it open to the LEDs right now. When I used my LED tester while it was open 3/4 of the LEDs would light up. The TV still worked but it shows you that with these edgelit Samsungs you can still "test" as if your LEDs are lighting up but that doesn't mean they are good. I have 142 volts from each set of strips. As rj says it's your backlight.

As a test couldn't you unplug the power board from backlights, hook up main board, turn it on and then take LED tester to the backlights and you'll see that some are out. I've done this with other tvs, never a Samsung.

**142 each side with new LEDs put in. Not the 3/4 LEDs, I tried to test that strip but no longer works cause I yanked it out. Good luck

[nomoresonys]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

So what should it be 142 or 192 volts?

[lotas]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

so what should it be 142 or 192 volts?

192v

[Davi.p]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

192 is the maximum so at full brightness..

[nomoresonys]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

Ok, another case of could have solved sooner had the legends of the powerboard been in focus, tried to read it but could not, thought 280 was too high for that tv.

[EazyBone]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

Here's some pics and voltage. My board also says 143 volts 65 inch.

[nomoresonys]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

So you had same situation, bad backlight but your voltage was 315 before replacement if I see it correctly, that should help lots of people if they find this.

[nomoresonys]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

You can add things to your signature if you think it will help and you don't have to keep going over the same shit, just tell them to look at your signature below if you enable it for a post.

[lotas]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

Here's some pics and voltage. My board also says 143 volts 65 inch.

No one measures voltage this way (there is no common gnd), you need to measure between + and - (led - and led+...)

[nomoresonys]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

Ok, I wasn't even looking at how it was done lol.

[EazyBone]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

No one measures voltage this way (there is no common gnd), you need to measure between + and - (led - and led+...)

See I learn every day lol

[Diah]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

measuring LED BL always depend on what kind of LED build in. LG had ziner build in, the way of it LED+ & GND chassis for each group. this also to any other brand which had LG panel. other with non ziner LED+ LED- to compare the high and low of each group.

[EazyBone]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

Here's a pic.

And thx diah for info, and lotas. Looks like 158v

Either way I still think this is backlight