Samsung UE49MU6400 acts dead until disconnecting backlights, then works.

[jesterace]

Hi everyone I have this TV which was dead one day when I tried to turn it on. I've had the cover off and there was just a pulsating/electrical clicking/ticking noise from the PSU. I disconnected the mainboard as sometimes this works to just fire it up with backlights on? That didn't help. Same noise. Then I disconnected the backlight cable from the PSU and the TV came alive. It appeared to respond to the remote and i'm pretty sure it's fully operational when there is no backlight connected.

However, I am in a dilemma.

I tested the backlight outputs with the lights disconnected and they only read 1-3v (mostly around 1.1v) on both outputs. There are two backlight channels.

I then tested the backlights with a backlight tester I have. They do light up, albeit with a bit of flicker initially and then they settle. The right side seems a bit dimmer than the left when looking through the port holes in the chassis of the TV. I tested them separately.

So I am not sure right now whether to pursue a new PSU (because there is no voltage output) or new backlights. Should the backlight output voltage be present even if there are no lights connected? Even just a brief voltage spike and then off when it senses no lights?

Where do you think I could go from here?

Thanks,
James.

[Diah]

LED BL

[Davi.p]

100% bad leds, already opened one of those. Usually one checks the voltage in the backlight tester, and you see the difference between channels.

[jesterace]

Ok thanks, I did find a PSU for about 8 quid so i grabbed one anyway that should be here next week to experiment. It just seemed odd that there is no voltage at all on the led output even when they are not connected. So i gather that is normal for this kind of set?

[Davi.p]

i already told you 100% bad led.. the led output is enabled by pulling to gnd one of the wires from psb>mb connector, that is.. pin 1 fail count..

[CapLeaker]

And the LEDs are current controlled, not voltage controlled.

[jesterace]

Thanks, I know you've told me that its bad led's and I value you're advice. I'm not trying to upset anyone who has more knowledge than me. I'm only trying to clarify some things because usually when i've probed the output in the past on other TV's there is usually voltage output on the LED connector whether there are LED's connected or not. Even if it's just a brief spike. On this TV I see nothing even when the LED's are disconnected. Maybe that's normal for this TV but this is why i'm asking these questions.

Take for example this video.

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

I think somewhere in this video around 6minutes, it shows that in that particular case there was voltage on the output with no LED's connected. This is what I was expecting to see with my TV. This is why I was originally cautious of thinking it's the LED's. Does that make sense?

I don't want to come across like I'm being an asshole. I just like to learn and understand the details why things happen. Maybe the TV in the video just operates differently but I don't know, that's why I'm here asking people who know more than me.

If you can help me to understand that mystery I'd be grateful.

[Diah]

yours BL are Edge LED, this kind of LED each unit build in zener Diode in reverse way,,, LED Tester will light them even one or more had shorted or open zener diode,, but the PSU will remark the short at zener because it will pull unusual currents so will be shutting down

[Davi.p]

Fail count pin tells nothing to you? I'm not sure but it is almost similar to the previous Sharp tv, where if leds turn on failed a certain amount of times the main board settled an error flag that inhibites tv turning on, the same your situation was for the tv of my brother, you don't even have tried voltage on the led tester as i said, what do you wait? If you don't trust me then try...

[jesterace]

I'm not an expert but I have a brain and like to learn. You telling me about fail count tells me that the TV is monitoring if the LED's were able to be illuminated within expected current ranges and if it detects a fault then increments a counter, and when the counter is exceeded the output is turned off? If thats the case then all that i'm trying to say is that wouldn't it be sensible to expect to see some voltage on the led output while it is checking the LED's and incrementing the fail count? or am I to expect that (on this model) I would not see any voltage until good LED's are connected? Perhaps because it doesn't output anything until it senses a load or simply because the fail check happens so quickly...

To be honest this is such a simple question that I have no idea why i'm going into this much details. All i was asking was:

If I disconnect LED's. Then connect multimeter to LED output. Then plug in power. Then switch on power and check multimeter immediately, should I see any high voltage on a good TV of this type? Such as 50,60,70+ volts?

If its not possible to know the answer because nobody has actually tried this then I completely understand, but really this was all I was asking. This is not about trusting anyone, it's about learning, so please don't think that i'm not trusting your advice.

Why do you think I bought another PSU? not because I don't trust the advice. I bought it because I want to know if I should see voltage at the LED output so I can learn for the future because I haven't been able to get that answer here. The only way i'm going to learn is from someone who knows the answer for sure or by getting a board which is "known good" and checking myself.

I do appreciate your help. In my original message I did ask this question though and nobody seems to directly be able to answer that.

[Diah]

of course you will see high V and then low on the LED socket when LED connected and Mainboards disconnected from PSU.

post photo of yours PSU BL circuit from the top and downside of the PSU

[Diah]

one things more .. you write when you plug out the LED socket TV work and response to RC,,, and you wrote too you tested the LED strips with tester... so unplug the LED socket and turn on the tv to work and light up one Chanel by one with yours tester,,,, so you will be able to see if there one shadow of defect LED in picture

[Davi.p]

The Fail count is to stop the software, not to turn off backlights , but indirectly stops also the attempts to turn them on, yes i guess almost surely a digital multimeter is not suitable to capture the attempt, an analog one probably do, if yours has Max function you can try it, or you can try in ACV, is it enough as an answer? If you continue doubting without trying the voltage with the led tester means you don't catch the problems, a strong difference between bars tells the bars are kaputt, also different light tells them are kaputt, i will not answer anymore until i see you change the bars, sorry..