Samsung un50nu7090 standby light blinks once

[liquidzorch]

Hello. I have an led tv here that I am trying to troubleshoot. When plugged in the standby light goes red, if I press the power button the light turns off then back on within less than a second. This tv has a power board integrated into the motherboard, so replacing components just to diagnose is a bit expensive. It was taken to a repair shop, and they said it was the LED strips, but they did not have them in stock so gave it back.

I normally would doubt that LED strips would cause a no power situation, but I have never come across a board like this. Maybe the led controller detects bad leds (open circuit) before turning on?

I can't seem to find any missing voltages around the motherboard side. There are 3.3v, 1.2v, 1v. THe power supply outputs two different voltages, 12v that goes towards the motherboard side, and around 130v that goes directly into the led strips. Both are present while plugged in.

Now if the led strips are plugged into their positive voltage terminal, and the negative sides are disconnected, I should see a voltage difference between these loose wires (LED negatives) and common ground right? Because there is no voltage, what seems like an open circuit on both LED strips (this model has two strips).

Any ideas? Does this model somehow verify LEDs before turning on?

[Andrew F. Ali]

Re: Samsung un50nu7090 standby light blinks once

I would agree with them that the Backlights are the fault.

[liquidzorch]

Re: Samsung un50nu7090 standby light blinks once

Well, I removed the screen, there were no LED strips in place, the shop removed them. But there was a little bit of char on one of the cable connectors on the LED strip side, so I decided to order the LED strips, got them today, installed and same issue. The red light just blinks randomly. I thought it only blinked when pressed (still does), but if I stare at it for awhile, it will blink once quickly at random intervals. Nothing on the board seems charred or even slightly wrong, and I cant seem to find any missing voltages, but I also can't find any info about this board online.

[liquidzorch]

Re: Samsung un50nu7090 standby light blinks once

Update.

When the power cord is connected, the mosfet that feeds the 12v into the mainboard turns on and I get all standby voltages, 5v and 3.3v, including the 1.5v and 0.97v that seem to go to the main processor. This stays on for about 15 seconds, then the mosfet turns off for less than a second and back on again. This process repeats infinitely.

[liquidzorch]

Re: Samsung un50nu7090 standby light blinks once

Okay so I found that the 5v and 3.3v stay on always. If I remove the 5v coil to disable motherboard from turning on, I get a flickering backlight, I guess that means I can rule out the power supply side of the board. So something from the motherboard side is bad. I hate that the motherboard and power supply are on one board, I can't even find a replacement where I live.

So far I have tested most ceramic capacitors for shorts all around the motherboard side, nothing. Everything would appear fine. Any ideas?

[liquidzorch]

Re: Samsung un50nu7090 standby light blinks once

Ok. So I can confirm that the power supply side is working. I removed the backlight pwm resistor (to separate the LED controller from the motherboard side), and got a solid on backlight.

The motherboard seems like it is trying to oot up for 15 seconds, then restarts, but during this there is no image at all. Just a solid red light, and each time it reboots the red light goes off for a split second.

It seems as if it was a bad flash.. maybe. Any ideas? I can't find this board where I live to buy a new one and I have spent hours on it trying to diagnose each part.

Ps. If I enter 1 8 2 mute power, the led starts to blink, and does so until the motherboard resets. If I was already 10 seconds into the boot cycle, then in blinks for the remaining 5 seconds. If I was 1 second in, it blinks for the remaining 14 seconds. Then back to solid red.

[Davi.p]

Re: Samsung un50nu7090 standby light blinks once

Post clear picture of the mainboard. The problem can be software or on the panel, try to boot the tv whitout tcon connected, or panel connected..

[liquidzorch]

Re: Samsung un50nu7090 standby light blinks once

The board does the same thing without the lcd connected. It does the same thing no matter what components I disconnect. I have tried without speakers, without wifi board, without lcd, and even without the power button (I can tell it does the same thing because of a faint power supply hum that pauses each time the motherboard resets).

Here is a photo without the heatsink.

https://imgur.com/a/eilMlqB

[Davi.p]

Re: Samsung un50nu7090 standby light blinks once

That pic is very low res on my phone, can you post board BN code? or are you able to locate possible microcontroller and flash mems?

[liquidzorch]

Re: Samsung un50nu7090 standby light blinks once

Sorry, It was taken with low light. It is a bn9649475a board. I don´t have the other information with me right now.

[Davi.p]

Re: Samsung un50nu7090 standby light blinks once

There are no good pics in the web, i can't help you without good pic. Or maybe Multiple pic, to see chip markings..

[liquidzorch]

Re: Samsung un50nu7090 standby light blinks once

Here are some high res photos I took. The links don't provide as much resolution since you can't zoom in, so I will add a link to my google photos. You should be able to read everything on the board.

https://photos.app.goo.gl/t9cCiKC4APqPtvV66

[Davi.p]

Re: Samsung un50nu7090 standby light blinks once

I see very out of focus the zone where there are those three sop8 chip near the metal clip on top right of first picture, anyway freeze those 3 chips plus ic503 and U23, with freezer spray or butane gas bottle etc.. wait 5 seconds then turn on tv...

[liquidzorch]

Re: Samsung un50nu7090 standby light blinks once

I have tried both freezing and heating, neither made any change.

[SMDFlea]

Re: Samsung un50nu7090 standby light blinks once

I can see a little solder blob or some damage near the 2 caps,looks like its bubbled up a bit around it from the heat

[Davi.p]

Re: Samsung un50nu7090 standby light blinks once

I instead have seen a resistor not right in place, near L406 inductor..
have you checked tension across all inductors in low volt section except the four in a row?

[liquidzorch]

Re: Samsung un50nu7090 standby light blinks once

I can see a little solder blob or some damage near the 2 caps,looks like its bubbled up a bit around it from the heat

Thanks, but that was me, I soldered a cable there to connect my oscilloscope probe clip. It is just excess solder on the 12v line.

[liquidzorch]

Re: Samsung un50nu7090 standby light blinks once

I instead have seen a resistor not right in place, near L406 inductor..
have you checked tension across all inductors in low volt section except the four in a row?

That was my latest attempt to diagnose. I saw that the coil was shorted to ground, so I de-soldered it to test which side was shorted. Turns out it goes straight to ground. Later I found out that the IC near it is a step up buck converter (sort of) so I guess it need a coil to ground. The resistor and diode next to it moved, but they are on their correct pads.

[Davi.p]

Re: Samsung un50nu7090 standby light blinks once

If you say coil to ground it means negative output dc-dc, so i guess this has an integrated tcon also, or not?
So what do ou say about the tensions on coils? If tcon integrated, it is highly common a fault in the dc-dc stages of tcon, i guess especially boost one.

[liquidzorch]

Re: Samsung un50nu7090 standby light blinks once

If you say coil to ground it means negative output dc-dc, so i guess this has an integrated tcon also, or not?
So what do ou say about the tensions on coils? If tcon integrated, it is highly common a fault in the dc-dc stages of tcon, i guess especially boost one.

Yes, it has an integrated tcon. If by tension on coils you mean a voltage, there are no voltages on the tcon coils. The tcon ic is not powered up. There are two p-channel mosfets that are on an off state, the gate is never pulled down (to power them on), theese two mosfets when on would deliver 12v to the ic. I imagine that the processor never gets to the part where it powers them on.