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do not have a standby light

Hi @oldturkey03, I have the same issue as described in the original post. I have also replaced both my main board and power supply and still do not have a standby light or any activity when I plug in the TV or press the power button. I have taken the voltages on both the old power supply and new power supply while they are plugged into the wall. I measured on the connector that goes between the power supply and main board. The original power supply had voltages of .93 across the 3 middle pins on the top row. The same pins on the new power supply are at 1.12. Seems like a slight difference, but with both power supplies plugged into AC power and unplugged from the main board, I am not getting the backlights to light up like others have troubleshooted before. So I am still unable to determine if it is a bad power supply or main board. Do you have any further troubleshooting tips?

Update (09/18/23)

In the pictures, the power supply that is installed in the tv with the wires coming out of it is the new replacement board that I bought. The other picture of the uninstalled power supply is the first one that was in the TV before I replaced it.

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@thill44 we need you to clarify what model your TV is. Your original comment was posted under a AU8000

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TV is a UN75AU8000BXZA

Version No : UA01 AU8000B-U75AB1

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Hi @thill44

Here's a zoomed in image taken from the 1st image you posted above (I assume it will be the same for the 2nd but it isn't as clear) showing the Standby voltage ref point on the power board.

Connect everything up so that is is all connected as per normal and then connect power to the TV and check if you get a voltage reading at the reference point.

If it is 7.5V then assuming that the cable to the mainboard is OK then it is possibly the mainboard. If <7.5V it is the power board.

If it is 12.35V then the power board is turned on, so again it may be the mainboard

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Thank you very much for the reply, will check on this and update soon.

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@jayeff I tried my multimeter on the standby reference point and both power boards are around 1 volt when connected to the wall. Out of curiosity, I removed the power board from the TV and disconnected the main board and what I’m assuming is the power connector for the backlight/LCD. With the power board plugged into the wall by itself like this, the voltage at the reference point was a perfect 13 volts! So it must be the backlight/LCD connector shorting it out and giving it low voltage. I’m afraid that if this is the case then the panel is not able to be saved?

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I just confirmed that if I connect everything on the tv as usual except the large connector on the power supply (not the small connector between the power supply and the main board, this is the only other connection on the power supply) then plug the power supply into the wall, I get a red standby light, and am able to turn on the TV and hear the startup chime. Of course, the tv has no picture or back light because I disconnected that from the power supply intentionally. what could be causing this to short out?

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@thill44

When you set it up so that you can start the TV and hear sound, try shining a flashlight at an angle close to the screen and check for images. They will be very faint so try this in a darkened room if possible.

If they are there then it may be the backlights.

Disconnect the power to the TV and then disconnect the backlight LED power cable where it connects into the CNL801 socket on the power board.

Using an Ohmmeter, check the resistance between the following pins on the backlight LED power cable plug

pins 1 & 3, 4 & 6, 7 & 9 and 10 & 12.

If you don't get a reading or it is a very high resistance reading reverse the meter leads and check again. By rights they should measure nearly all the same with the leads in one direction and then all the same with the leads reversed i.e. red on 3 and black on 1, red on 6 and black on 4 etc should all be the same and then with black on 3 and red on 1 etc should all be the same but most likely will be different (or should be) to the first set of measurements with the leads the other way.

This is just checking to see if there's a very low resistance path on one of the 4 LED backlight strips that may be dragging down the voltage on the power board.

Also check if there's a low resistance path between any of the pins in the LED power cable and an earth point (screwhead) in the TV just in case there's a short to earth on the strips

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Want to make a late update as I was able to solve the problem. I found out that this TV had been showing signs of failing backlights before it failed, with what I would describe as a dim column on about a quarter of the TV. I was not able to measure the resistance as described above, but I decided to tear down the TV and check the backlights instead. Once I could get a look at them I immediately noticed that one of the 4 strips of backlights had burned and melted entirely inside of the tv, which was shorting the power supply and restricting the tv from turning on unless they were disconnected. So I unplugged that led strip and plugged the power supply back in and all of the other backlights immediately lit up like oldturkey03 mentioned they would. Since then I replaced the LED and the TV is repaired. Thank you all for helping me diagnose the issue.

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@thill44 We need you to add some pictures of your boards and interconnect cables. https://www.ifixit.com/Guide/Adding+imag... happens when you disconnect the backlight LED Connectors. Any difference in presentation? If there is no standby light and you disconnected the main board,something else is either shortening out the "cold" side of the power board. After all by disconnecting the mainboard you also disconnect the IR board etc. Or this is another bad power supply, which would be very strange. Let's see what you got going on here.

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Hey, I have updated the original post with more info about my TV and with pictures of the original and new power supply. I have tried connecting everything except the large ribbon cable to the backlights and still nothing. Still no status light on either. Now is a good time to mention that I have tried some of the easier solutions, tried the outlet with a lamp and it is fine, tried a different outlet, tried the physical power button and the remote power button, ensured the remote has batteries, and tried a different power cable. The TV stopped working before I moved, and then after I moved I had broken the IR sensor/power button/status light module and it was the first thing that I replaced. The part numbers matched up exactly so I am sure it is not that.

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@thill44 just to clarify, it was already not working when you replaced the IR sensor/power button/status light module. I agree. What makes me go "HM" is this "UN75AU8000BXZA" Samsung has trouble with the large panels shortening out the system. Try and reconnect everything but the LCD and see if that makes a difference. If you can, post some pictures of the rest of the board"s" and wiring.

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Thanks for the reply, yes it was broken before I moved and I just got the opportunity to fix it. I’ll upload some more pictures later and try it out without the LCD plugged in.

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I have tried every combination of connected and disconnected that I can think of and still am not receiving any indications of life. I wouldn't think that the power supply I ordered is also bad, but I do not know of any way that I can definitively test it. Any ideas?

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@thill44 disconnect the power cord, disconnect the main boards. Reconnect the power cord and the backlight LED‘s will come on if your power supply works. Let us know what you find out.

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