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Samsung TV backlight always on

Hi, I got a Samsung TV UE28F4000AW where the backlight stays always on, if I unplug the TV the backlight stays on until I press the power button. As soon I plug it back the backlight turns back on.

what is the problem?

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The Power unit of the TV first store the power then supply next to the parts. In case of power outage it protects devices from sudden power failure. Might possible the light you can see there it from capacitors and will turn off after a few minutes. Although you should confirm with brand customer service because there is no image which define the exact issue.

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Hi @francy66 ,

Is the red power standby light on or off when the TV is turned off by the remote or the power button on the TV?

There may be a problem with the cable connection between the mainboard and the power board or a problem with the power board or the mainboard. Further testing would be required to determine where it is.

Sorry to be so vague but when power is supplied to the TV the power board sends a voltage signal to the mainboard (standby voltage) that indicates to the mainboard that there is power available. It is the mainboard that turns on the red standby power light and it also maintains a signal to the power board not to turn on full power output i.e. stay in standby mode until the TV is turned on

If there is no signal on the PS on/off lead at the power board for whatever reason then the power board turns on out of standby and the backlights turn on full intensity. (Look for the PS On/Off lead at the mainboard - power board cable connector on the power board)

Be safety aware when working in the back of a TV when the power is connected as there is exposed lethal AC voltage on the power board. Disconnect the power from the TV unless testing for the various voltages

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@jayeff

Yes, the standby light is ON when the TV is turned off

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@francy66

OK then you'll still need to check if there is a signal on the PS on/off wire on the power board or even if the cable is connected properly at both ends - mainboard and power board.

Note the safety message, even though the voltages you're testing for are low DC voltages there is still AC voltage nearby, so unless testing with a DMM, disconnect the power to the TV.

The standby light question was just to see if other parts of the standby mode were working OK

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@jayeff

I checked the main board - power board cable and it seems connected properly, however I’m not sure how to check the PS on/off signal

por

@francy66

I can't find a schematic for the power board (is it a BN44-00644D?) so it will be difficult as I don't know the TV and the power boards and mainboards are not all the same as per the voltages.

Maybe the only thing to try is to measure the voltage at the PS on/off lead at the power board with the mainboard -power board cable connected to the power board and then do the same with the mainboard cable disconnected from the power board and see if they are the same or different. Remember to disconnect the power from the TV before removing the cable. Also if you can't easily access the PS on/off pin on the connector then you may have to trace the track on the board from the pin to the nearest accessible component on the power board that you can connect the meter to . As I said it is not easy without a schematic

Normally they would be different because when you disconnect the cable from the power board the backlights should turn on so if they are the same voltage at the power board then it means that there is no current flowing from the mainboard when the cable is connected which would alter the voltage value at PS on/off.

You may have to just order a board from a supplier that accepts returns if the replacement board doesn't resolve the problem.

Given that the TV works OK I'm leaning towards a mainboard problem and not a power problem but this is only a guess.

Also maybe check that the PS on/off wire in the mainboard - power board cable has continuity from end to end by testing with an Ohmmeter, just to eliminate it as a possibility. Do this with the power disconnected

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