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Why won't my TV turn on? No standby light

I have a Samsung 55” UN55MU6300 4K Ultra HD Smart LED TV.

Today I noticed the TV would not turn on and the standby light is off. The remote has batteries and is working. The outlet has power. I unplugged the TV, pressed power button for 30 seconds, plugged it back in and it still does not work.

I tried to check the power board (L55S6 FHS BN44-00807A) but have no idea what I am doing. I have attached a picture of it.

Block Image

Is there something wrong with the power board? Ordering and switching out the power board seems simple enough for me to do and something I am willing to try before requesting professional service.

Please help!

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Hi @alanmajors ,

Check that the fuse is OK. (play it safe and disconnect the power and use an Ohmmeter to test it).

If it tests open circuit then there’s always a reason why a fuse blows. You could try replacing it and see what happens (power surge perhaps caused it to blow). Search for 250V T5A fuse. T= Timed (or slow blow) so make sure that you get a slow blow fuse as a standard fuse with the same rating will blow straight away when the power is connected.

If the fuse tests OK i.e. short circuit, then reconnect the power to the TV and then check that the voltage at the Standby Voltage (STB) reference point on the power board is OK. It should read 8V DC in standby and 13V DC when the TV is turned on.

Be extremely careful when testing at the reference point because you don’t want the meter’s test probe to bridge between the pins of the power transistor and short them out. Also take note of @oldturkey03 advice as there is exposed lethal AC voltage on the power board.

If there is no voltage at the reference point then there is a problem with the power board. If there is a voltage and it seems to be approx. the correct value try to find it on the cable going to the mainboard (CN803) and check if it gets to the mainboard OK.

If it does then it may be a mainboard problem, as it turns on the standby power light when it gets the standby voltage signal from the power board.

Here’s an image taken from your board image to show where it is located.

Block Image

(click on image to enlarge for better viewing)

Also try the following to see if the power board is working to some degree:

Disconnect the power from the TV.

Disconnect the cable between the power board and the mainboard.

Reconnect the power to the TV.

The screen backlights should turn on and stay on. There will be no picture because the mainboard is disconnected.

If it does this then the power board is functioning enough to be able to supply the backlights i.e. power OK from AC power input through to DC power output to backlights, but this doesn’t mean that it is supplying the mainboard if you get what I’m saying

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@alanmajors it could be power board issue but remember that it is the main board that tells the power board to turn on. Use a multimeter and check the power going into the PSU (power supply). the fuse right above it should test for continuity as well as for power. (multimeter to AC voltage (220V or 110V depends on where you are) black to ground and red to either end should get you a power reading. It’s hot so you don’t wont to touch it with your fingers. Could be a shocking experience! After that you will have to take measurements at the connector to the main board. The values that are expected will be printed on the board. Most common reason for no standby voltage etc. is a bad PSU :-) So if you can get one relatively cheap with a good turn policy just in case you need to do so, go for it. Let us know what you find out.

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Hello,

i had a powersupply that had standbyvoltage (strobing) but no backlight. The backlight voltage was too low. In my case one of the two boost Diodes for the Backlight had a short. Easy to measure with a Diode tester. After replacing it was working again.

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@legotechniker absolutely the right thing to test. That just shows the importance of a good formulated question. This from the OP "Today I noticed the TV would not turn on and the standby light is off" is indicative of a bad power supply vs. anything else.

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