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Samsung TV won't turn on, no standby light

Hi all,

I have a Samsung 4K TV, model code UN65MU6300FXZC, version number FA01.

The TV will not turn on and the red standby light does not turn on either. Checked the outlet, outlet is fine.

My first thought was the power supply board but when I tried unplugging the main computer board from the power supply board and then plugged the TV in, the back-lights came on right away. Of course no image on the display because the main board was unplugged, but because the back lights were on, this made me doubt that the power supply board is the issue.

Anyone have any idea if this indicates a main board problem or a power supply problem?

Thanks!

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Hi @ethanh and anyone else that is interested.

Just wanted to let you all know that I replaced the main board on my TV today and the TV is working again!

Thanks to @jayeff for confirming that if the back lights come on when you unplug the main board from the power supply board, the problem is the main board.

Not sure if we are allowed to recommend parts suppliers but ShopJimmy or TvPartsToday would be my recommendations. I bought my main board from TvPartsToday because ShopJimmy was out of stock (ShopJimmy was my initial first choice). In the end had no issues with TvPartsToday.

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@drowdarrow Hey glad to hear you got it working! Thanks for the update. I've been slacking on doing the final tests on mine but @jayeff has also been really helpful in giving me instructions on that. I'm hoping I can get a part ordered for next week.

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Hi @drowdarrow

Check on the power board , there should be a test point marked STB (standby) with a voltage value or it may be printed in a box indicating a pin voltage in the cable to the mainboard which shows the expected voltages

The STB voltage is usually 7.5V when the TV is off and 11.8V when the TV is on but it may be different in your model.

With the TV in standby mode check the value of the STB voltage, as shown on your power board,

If the voltage value is correct then most probably the problem is in the mainboard as it turns on the standby light if it gets the correct STB voltage from the power board.

If the STB voltage is not the correct value, it most probably is the power board

If the standby light was on and the TV doesn’t start then that is another problem

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Thanks, I will look into checking that. I do plan on getting to the root cause but I may just end up ordering both boards due to parts being available and relatively cheap. That way I have some spare parts for the future when boards aren't as available or as cheap.

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Hi @drowdarrow ,

As a safeguard for the future, if you haven't already done so , connect the TV to the power outlet via a quality surge protector, i.e. not a cheap one.

The better ones indicate if they have operated due to a power surge and may have to be replaced and some even come with a manufacturer's warranty covering replacement of the appliance if the appliance fails due to the surge protector not protecting it from a power surge.

The cheap ones are usually of the one time sacrificial type and give no indication at all that they have operated unless there is no power available through them after the surge has passed. In some instances they will still supply power after a surge but will offer no further protection unbeknownst to you.

Just my experience with surge protectors and TVs

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@jayeff Great tip, I did have it plugged into a surge protector but not a quality one. I'll be sure to replace it with a better one before I hook the TV back up.

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Hi Andrew, have you had any luck with this? I also have a Samsung UN- but 50". I get no power and no standby, but in my case when unplugged from main board I get no lights, even though the voltage tests I've run so far seemed normal. I need to test that standby when plugged in. Did you test yours?

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Hi @ethanh I did not diagnose exactly what’s wrong but I did order a new main board and a new power supply board. I probably should spend more time with a meter to see exactly what’s wrong but I figured it’s going to result in replacing at least one of those boards. Those boards should be arriving in the next few days, I’ll update this post with the results!

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Hi @ethanh

Apologies if I confused you but I did say to find the STB reference voltage on the board. I didn’t say that it would be in the CNM801 table on the board

Here’s an image taken from the image of the board in this link where I’ve highlighted where the STB reference voltage can be checked.

Block Image

(click on image to enlarge for better viewing)

You didn’t give the make and model number of the TV, but if it is a Samsung TV try the following:

Disconnect the power from the TV and then disconnect the cable between the mainboard and the power board.

Reconnect the power and check if the backlights turn on and stay on.

If they do then there’s a problem with the mainboard or the cable between the mainboard and the power board.

If they don’t come on check the voltage at the LED cable connector on the power board. If it is very high DC voltage then there is a problem with the LED backlights, the distribution pcb that the LED strips are connected to or the cable feeding the power to the backlights. There is an open circuit somewhere and no current is flowing through the backlights which would lower the voltage

If it is a low DC voltage and the backlights aren’t on then there is a problem with the power board.

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@jayeff Ah okay, my mistake. I did see that STB on there when I initially pulled the board a few days ago. I don't know why I was looking for it on CNM801 though.

I'm going to try out your suggestions today. I can't thank you enough for all of your help. I'll post back with my results!

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Samsung smart Tv had the same issue, No power no Red LED on. Completely dead. Then i found the solution in this video. The output rectifier was damaged and I replaced it as shown in this. It starts working. :-) (Y) > https://www.youtube.com/watch?v=tJxs9LUF...

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Sounds like there was a short on the mainboard that was pulling the PSU board down, isolating the mainboard from the PSU and testing the unit that way is always the way to go. Congrats on fixing the unit!

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Andrew wird auf ewig dankbar sein.
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