Samsung UN55HU6840 Solid red light, won't turn on

[AlexVaq]

Hi guys, I hope you can help me out with your experience.

I have this wonderful Samsung 4K TV that shows a solid red light when connected to power (standby, I guess), but refuses to turn on. When I press the physical button to turn the TV on (on the back), it does nothing. So I opened it and try some tests. So far I have checked:

- That the button works properly.
- That the backlight turns on if I disconnect the main board from the power supply.
- That the power supply board (a BN44-00755A) gives the right output without load on the CNm803 connector (the one that links to the main board). I used this posts to find out the voltages:

https://www.badcaps.net/forum/showth...ght=un55hu6840

where user @2smoovedaddy kindly put the expected output in a post.
- I took the repair guide from a russian page and began the troubleshooting for "no power" (see attached pic). I get:

BD213 --> Fine (~5V)
BD200-208 --> Fine (~12.8V)
BD228 --> Wrong (gives ~0V)

Of course, the power output of the main board in the next step is all screwed up. IC209 is fine, but the rest are totally wrong.

So I try to find a short in BD228, but I don't see anything. At this point I think the power supply is broken, and although the output is right without any load, that's not very meaningful, and under load doesn't work very well.

But there are a couple of things that make me be suspicious:

- BD228 is connected to BLU_ON on the power supply, which is 3.13V (no load) according to the aforementioned post. Why should I expect 5V there (according to the repair guide)?.

- The power output of the main board is totally wrong. I would expect a few things to go fine with solid 13V input, but I'm wild guessing here and I can be completely wrong.

Can you give me an advice?

[petehall347]

Re: Samsung UN55HU6840 Solid red light, won't turn on

is the BD228 a regulator transistor .. if so check its collector emitter and base voltages

[AlexVaq]

Re: Samsung UN55HU6840 Solid red light, won't turn on

Thanks for the answer! No, it is not a regulator transistor. It looks like a resistor or a small capacitor, but honestly I don't know what it is. I'll upload a photo when I get back home.

[AlexVaq]

Re: Samsung UN55HU6840 Solid red light, won't turn on

BD228 is the one next to the 23. Can this be a diode?

[AlexVaq]

Re: Samsung UN55HU6840 Solid red light, won't turn on

Got more info. The power on sequence is:

- Main board gets 5V (stand by). This seems right.
- Upon pressing the power on button, the voltage on pin PS_ON raises to 3.3V, and the PS returns 12.8V on the 13V lines. I think Vamp should be ~13V as well, but I'm not sure (other models are 18V).
- Then, the main board returns something on BLU_ON, up to 3.3V. This controls the backlight, 3.3V is maximum brightness.
- Then other stuff happens, which is not very relevant to the discussion.

My situation is that the PS is always getting 3.3V on PS_ON when I connect the TV, and I'm not sure if this is expected (I should try to ground PS_ON to see if the PS turns off). But anyway the main board is not sending anything to BLU_ON, it stays at 0V. I'm strongly suspecting the main board, but I would appreciate some advice.

In case you have some wiring diagrams to share, or specific instructions to troubleshoot the boards, here are some P/N:

PS: BN44-00755A
MB: BN94-08076A

[AlexVaq]

Re: Samsung UN55HU6840 Solid red light, won't turn on

Ok, if I connect to ground PS_ON, the PS turns off. As soon as I remove the ground connection, the MB turns on the PS, which I think is utterly wrong.

I'm tempted to buy a new MB, but it would be great if you had any ideas on how to repair this one. It's a pity to throw away the whole main board just because a capacitor/transistor etc is wrong, but I understand it can be extremely difficult to find the culprit.

[dick_barton]

Re: Samsung UN55HU6840 Solid red light, won't turn on

Some power supplies require PS_On to be taken to 0v to turn them on some maybe yours does too.

[budm]

Re: Samsung UN55HU6840 Solid red light, won't turn on

Got more info. The power on sequence is:

- Main board gets 5V (stand by). This seems right.
- Upon pressing the power on button, the voltage on pin PS_ON raises to 3.3V, and the PS returns 12.8V on the 13V lines. I think Vamp should be ~13V as well, but I'm not sure (other models are 18V).
- Then, the main board returns something on BLU_ON, up to 3.3V. This controls the backlight, 3.3V is maximum brightness.
- Then other stuff happens, which is not very relevant to the discussion.

My situation is that the PS is always getting 3.3V on PS_ON when I connect the TV, and I'm not sure if this is expected (I should try to ground PS_ON to see if the PS turns off). But anyway the main board is not sending anything to BLU_ON, it stays at 0V. I'm strongly suspecting the main board, but I would appreciate some advice.

In case you have some wiring diagrams to share, or specific instructions to troubleshoot the boards, here are some P/N:

PS: BN44-00755A
MB: BN94-08076A

This power supply board uses Active high = on, it also has pull-up resistors so that is why when you disconnect the board from main board (self test)the backlights come on.
So at this point, if you use the remote to turn the TV on and off, do you see the PS-ON toggle high and low.
If you check the 12vdc at the T-CON board, does it go to 0V when TV is turned off.
The TV will remember the last state of the TV, if the TV was on and then you unplug the AC cord, when you plug the AC cord back in the TV will turn itself back on.

[AlexVaq]

Re: Samsung UN55HU6840 Solid red light, won't turn on

Hi budm, thank you for your help!

I do not have the remote, so I grounded the PS_ON using the button on the back of the TV. I connected PS_ON to arrow up, both of them are ~3.3V, and upon pressing the button up, everything goes safely to ground. The PS turns off when I do this, but as soon as I stop pressing the button (and stop grounding PS_ON), PS_ON raises again.

I'm 99% percent sure that pressing arrow up won't turn on the TV (actually, the manual says "long press power on, and that's not even a long press). So I totally suspect the MB.

Also, a very simple test: the TV won't turn off when I long-press the power on/off button.

[budm]

Re: Samsung UN55HU6840 Solid red light, won't turn on

No need to connect the PS-ON to the joy stick on the back of the TV.
At this point even though the PS-ON is stuck on it doe not matter at this point, if it is only PS-ON problem but if other circuits are fine then the backlights should come on and you should see the pictures and hear the sound and all other function as well when you do long press power on but you are not seeing that, correct? so yes the MB has problem.
BTW, what are the Voltage on the BL-ON, PWM-DIM when you measure them with all connector in place?

[AlexVaq]

Re: Samsung UN55HU6840 Solid red light, won't turn on

BLU_ON is 0V, unless I remove the MB, that becomes ~3.3V. P_DIM1 is always 3.3V, with and without MB.

Yes, I don't see any image or sound, not even backlight. I get the backlight only if I remove the MB.