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Old 05-11-2019, 04:51 AM   #1
Steradiant
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Default Samsung UE55H7080 black screen (sound is working)

Hello,

some days ago my Samsung UE55H7080 broke. When I switched on the TV, the screen stayed black (2 hours ago I was still watching TV). The sound is still working, I can hear how I can switch channels, go into the menu and so on. I've already tried to plug the TV off for the whole night and plug it back on the next day and it didn't help.

I decided to open the TV because I thought it might be a problem with the power supply for the backlight. Attached you can find some pictures of the inside of the TV.

What I've already checked:
- The fuse on the supply board is ok
- I can't see any broken wires, bulged capacitors or similar.
- I've checked the voltages of the LED supply; they are as follows
CNL801A:
Pin 1 (N.C.) 0V
Pin 2 (N.C.) 0V
Pin 3 (L4+) 16.8V
Pin 4 (L4-) 0V
Pin 5 (L3+) 54.7V
Pin 6 (L3-) 0V
Pin 7 (L2+) 54.7V
Pin 8 (L2-) 0V
Pin 9 (L1+) 16.9V
Pin10 (L1-) 0V

CNL801B:
Pin 1 (N.C.) 0V
Pin 2 (R1+) 39.8V
Pin 3 (R1-) 16.9V
Pin 4 (R2+) 127.4V
Pin 5 (R2-) 54.7V
Pin 6 (R3+) 127.4V
Pin 7 (R3-) 54.7V
Pin 8 (R4+) 39.8V
Pin 9 (R4-) 16.8V
Pin 10 (N.C) 0V

What makes me wonder a little bit when looking at those voltage levels is, that they are not the same for left and right but I sadly haven't found a circuit diagram of the BN44-00715A in http://www.toms-service-manuals.com/ or elsewhere.

I also can't see any light coming through the tiny holes in the back of the tv when I plug it in and switch it on. I haven't tried that in the very dark yet though.

Do you have any suggestions what to check further? Mainly it would be interesting for me, where the failure is. If it is "only" the backlight or the screen.

Thanks in advance.
Attached Images
File Type: jpg DSC_0243.jpg (421.0 KB, 32 views)
File Type: jpg DSC_0244.jpg (435.3 KB, 22 views)
File Type: jpg DSC_0245.jpg (389.0 KB, 18 views)
File Type: jpg DSC_0246.jpg (445.7 KB, 16 views)
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Old 05-11-2019, 06:48 AM   #2
yokoono
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Default Re: Samsung UE55H7080 black screen (sound is working)

Disconnect the connector between the motherboard and the source. The backlight must light up. If not most likely you have broken LEDs. With a stronger light, you can also see if it is an image.
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Old 05-11-2019, 06:53 AM   #3
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Default Re: Samsung UE55H7080 black screen (sound is working)

Quote:
Originally Posted by yokoono View Post
Disconnect the connector between the motherboard and the source. The backlight must light up. If not most likely you have broken LEDs. With a stronger light, you can also see if it is an image.
Thanks for the advice but could you please tell me which board you mean with "motherboard". The one at the bottom I suppose?
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Old 05-11-2019, 07:01 AM   #4
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Default Re: Samsung UE55H7080 black screen (sound is working)

Unplug this connector to see if backlights turn on.
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Old 05-11-2019, 07:20 AM   #5
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Default Re: Samsung UE55H7080 black screen (sound is working)

Quote:
Originally Posted by tvtimmy View Post
Unplug this connector to see if backlights turn on.
Thanks for the explanation. Yes, the backlight turned on after unplugging this connector. What does this tell us?
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Old 05-11-2019, 07:42 AM   #6
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Default Re: Samsung UE55H7080 black screen (sound is working)

The motherboard does not command backlighting. Measure if there is voltage on the BL-ON pin when turning on the TV.
Theoretically when it is in stb on bl / on it must be + 3v approx. and start at 0.

Last edited by yokoono; 05-11-2019 at 07:47 AM..
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Old 05-11-2019, 08:12 AM   #7
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Default Re: Samsung UE55H7080 black screen (sound is working)

Quote:
Originally Posted by yokoono View Post
The motherboard does not command backlighting. Measure if there is voltage on the BL-ON pin when turning on the TV.
Theoretically when it is in stb on bl / on it must be + 3v approx. and start at 0.
Sorry for asking so much but where is the bl_on located? I only find BLU_ON on the power supply board (on the right hand side).

The voltage at the wire which is labeled with STB and ON changes from 0 to 12.63V when I switch the TV on.

I have to note, that every external connection to my TV is disconnected, only the power is available at the station where I measure stuff.
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Last edited by Steradiant; 05-11-2019 at 08:37 AM..
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Old 05-11-2019, 08:46 AM   #8
yokoono
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Default Re: Samsung UE55H7080 black screen (sound is working)

I think BLU is the command for backlight ignition. With the TV in the socket, measure the voltage there (theoretically + 3v approx.). Press power on and see what voltage it looks like (theoretically 0v).

Last edited by yokoono; 05-11-2019 at 08:56 AM..
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Old 05-11-2019, 09:09 AM   #9
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Default Re: Samsung UE55H7080 black screen (sound is working)

Like I said, I don't have any connection to satellite or similar on my measurement place. BLU_ON raises from 0V when the TV is off to 4.6V when I switch the TV on.
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Old 05-11-2019, 09:28 AM   #10
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Default Re: Samsung UE55H7080 black screen (sound is working)

OK. If that is the pin you are looking for, it means that the motherboard gives the command to backlight. The command follows a route and arrives at the LED driver. Measure if that command arrives and if something is not interrupted.
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Old 05-11-2019, 09:40 AM   #11
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Default Re: Samsung UE55H7080 black screen (sound is working)

Quote:
Originally Posted by yokoono View Post
OK. If that is the pin you are looking for, it means that the motherboard gives the command to backlight. The command follows a route and arrives at the LED driver. Measure if that command arrives and if something is not interrupted.
You write "if". I honestly can't tell if this is the pin I'm looking for.

Furthermore, you wrote that the voltage should go from high (approx 3V) to low (0V) when switching on the TV. This pin shows the opposite behavior though.
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Old 05-11-2019, 11:40 AM   #12
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Default Re: Samsung UE55H7080 black screen (sound is working)

Another pin for bl-on does not see what it would be like. If the voltage increases at startup is correct because I think that the pine and the input to the integrated driver must be at least one transistor. Start from the bl / on pin to the integrated driver and see if the command reaches its starting pin. I do not know what is integrated and I do not even have the service manual.
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Old 05-11-2019, 12:14 PM   #13
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Default Re: Samsung UE55H7080 black screen (sound is working)

Sadly I also don't have a service manual. Is there any way to get one?

What makes me suspicious is, that the BLU_ON bin shows 3.2V when the connector is disconnected (hence, the LEDs are on; as explained above) and the 4.6V when the plug is connected and the TV is switched on.

Should the integrated driver be a IC on the power supply board?

So to follow the path I would have to remove the power supply board from the TV and follow the paths on the bottom. Or have you thought of any other way?
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Old 05-11-2019, 03:01 PM   #14
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Default Re: Samsung UE55H7080 black screen (sound is working)

https://www.badcaps.net/forum/attach...3&d=1557584044
1) The power supply/LED driver board has pull-up resistors for the PS-ON (CNM803 pin 14), BLU_ON/OFF (same as BL-ON, CNM803 pin 12), P_DIM 1 ~ 4; the pull-ups are there to keep those control pins HIGH for power supply/LED driver board self test. When the main board is connected to the power supply board and TV is not yet turned on, those controlled pins will be kept low by the main board, and the PWM in this case will be kept low by the T-CON board since T-CON board controls the backlights level in this TV model, you can see the wires from T-CON board to the power supply board connector CNM803, Samsung calls this "Samsung’s segmented dimming technology" so they can claim fake high dynamic contrast ratio.
2) When all the connectors are back in place you need to report the Voltage of each pin of the CNM803 of the power supply board so we can see if all the control signals are present or not, right now the BLU_ON/OFF is OK but we do not know if we have the PWM signals or not, these PWM 1 ~ 4 are used for controlling the backlights level, so report all the pin's Voltages, you MUST, REPEAT MUST put down the pin name and follow by the Voltage reading for each pin when TV is turned on so we can easily see what you have and do not have on those pins.
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Last edited by budm; 05-11-2019 at 03:06 PM..
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Old 05-11-2019, 05:18 PM   #15
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Default Re: Samsung UE55H7080 black screen (sound is working)

Just for fun check the tcon fuse, make sure it isn't blown and see if you have voltage on each side, negative probe on ground positive on each side.
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Old 05-12-2019, 12:04 AM   #16
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Default Re: Samsung UE55H7080 black screen (sound is working)

I've checked the voltages on the CNM803 now after switching on the TV
N.C. 0V
P_DIM4 0V
P_DIM3 0V
P_DIM2 0V
GND 0V
P_DIM1 0V
A13V 12.6V
OD_ON/OFF 3.2V
A13V 12.6V
OD_LEVEL 0V
A13V 12.6V
BLU_ON/OFF 4.8V
A13V 12.6V
PS_ON 3.2V
A13V 12.6V
GND 0V
GND 0V
GNV 0V
Vamp 12.6V
Vamp 12.6V
Vamp 12.6V
GND 0V

I have to note that I don't have an oscilloscope at home right now, so I only can check the voltages with a simple multimeter. What makes me wonder is, that the P_DIMx Signals are 0V. But if they are actually PWM signals I might have problems measuring them with the simple multimeter?

I've checked the fuse at the T-con board (at least I think that this is the fuse; attachment) and this one is okay. The two LEDs on the T-con board light up blue after switching on the TV.
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Old 05-12-2019, 08:09 AM   #17
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Default Re: Samsung UE55H7080 black screen (sound is working)

disconnect, one at a time, the cables going from the tcon to the panel see what the result is (note: cable changes with POWER OFF).
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Old 05-12-2019, 08:39 AM   #18
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Default Re: Samsung UE55H7080 black screen (sound is working)

When I disconnect one of the top cables (see red areas in attachment) of the tcon board, the backlight turns on as I switch on the TV. The backlight also turns on if both cables are disconnected. By disconnecting the bottom cables, nothing (obvious) changes.

If you need any information about voltage levels, just let me know in which ones you're interested in.
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File Type: jpg t_con_cable.jpg (885.3 KB, 19 views)

Last edited by Steradiant; 05-12-2019 at 08:41 AM..
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Old 05-12-2019, 08:44 AM   #19
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Default Re: Samsung UE55H7080 black screen (sound is working)

Panel Failure, H 7000 and 8000 series from Samsung suffer from bad design and display driver failure.
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Old 05-12-2019, 08:51 AM   #20
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Default Re: Samsung UE55H7080 black screen (sound is working)

You need to disconnect the ribbon cables with the flip lock one at a time to see if half of the panel is working. This thread may apply https://www.badcaps.net/forum/showthread.php?t=50646
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