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SAMSUNG UN556203 TV Black Screen

My Samsung 55" LED TV has the dreaded black screen issue like others here. I have been following some of the answers and have a question in the diagnosis process.

Have taken the back off exposing the boards. When plugged in I have the standby light. When turned on I only have black screen. No backlights. However, the flashlight test shows the startup image (not hooked to another video source). when powered on.

Next step though is where I can's seem to find the answer I need. When I disconnect the cable between the power board and the main board and plug the TV back in I lose my standby light and cannot turn the TV on. If I plug the power board / main board cable back in I have standby light again. Can someone tell me what that might mean and what to check next?

Update (01/27/24)

Finally

Finally received the correct 10 pc. set of replacement LED strips. I goofed on the first order. Ordered just 1 strip instead of a complete set. Second order the vender mixed up my order and sent me wrong LEDs. 3rd time received the correct ones.

Snapped them together and tested each one before installing them. They all worked and I was able to test them as a complete system by unplugging the main board from the power board as before. So I knew they were working before re-assembling the screen and trim.

Surprisingly the diffusion panels and screen panel trim snapped together a lot easier than taking it apart. So the TV is back in working condition.

A couple of things I would like to mention in case anyone is following this. To lift the screen off and replace it I found 2 suction cups at Harbor Freight. Their small ones were less than $2 each. I purchased my backlight LED tester off of ebay for $15 shipped. The replacement LEDs also came from ebay. They didn't appear to be Samsung but fit all of the mounting holes. To make it easier to work on I threaded 4 - 2" screws into the rear threaded inserts for the wall mount. This allowed me to have the disassembled TV flat on the floor without having to rest on the power board or main board.

Thank you again for all of the advice jayeff.

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Hi @pippa4368

When I disconnect the cable between the power board and the main board and plug the TV back in I lose my standby light and cannot turn the TV on.

This is normal. Disconnecting the cable connection between the mainboard and the power board means that the mainboard can't operate i.e. no power from the power board. It is the mainboard that controls the TV either through the use of the TV controls or the remote control.

When the power is connected to the TV, the power board sends a standby voltage signal to the mainboard to indicate that the correct power is available for the TV if it needs to be turned on. Upon receipt of this voltage signal, it is the mainboard that turns on the red Standby power light.

In standby mode the power board is just supplying minimal power to the main board so that it can start to work upon command by the user. When the TV is turned on, the mainboard turns the power board full on, to supply all the power necessary for the correct operation of the TV.

If you don't get the TV screen backlights turning on when the mainboard- power board cable is disconnected and the power is connected to the TV then there's either a problem with the power board or the backlights. There will be no images on the screen or audio etc as the mainboard has been disconnected.

Since you are seeing images on the screen seems to indicate that the mainboard and tcon board are both OK and also the power board to a degree as well.

First disconnect the power from the TV and then disconnect the LED power cable from the power board, reconnect the power to the TV and then use a DMM (digital multimeter) to measure between any LED+ve pin on the power board's LED power cable connector and an earth point e.g. screwhead. You should measure anywhere between 150-300V DC. If you measure <50V DC the power board is faulty.

If you measure the correct voltage then the backlights are faulty and the backlight strip (or strips) need to be replaced. A single faulty LED in a strip can mean that the whole strip doesn't work as they are connected in series connection.

Alternatively get a TV backlight tester (examples only) and disconnect the power to the TV and the LED power cable from the power board and connect the tester between the LED+ve and LED -ve cable feeds in the cable, for each strip and check if the LEDs works or not. The tester supplies the power to turn on the LEDs and not the power board.

If the strips are faulty search online for (insert TV make and model) backlight strips to find suppliers that suit you best. BTW is the model number a UN55H6203?

Also look on YouTube to see how to replace the strips in Samsung TVs.

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Thank you for the reply. I will measure voltages at the LED connector and report back.

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Had a chance this morning to check voltages on the power supply board. All of the measurements were made with the power board disconnected from the main board.

Voltages at the LED connector read 300v at only one terminal. The rightmost terminal read 300v DC. All of the others (and I realize every other one is a ground) read zero voltage. Marked 1. in my picture.

Voltage at location marked 2. read 13v DC

Voltages at location marked 3. read zero on both sides (not sure what that part is or even if it should have voltage. Just saw a 250v note on the board.

So is the next step checking the LEDs with a backlight tester? I don't have one but they don't seem to be very expensive.

Please let me know what you think.

Jeff

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Hi @pippa4368,

Looking at a close up image of the power board for the TV (is it a BN-44-00774a?) I think that there should be more voltage readings that what you read at Step.1.

The pinout that I can make out (allowing for my crappy eyesight) is:

pin 1 = M1+

pin 2 = M1-

pin 3 = M2+

pin 4 = M2-

pin 5 = C

pin 6 = M3+

pin 7 = M3-

pin 8 = C

pin 9 = M4+

pin 10 = M4-

pin 11 = M5+

pin 12 = M5-

To me this indicates 5 LED strips so there should be voltages on all the +ve leads, not just the one but it may depend on how they're wired.

Maybe measure between a +ve lead and a "c" lead.

The 13V DC at Step.2 is OK

At Step 3 did you change the DMM to read AC volts?

If not then this is why you measured 0V. Step.3 is the main AC supply fuse (i.e. 120V AC so be careful- lethal voltage).

If you change the DMM to read AC volts, you should measure the correct supply voltage when connecting the meter to either side of the fuse and earth (not when connecting across the fuse as there is no voltage drop on a fuse).

You shouldn't have to do this in any case as you have measured the correct voltage at the 13V point and also (albeit only at 1 point) on the LED power supply point on the board. No AC supply would mean no DC supply to the rest of the TV

It is looking like faulty LED backlight strips but a backlight tester would allow you to know for sure, before having to disassemble the TV to verify that it is.

由完成的

Thank you for the reply.

Re-checked my voltages. With the main board disconnected from the power board and the TV plugged in I still only have voltage (300vDC) at the rightmost terminal of the LED connector. I tried using one of the C terminals as a ground but no difference. In fact I checked continuity between the C terminal and a chassis ground and it is open. I also turned my multimeter to AC and checked both sides of the 250vAC fuse and read 0 voltage on both sides to ground. Not sure if that really means anything.

I looked at all of the capacitors and didn't see any leaking or deformed caps.

As you suggested I am going to buy a cheap LED backlight tester and test the LED strips. More curiosity now than anything else.

Thank you again.

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@pippa4368

Regarding the AC voltage it seems as though the chassis earth is not connected to the AC earth.

Is there a two wire or 3 wire power cord from the wall outlet i.e. 2 pin or 3 pin plug on the cord?

If only 2 pin this is why you don't see any voltage between the fuse and a chassis earth.

If you measure between the fuse and the power cord white wire you should see the 120V AC supply.

Apologies. Every country has different AC wiring regulations and I forgot. Was just thinking about what it is where I am.

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OK so I received my LED Backlight Tester and had a chance to check my LED strips. As you said earlier my TV has 5 strips. With the 120vAC power unplugged I checked each pair of +/- terminals using the tester. Results as follows:

pin 1 = M1+ / pin 2 = M1- tester read 300v with no backlight turning on

pin 3 = M2+ / pin 4 = M2- tester read 34v and backlight turned on

pin 6 = M3+ / pin 7 = M3-tester read 33v and backlight turned on

pin 9 = M4+ / pin 10 = M4-tester read 34v and backlight turned on

pin 11 = M5+ / pin 12 = M5- tester read 33v and backlight turned on

So it looks like 4 of the 5 backlight strips work when tested individually. I then plugged the TV into 120vAC power and plugged the power board into the main board. The backlight plug was still unplugged. I turned the TV on and when I applied power to each pair of backlight terminals I was able to see the Smart TV startup logo and / or "no input signal" message. It was dimmer than usual because I assume it was being illuminated by only one backlight strip at a time. But the 4 working backlights all illuminated the message.

So is it difficult to get to the backlights? Any advice? Can we rule out a defective power board, main board, and tcon board?

Thank you again for your continued help.

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Hi @pippa4368,

I would start by replacing the backlight strips. It needs to be replaced anyway and after that if there are more problems, it's a lot easier to change a board. Given what you tested the boards seem OK.

Seems as though there may be 10 strips and not 5 i.e. two strips on each power feed. the LEDs are connected in series on a strip so it only takes one to go faulty for the whole strip(s) not to turn on,

Here's just one supplier for strips (if that is your model - I assumed it is a UN55H6203 and not a UN556203 because I couldn't find any info for the model number that you posted

Search online for "(insert the correct model number) backlight strips" to find suppliers that suit you best.

It's not really that difficult to get to the backlights but you must take care.

Here's a video that shows what to do.

If you decide to do this then also ensure that you don't mix up the order of the diffuser sheets or reverse them when reinstalling.

由完成的

I ordered the replacement backlight strips so we'll see how that goes. Before I started disassembling the screen to expose the LED strips I plugged my ROKU in, energized one of the backlight strips, and was able to watch a little bit of a show. It was a little dim since it was only running one backlight strip but hopefully this shows that everything except maybe the power board is functioning.

Also I energized each LED strip after removing the screen. One strip didn't light up at all which was what I found earlier, but once I could see the individual LEDs 3 of the remaining 4 strips each had one or two LEDs that did not light. None of them appeared burnt.

A little history on the TV. It is about 7 years old. Recently we moved 600 miles to a new location. While I tried to pack it carefully it is possible the jostling during the move (was packed inside a container and transported by flatbed truck) could have played a part in the LED failures. Who knows. Thanks again.

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@pippa4368

The failed LEDs on the other strips could have failed "short circuit" with no visible damage allowing the other LEDs on the strip to still function and not have burnt out (usually noticeable damage) thereby going "open circuit" causing the whole strip to not work.

As to why, as you say who knows.

I don't think that there'll be a problem with the power board but time will tell.

Good luck

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@pippa4368

3rd time lucky ;-)

Enjoy your working TV

Cheers

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