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My Toshiba 43LF621U19 is not powering on

My Toshiba 43LF621U19 is not turning on. The blue standby light that used to glow when plugged in is not turning on.

I tested the power board and the main board to the best of my knowledge and they seem to be working fine. I was able to read in 5V for the standby voltage. The power going in to my standby light board is ~3V. There is one connection from the powerboard to my main board and another that is labelled as 1. VLED+, 2. NC, 3. NC, 4. NC 5. VLED-. When the powerboard is ON, I tested the voltage difference between VLED+ and VLED-. It is close to 0V (~0.43), and doesn't change when I press the power button on the TV. Is this expected? If so, what else might be going on and any ideas on how I can fix this?

Thanks in advance.

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Update 04/17/2022

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Vittal Premachandran VLED+ and VLED- are your backlight Anode and Cathode. The voltage will only increase if the main board "tells" your TV's power board to turn it on. Check the BKL voltage on your main board. Let us know what that shows you.

Toshiba is an extremely secretive and paranoid company. Service manuals and schematics are hard to obtain.

Do you have all the other 12V and 5V voltages coming from your power board/main board connector?

We need a clearer picture of your main board as well as the T-con board (blue board). Do you have power on the T-con board. check the F1 fuse for 12V

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@oldturkey03 Thanks for your suggestions. So, if I do not detect any change in VLED voltage, then the problem could either be in the main board, or in the power board, right?

"Check the BKL voltage on your main board. Let us know what that shows you." - Sorry, but what is BKL voltage and how do I check it?

"Do you have all the other 12V and 5V voltages coming from your power board/main board connector?" - Yes. I am able to read the correct voltage from each pin that is labelled with 12V or 5V or 16V, both on the main board side and the power board side.

I uploaded other pictures of my T-con board and main board. Let me know if you need me to split them up into parts to help zoom into the components more effectively.

"Do you have power on the T-con board. check the F1 fuse for 12V" - I saw a tiny component labelled F101, which looks like a miniature fuse. I tested it and it seems to be open when unplugged. How can I test it for 12V? Btw, the board is labelled with 32V.

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@Vittal Premachandran F101 sounds right. Hard to see with my old eyes. If the fuse is open then it is most likely your T-con board. wht happens when you unplug it and try to start your TV? I'll see if there is anyway that we can start the power supply with a pull down or pull up resistor. Let's see what I can get on this Toshiba

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Sorry, I might have used the wrong vocabulary. When I tested the fuse (F101) for connectivity, I got a beep on my multimeter. That means that the two end points are connected and the fuse is working fine, right?

I unplugged the tcon board and tried starting the TV. No luck. Standby light doesn't turn on.

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@Vittal Premachandran Oh okay. Yes you fuse has continuity and it is good. Still looking for Toshiba stuff :-)

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@Vittal Premachandran

Easiest way is to disconnect the power from the TV and then disconnect the LED power lead from P803 on the power board and connect a LED backlight tester to the LED power lead and check if the LEDs all work.

If not then the LED strips are the problem, if they do then it is the power board or the mainboard telling the power board not to supply power to the LEDs.

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I placed an order for the LED backlight tester. Should arrive in 2 days. Meanwhile, I was watching https://www.youtube.com/watch?v=Z1DEkSgu... and at 3:34 in the video, he says that if BL ON does not show 3.3V on standby, then the problem is with the main board and I should replace it. My BL ON does not show 3.3v. I read 0.2V out of that pin when the power is ON. Does that mean that my main board is the one that is faulty? Around 5:12 in the same video, he says that if the LED strips are bad, VLED should read ~130V. But I am reading 80V from that pin. Do we still think that the LED strips are the problem or should I focus on main board?

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@Vittal Premachandran

Wait until you get the tester then if the LEDs are faulty you won't have spent time chasing problems that aren't there.

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I received the tester today. I tested the LED strips as you recommended and I was able to see that the backlight LEDs started glowing. Now, I am not sure how many strips there are behind the panel, and whether all the LEDs are glowing or only a subset of them. Any suggestions for next steps?

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@Vittal Premachandran

Looking at an image of the full set of LED strips for your model, there are 8 strips having 4 LEDs in each strip so maybe check if there are any "darker" areas between any rows or LEDs in a row.

The strips would be evenly spaced as well as the LEDs to give an even full coverage of the screen area.

Not sure if the LEDs in a strip are in series connection or not but would suspect that they are, so if one LED was out in a strip the whole strip would be dead but maybe not I don't know.

Check what the voltage is when measured between VLED + and an earth point on the power board (e.g. screw head). Measuring between VLED + and VLED- is no good if the strips are open circuit as there would be no current flow through them.

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The voltage between VLED+ and ground is 80V. I read 0V between VLED+ and VLED-

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Vittal Premachandran さん、ありがとうございました！
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