TCL 55S401 - No Backlight (powers on, sound, flashlight image)

[Chris24]

Hey everyone - I have a TCL 55S401 with these symptoms: powers on, has sound, and a black screen.
I tested with a flashlight and can see the TCL logo and setup screen. I did a factory reset and with a flashlight I can now see "select language."

Following instructions from Budm in a recent post for a TCL 55S405 without backlight, I did the following tests:

Power Supply to LED (pin label is under board)
Tested w/ multimeter at 200V and tv unplugged to plugged in to power on:
P10 LED1+ : 0
P9 LED2+ : 0>48~60>0.69
P8 NC
P7 LED2- : 0 > 0.01
P6 LED1- : 0 > 0.01
P5 LED4+: 0 > 0.01
P4 LED3+: 0 > 0.01
P3 NC
P2 LED3-: 0>22~25>0
P1 LED4-: 0

I also tested the PS to MB and will post later, but pretty much had 12V everywhere labeled 12V and 3.3V on P_Dim and 3.5V BL_ON

Another random symptom is that the TV turns on immediately after plugging into the socket instead of going straight to standby and waiting for me to push the power button. Not sure if that matters

Pics to follow:

[Chris24]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

Images attached. Any help figuring out if its the LEDs or something else is MUCH appreciated

[neilc6]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

Those TCL have lots of backlight failures. Most TVs will remember what state they were in if unplugged so if it was on, it will turn on automatically. If you want to repair TVs, time to buy a LED tester.

[Chris24]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

Thanks Neil, I’ve heard that is common. Figured I would rule out the PS at least first and since I didn’t get a jump in voltage was wondering what the deal was. What do you think about testing on the PS?

I’ve heard these TV LEDs are a little easier to swap so if that’s the case I’d give a go

Great point about on/off

[neilc6]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

https://youtu.be/I32MBlbRWMs

https://youtu.be/2uJzGlAaM0M

Replace all your strips with new and turn down the backlight level.

[handsworth]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

Just be careful removing the screen. Those babies are glued down.

[budm]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

Based on your reading in post #1, it looks like you have open circuit in LED #2. this TV has LED 2 string connected in series with LED 3 string.
If you leave the LED connector out and then keep the black probe on the chassis, red probe on LED2 + pin of the power supply board then turn on the TV while watching the meter to see how high the Voltage jumps up to.

[Chris24]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

With LED connector unplugged from PS I get:

P9 LED2+ : 0> 68~101v > 0

and

P2 LED3- : 0 > 78~90V > 0

The PS clicks once and I get a 90v or so reading and back to 0 and then a few seconds later it clicks again and I get a 65V or so reading then back to 0

[budm]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

It goes into protection shutdown since no load (LED strings) are not connected.
So do you have LED tester to test the LED strings?

[Chris24]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

Opened her up and tested the LEDs. Had two LEDs out, one top strip and one on bottom strip on left side of screen. Got a replacement set of light strips (not skilled enough to replace a single LED) and went ahead and replaced them all, and now the TV works, nice and bright. Will turn the backlight down.

Thanks to bud, Neil, and hand for your responses. Always learn something new on here

[budm]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

Thanks for the follow-up on the repair.

[neilc6]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

I got a TCL 55S405 with no backlights and cracked the screen removing the panel. I scanned this shopjimmy video to see if the panel was taped to the screen which it was. https://youtu.be/KaKi5aC9fUQ I then followed their procedure of running a plastic sheet along the edge to separate the tape. No problems with the bottom and side edges but the top edge uses stickier tape which is how I ended up damaging the panel. Better to use this procedure from bigdog8882 https://youtu.be/QdxSHeQFuwY

Sometimes with taped panels, I leave the frame attached and just separate it from the chassis but just assumed Shopjimmys method would work. On this model, the frame on the top sits inside of the chassis so a bit harder to unclip. I should have read all the comments from that video.

[DXseekerMO]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

I got a TCL 55S405 with no backlights and cracked the screen removing the panel. I scanned this shopjimmy video to see if the panel was taped to the screen which it was. https://youtu.be/KaKi5aC9fUQ I then followed their procedure of running a plastic sheet along the edge to separate the tape. No problems with the bottom and side edges but the top edge uses stickier tape which is how I ended up damaging the panel. Better to use this procedure from bigdog8882 https://youtu.be/QdxSHeQFuwY

Sometimes with taped panels, I leave the frame attached and just separate it from the chassis but just assumed Shopjimmys method would work. On this model, the frame on the top sits inside of the chassis so a bit harder to unclip. I should have read all the comments from that video.

I just installed backlights in this model 3 days ago. I used a hair dryer on the top edge of the screen and an old driver license slid between the screen and screen rest to separate them. Seems to work like a charm for me!

[david37g]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

I have a TCL 55S405 with no picture, only sound. The white light on the bottom came on when I pressed the remote power button. I wanted to check the backlight power supply. I tried the suggestion of Bud M to test led voltages with the led cable unplugged. When I put the power on the PS fuse blew. Does this mean the PS is bad?

[david37g]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

Found 2 bad mosfets on back of power board aod9n50 and two bad led back lights. Replaced the mosfets and all the led strips and the tv works.

[nwcs]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

Can you tell me how you got the panel off without breaking it. The top is glued down and I do not want to damage it getting it off.

[handsworth]

Re: TCL 55S401 - No Backlight (powers on, sound, flashlight image)

I use a feeler gauge. I believe it was at .25mm. Strong enough to break the seal but flexible enough go over the bezel humps. GO SLOW!