TCL LED backlight voltage drop

[Alex9]

Hi, fixing tv's is my new hobby and I have this TCL LE32HDF3010 that turns on but has no backlight.

First I checked all the caps on the Cold side of the board cause that's one of the only things I know how to do so far lol. They're all fine to my knowledge. Then I opened up the screen so I could use my fancy new LED tester and all the LEDs are lighting up and all 3 LED strips are wired together straight into the power board (ES282C2). I'm assuming the LED tester showing that they're running at 60v means that's how much they require. The power board is only putting out 46v whether the LEDs are connected to it or not. Also when the TV is on standby the voltage stays at about 40v and I thought it was odd to be getting that much power on standby, but I don't know very much .

So I sat there for a few hours attempting to trace the 46v to a source thinking I could narrow it down to a component but I wouldn't dare cross into the Hot section of the board because I can't make out where AC ends and DC starts cause there's no obvious bridge rectifier.

I figured out that in a dark room the backlights do come on for a second only but not bright enough to see with the lights on. I measured the voltage while this happens and it goes up to OL first and then 80v, 70v, 60v, 46v over a couple seconds.

If I turn the tv on and have the LED tester powering the LED's instead of the power board, the tv seems to work as it should.

I could just spend $25 on a used board but I'd really like to get to the bottom of this if I can, and learn as much as I can in the process. Thanks for any help!

[dick_barton]

Re: TCL LED backlight voltage drop

The bridge rectifier is made up of the 4 diodes alongside each other. The area marked live is the HOT side of the power supply where you make your measurement with caution.
The COLD side is the safe side which then connects power to the main board and other circuits where applicable.
There's a thick black line that clearly separates the HOT and COLD side of the circuitry.

Photo's of both sides of your power supply could prove helpful.

[Alex9]

Re: TCL LED backlight voltage drop

Ok I see the diodes now. Here are pictures of both sides of the board. The last one is my assumptions about the bridge rectifier. Thanks.

[budm]

Re: TCL LED backlight voltage drop

https://www.badcaps.net/forum/attach...6&d=1475179297
You may want to inspect the solder joints of all those pins of the black connector, they are solder to single-sided board with not much mechanical support.
It can be that the BL-ON is not present so the it is stuck at 46VDC, I am not 100% sure yet until I can trace out the circuit on the bottom side of the board.

[Alex9]

Re: TCL LED backlight voltage drop

https://www.badcaps.net/forum/attach...6&d=1475179297
You may want to inspect the solder joints of all those pins of the black connector, they are solder to single-sided board with not much mechanical support.
It can be that the BL-ON is not present so the it is stuck at 46VDC, I am not 100% sure yet until I can trace out the circuit on the bottom side of the board.

Sorry I don't see any pins to black connectors that don't solder through to the back of the board

[dick_barton]

Re: TCL LED backlight voltage drop

I believe Budm may be referring to P802. (black connector)
If you could measure the DC voltage at connector P802 - pin marked BL_ON to chassis when you switch on out of standby.

What is the number written on U801

[Alex9]

Re: TCL LED backlight voltage drop

I believe Budm may be referring to P802. (black connector)
If you could measure the DC voltage at connector P802 - pin marked BL_ON to chassis when you switch on out of standby.

3.232v on both.

What is the number written on U801

Attached a picture.

Also there doesn't seem to be any play at the solder joints of the black connector.

[dick_barton]

Re: TCL LED backlight voltage drop

3.232v on both.

Sorry but are you saying that the "Backlight On" voltage is 3.232V when in standby and out when of standby.

If that's the case what is the voltage on the STB pin when in standby and when taken out of standby?

[budm]

Re: TCL LED backlight voltage drop

"I figured out that in a dark room the backlights do come on for a second only but not bright enough to see with the lights on. I measured the voltage while this happens and it goes up to OL first and then 80v, 70v, 60v, 46v over a couple seconds."
Can you set the meter to higher range? It shows OL, that means the Voltage went over the scale.
And when you measured that Voltage, did you use the chassis as the GDN REF and Red probe on P801 pin 9 LED +?
It seems to me that the Voltage booster circuit is working and able to boost the incoming 46VDC up to higher Voltage to run the LEDs.
I also need the see the pictures of the LED connect back into P801 so I can see how many wires it has so I can figure out of the LED strings are connected in series or not. Also straight shot pictures of the WHOLE bottom side of the power supply board board
So there are 3 LED strips, how many LEDs per strip? So far it does not look like each LED strip return is sinked through the LED Controller IC OB3362.
You also need to check the DCV dimming control signal.
The IC is made for sinking 4 strips of LEDs through:
IC Pin 7 (P801 connector pin 1)
IC Pin 8 (P801 connector pin 2)
IC Pin 9 (P801 connector pin 3)
IC Pin 10 (P801 connector pin 4)

[Alex9]

Re: TCL LED backlight voltage drop

Sorry but are you saying that the "Backlight On" voltage is 3.232V when in standby and out when of standby.

If that's the case what is the voltage on the STB pin when in standby and when taken out of standby?

Sorry. I'll be specific

[budm]

Re: TCL LED backlight voltage drop

BL-ON command signal is fine (3.2V = ON is OK)
Per spec sheet:
"A DC voltage ranging from 0.2V to 2.2V at DIM pin
corresponds to a LED brightness of approximately
0% to 100%" So check the DCV at dim control pin 2 of the IC.
'Internal PWM burst mode dimming mode is
obtained by connecting a capacitor from the
BF/DMOD pin to ground" the circuit is Set to internal PWM (PIN 6 C804 to GND).

[Alex9]

Re: TCL LED backlight voltage drop

"I figured out that in a dark room the backlights do come on for a second only but not bright enough to see with the lights on. I measured the voltage while this happens and it goes up to OL first and then 80v, 70v, 60v, 46v over a couple seconds."
Can you set the meter to higher range? It shows OL, that means the Voltage went over the scale.
And when you measured that Voltage, did you use the chassis as the GDN REF and Red probe on P801 pin 9 LED +?

I did use the chasis as the gnd and red probe on P801 pin 9. At a higher DC range on the DMM, when turned on it reads a MAX of 82.9v and drops steadily down to 46v over about 8 seconds.

So there are 3 LED strips, how many LEDs per strip?

7 LEDs per strip. (by far the hardest answer to get )

So far it does not look like each LED strip return is sinked through the LED Controller IC OB3362.
You also need to check the DCV dimming control signal.
The IC is made for sinking 4 strips of LEDs through:
IC Pin 7 (P801 connector pin 1)
IC Pin 8 (P801 connector pin 2)
IC Pin 9 (P801 connector pin 3)
IC Pin 10 (P801 connector pin 4)

I'm a bit confused about this question. P801 has only 6 pins and I'm pretty sure 1 & 2 are Negative and 5 & 6 are Positive. 0vdc from pins 1 & 2 and 46v from pins 5 & 6.

[Alex9]

Re: TCL LED backlight voltage drop

Forgot these.

[budm]

Re: TCL LED backlight voltage drop

Sorry my mistake, P801 only has 6 pins.
Pin 1, 2, 3, and 4 are the Cathode return pin of each LED strips, since you only has 3 LED strips so that means one of the 4 pin is not used (yhat is why I want to see the pictures of the LED connector to see which pin is not being used). The return current through the LEDs go through the internal circuit that control the constant current through each LED string and then return to the GND pin of the IC.
Pin 5 & 6 is where the LED+ Voltage is fed to the Anode pin of all 3 LED string.
75~80V feeding the LED strings is about right, something is telling the circuit to go into shutdown that is why it drops back down to standby Voltage of around 40V.
Can you take the reading of pin 1~4 of P801 one at a time each time you turn on the TV? I want to see what dcv you have on each Cathode pin of each LED string before it goes to standby mode.

[budm]

Re: TCL LED backlight voltage drop

https://www.badcaps.net/forum/attach...5&d=1475203204
That does not make sense top have only 4 wires for 3 LED strings. I wonder how the LED strips are wired up inside the panel. Pin 5 and 6 are tied together for the LED +.
I see 6 connectors total on the LED strips. 22 LEDs total so you have two strings of LED, each string has 11 LEDS based on the wiring of the connector P801.
You said using you tester you get about 60V, how did you hook up the tester to the LED strings?
22 LEDs *3V = 66V if they are connected as one big string, but looking at the connetor wire termination it does not look like they are connected as one big string.

[budm]

Re: TCL LED backlight voltage drop

https://www.badcaps.net/forum/attach...9&d=1475179297
OK, I see it now, pin 1 and pin 2 of P801 are shorted together through R946 0 Ohm jumper so the IC sink pin 8 and 9 are tied together.
So the LED strips are connected in parallel, and they may be 6V LED (Dual LED in one body) not 3V LED.
So I need to know how the LED is connected to test the LED strips. All the LEDs light up brightly when using the tester?

[Alex9]

Re: TCL LED backlight voltage drop

Sorry my mistake, P801 only has 6 pins.
Pin 1, 2, 3, and 4 are the Cathode return pin of each LED strips, since you only has 3 LED strips so that means one of the 4 pin is not used (yhat is why I want to see the pictures of the LED connector to see which pin is not being used). The return current through the LEDs go through the internal circuit that control the constant current through each LED string and then return to the GND pin of the IC.
Pin 5 & 6 is where the LED+ Voltage is fed to the Anode pin of all 3 LED string.
75~80V feeding the LED strings is about right, something is telling the circuit to go into shutdown that is why it drops back down to standby Voltage of around 40V.
Can you take the reading of pin 1~4 of P801 one at a time each time you turn on the TV? I want to see what dcv you have on each Cathode pin of each LED string before it goes to standby mode.

P801 pin max readings before standby mode:

pin 1 mostly reads 1.1v to 1.5v
Other times I got these results:

1st test - 6.266v
2nd test (happened two seperate times) - 9.11v then shot up to 14.2v and the backlight came on for 10 seconds then went back off.
3rd test - 24v
4th test - 23.9v
5th test - 3.76v and a backlight flash


pin 2 - 3 tests - 1.1v, 1.3v, 1.5v

pin 3 - 3 tests - 1.27v, 1.29v, 1.35v

pin 4 - 3 tests - 1.54v, 1.31v, 1.5v

I'm going to open up the lcd side later tonight because I have some stuff to do. I'll post back a picture of the LED wiring configuration later. Thanks for the help btw .

[budm]

Re: TCL LED backlight voltage drop

P801 pin max readings before standby mode:

pin 1 mostly reads 1.1v to 1.5v
Other times I got these results:

1st test - 6.266v
2nd test (happened two seperate times) - 9.11v then shot up to 14.2v and the backlight came on for 10 seconds then went back off.
3rd test - 24v
4th test - 23.9v
5th test - 3.76v and a backlight flash

pin 2 - 3 tests - 1.1v, 1.3v, 1.5v

pin 3 - 3 tests - 1.27v, 1.29v, 1.35v

pin 4 - 3 tests - 1.54v, 1.31v, 1.5v

I'm going to open up the lcd side later tonight because I have some stuff to do. I'll post back a picture of the LED wiring configuration later. Thanks for the help btw .

I traced out the board in pots 16, Pin 1 and pin 2 should have the same reading since they are tied together though 0 Ohm jumper, please verify that.

[Alex9]

Re: TCL LED backlight voltage drop

https://www.badcaps.net/forum/attach...9&d=1475179297
OK, I see it now, pin 1 and pin 2 of P801 are shorted together through R946 0 Ohm jumper so the IC sink pin 8 and 9 are tied together.
So the LED strips are connected in parallel, and they may be 6V LED (Dual LED in one body) not 3V LED.
So I need to know how the LED is connected to test the LED strips. All the LEDs light up brightly when using the tester?

Every LED lit up nice and bright with the tester connected. The other way I connected it was to the connector at pins 1 and 6.

If you don't need me to open it up again please let me know. I'd be happy to do it all the same.

[budm]

Re: TCL LED backlight voltage drop

I would like you to try this, disconnect the female LED connector from P801 then use your LED tester on the female connector between pin 1 (Cathode, L-) and pin 5 or pin 6 to see if one set of 11 LEDs will light up, then do the same on pin 2 (Cathode, L-) and pin 5 or pin 6 to see if another set of 11 LEDs will light up, record the Voltage reading on the tester also.