No backlight on Insignia NS-50DR710NA17, need help with voltages

[jayanth_rk]

I have an Insignia TV (NS-50DR710NA17) with the power supply board PLTVFY411XAF7. TV works fine, but the backlight is not functioning at all.

The output connector to the LEDs from the power module is seen in the attached picture.

Between the chassis and +VLED, I'm only seeing about +36.5V DC. The -VLEDs show about 34-35V DC when the connector is plugged in. This is definitely too low, right? I could take apart the TV completely to figure out how many LEDs there are, and what the voltage should actually be, but I'd rather avoid that.
Is there anything else on the power supply board I can test with just a multimeter to confirm if it's good or not?

(Followup question, how exactly are the LEDs wired? Seems like the power supply is between the chassis and +VLED, so what are the -VLEDs for?)

Thank you very much for your time.

[budm]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

We need to see the pictures of the whole backside of the TV showing all the boards.
This looks like your board:
https://www.shopjimmy.com/insignia-p...ply-board.htm#
The panel has two LED strings.
The Power supply board connector that goes to the main board has label for each pin, we need to find oout what the Voltage for each pin when you turn the TV on, you must report the reading in this format:
CN9103 (The left column is the EVEN number pins, right column is for ODD number pins)
Pin 1, it has white dot for indicating pin 1 (DIM) = xx Volts
Pin 2 (left column) BL_O/F = xx Volts

https://www.shopjimmy.com/insignia-p...ply-board.htm#
The +VLED pins are for the Anode of the LED strings, the ANODE of the two lED strings are tied together.
The -VLED 1 is the Cathode return for the LED string # 1
The -VLED 2 is the Cathode return for the LED string # 2
The Cathodes return to ground via U8602 (for LED 1), 8603 (for LED 2)and current sensing resistors.

"Between the chassis and +VLED, I'm only seeing about +36.5V DC. The -VLEDs show about 34-35V DC when the connector is plugged in." There are TWO -VLEDs, are you 100% sure that '-VLEDs show about 34-35V DC'?

What is the Voltage rating of the cap C8623 by the LED connector?

[jayanth_rk]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

Attached a picture of the whole back of the TV. The connector in the earlier picture is at the top left of the board, and its thick black cable goes behind the back plate.

[budm]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

Please see my updated post #2.

[jayanth_rk]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

Thanks for the detailed response.

Voltages on CN9103 (Power supply board to main logic board):
Pin 1, DIM: 2.836 V
Pin 2, BL/OF: 3.264 V
Pin 3, PS_ON: 3.317 V
Pin 4, NC: 0 V
Pin 5, GND: 0 V
Pin 6, GND: 0 V
Pin 7, GND: 0 V
Pin 8, GND: 0 V
Pin 9, +12VA/+16VA: 15.93 V
Pin 10, +12VA/+16VA: 15.94 V
Pin 11, +12V/+16V: 15.92 V
Pin 12, +12V/+16V: 15.92 V
Pin 13, +12V/+16V: 15.92 V
Pin 14, +12V/+16V: 15.92 V
Pin 15, +3.5V/+5V: 0 V
Pin 16, +3.5V/+5V: 0 V

Looks like the LED voltage readings I took last time were when the TV wasn't properly turned on or in the right mode. This time I connected a HDMI input and verified using a flashlight that it was showing.

Voltages on CN8601 (Power supply board to LEDs):
+VLED: 52.85 V
-VLED1: 1.445V (slight fluctuations)
-VLED2: 1.55V (slight fluctuations)
Please disregard what I measured last time.

C8623 is a 47uF 100V electrolytic capacitor. Oh, so +VLED could never have been more than 100V to begin with.

Looks like Pins 15 and 16 on CN9103 are a problem? But they go to the main logic board, and everything there looks fine to me. Video and audio are working, it's just the backlight that isn't.

[budm]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

Pin 15, +3.5V/+5V: 0 V
Pin 16, +3.5V/+5V: 0 V

1) That power supply section is not stuffed with parts, that is why there is no Voltage there.
2) Put black probe of the meter on the chassis, red probe on the +VLED then turn on the TV while keeping an eye on the meter to see if the Voltage jumps higher than 60V or not, repeat the same test without having the LED connector in place and see what you see on the meter.
3) Look at the LCD panel model number sticker and tell us what it is, I cannot read it from your picture, I can see TPT????????. You can provide closed up picture of the sticker.

[jayanth_rk]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

1) Okay, I see the blank areas on the board.

2) No, there is no voltage spike. +VLED goes up to ~36 V when the power cord is plugged in (But TV is turned off), and ~53.8V when I press the power button. Whether or not the connector is in place makes no difference to the voltage.

3) Attached. Is that the sticker you mean? The number is TPT500U1-QVN03.U

[budm]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

https://www.shopjimmy.com/sharp-lb50...t-strips-8.htm
Notes: per shopjimmy, this model of LCD panel has more than one kind of LED set used in the panel.
I believe that your panel may be using these LED strips (32 LEDs total), so each LED string has about 16 LEDs, if they are 3V LEDs then Vf of each string is about 16 LEDs x 3V = 48V. 48V + Vdrops on the PWM controlled IC then 53.8V should be enough to light up the LED strings.
Are you 100% sure that there are no backlights at all?
Try turning on the TV ins dark and see if the backlights even flash on or not.

[jayanth_rk]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

Hmm yes, that would mean the voltage is right. There are no backlights at all, I just checked that again.

I think the only thing to do now is buy a pair of suction cups and have a look at the LEDs.

[budm]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

If you are going to do lots of TV's with LED backlights, you should invest in LED tester that you can get for < $30.
Taking the panel apart is the last thing you will want to do, especially the 50" and larger.

[jayanth_rk]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

This is definitely just a one-time project. Yeah I really wanted to avoid opening up the panel. Thanks for all the help, I'll probably end up opening it and verifying the LEDs.

What does the fact that there are no initial voltage spikes indicate? Could it mean that LEDs are fine after all?

[budm]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

What Voltage spike?
"+VLED goes up to ~36 V when the power cord is plugged in (But TV is turned off), and ~53.8V when I press the power button." The Voltage did go up from 36V to 53.8V as your report, isn't that true?
Right now we do not know for sure how many LEDs it has per LED string.
Do you have 47 Ohms 1/2 or 1W (preferred) resistor?

[jayanth_rk]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

Oh I was referring to your point, "turn on the TV while keeping an eye on the meter to see if the Voltage jumps higher than 60V or not". That doesn't happen.

Unfortunately I don't have any resistors with me at the moment. What did you have in mind?

[budm]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

I want to put resistor on -VLED and ground one at a time, this should cause the LED string to light up.

[jayanth_rk]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

Sorry, I don't quite understand that.

To my understanding, +VLED (53.8V) is positive terminal for both of the LED strings, and -VLED1 and -VLED2 are the negative terminals. -VLED is output by the power supply board to be about 1.5V, which creates roughly 52 volts of potential difference between the ends of the LED strings.

What happens when -VLED is connected to ground via a resistor?

[budm]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

-VLED1 (same for -VLED2) is the cathode return for the LED string, it goes to ground via PWM controlled MOSFET U8602 and current sensing resistor to maintain constant current through the LED string.
As I indicated, we do not know for sure if the LED string has 16 LEDs per string or not, if it does it should be lit up right not when -VLED is less than 2V but it is not lit up, so if we use the resistor and it still does not light up, then that < 2V may be error and it is fooling us that the LED strings do not have open circuit, we should not see any Voltage on -VLED if the LED string are open circuit, so at this point I like to check the -VLED without LED connector in place to see if it still show <2V on the -VLED pin on the power supply board or not, if it does then that Voltage is coming from the MOSFET not from the Cathode return of the LED string.

"What happens when -VLED is connected to ground via a resistor?" We are just bypassing the constant current circuit for the LED.

[jayanth_rk]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

Great explanation, I understand that now.

I just checked -VLED when connector is disconnected. It falls to zero cleanly, there is no residual voltage coming from the MOSFET. I will try to get a resistor and do what you described.

Strangely, the -VLEDs now read about 4.4V when the connector is in place, instead of the 1.5V I saw earlier. Not sure why that change would be. The potential difference would still be over the total Vf though.

[budm]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

The -VLED will change with backlights dimming level setting.
I am beginning to think that we may have more thane 16 LEDs per string because at 52V different between -VLED and the -VLED should cause the LED string to light up if we have 16 LEDs on the LED string, that is why we need LED tester to tell us what the Voltage of the LED string is.

[jayanth_rk]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

Yes that is possible, the ShopJimmy listing might not be the right one.

Alright, I'm going to have to keep this aside for a couple of days, I will let you know if there's something new to share. Thanks again!

[Aa1838]

Re: No backlight on Insignia NS-50DR710NA17, need help with voltages

.

So I am having basically the same issue. I don't know if the other didn't notice or mine is different in this aspect, but my backlight LEDs are coming on somewhat. When the tv is turned on in a dark room you can see the screen but it is extremely dim. With the back cover off you case see light from the holes. It's not a settings issue because I've factory reset it.

I'm just wondering if the power supply isn't supplying enough volts. If you look at the first picture of the LED connector right below the pin out it says 94V. That's the only mention I could find what the led output might be. But I'm unsure if that actually corresponds.

Any thoughts?