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Vizio LC420DUG-JFR1 Replaced LED strips now picture fluctuates.

I replaced the LEDs on this unit back in 2017, so when the same symptoms of black screen with sound happened again, I performed the flashlight test. I could see the picture. I ordered the LEDs from the same source and installed. They all lit up and were working, but I did notice they seemed to be getting slightly brighter, then dimming slightly. After putting it all back together carefully and and hooking it to a video source, the picture intensity / clarity keeps changing approx every 3 seconds Like the LED intensity is fluctuating up, down. The picture is clearly visible but the white areas have gray lines through them, then they slightly soften as it fluctuates and the lines disappear. There is also visible white at the top edge of the screen that goes away as the picture fluctuates. There are 5 LED strips. 3- R1/L1 and 2- R2/L2. They were all connected properly. When I tested the voltage, Vout 1 is 61 V and Vout 2 is 91 V. I thought they should be approx 80 and 120 v . I unplugged the TV and the the LED connector and retested. Vout 1 is approx 95 and Vout 2 is approx 140.

Posted by Stl Zou on

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1 Answer

Larry

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  • Vizio Master
  • 3,588 Answers

Are you measuring LED voltage using frame ground or the plus and minus terminals at the connector

Posted on Nov 24, 2021

  • 3 more comments 
  • Stl Zou
    Stl Zou Nov 24, 2021

    I'm measuring black to ground (gray metal case) and red to Vout1 and Vout2. Vout1 measure 61 v and Vout2 measures 91 v when the LED wire harness is connected. There are 10 led's per row. 5 Rows total. 2 rows on Vout1 and 3 rows on Vout2...I believe.

  • Larry Nov 25, 2021

    That's the mistake most make when working on sets with backlight issues. Measuring to ground won't tell much as it's the minus side that's regulated. See what you get measuring from the plus to minus terminals of the LED output, don' t use frame ground. Also check the backlight on terminal of the connector going to the main and make sure the CPU in the main is telling the backlights section of the power board to turn on, should see around 3 volts there.

  • Stl Zou
    Stl Zou Nov 25, 2021

    I'll be honest, not sure where to test. I uploaded an image of my board / connector



  • Larry Nov 26, 2021

    Test between pins 1 and 3 and 5 and 7, test it without an input connected and the voltage should be constant if things are working ok.

  • Stl Zou
    Stl Zou Dec 02, 2021

    Sorry for the delay. Voltage between 1 and 3 was 56.6. Voltage between 5 and 7 was 85.5. Tested DC V with power on, Led connected. I wasn't sure what you meant by test without an input.

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I have a weird problem: My TV's LED backlight pulses... it is generally off / very dim and periodically the LEDs pulse briefly to a reasonable intensity then fade back to dim every few seconds.

Could be a short in the strips test the strips In each row if you look at the row count the LEDs each require 2.8 dc volts add them up for each row example if you have 9 in the row you will need 25.2 volts dc to power test it .if you look at the right side of the strip you will see 3 dots a plus two minus use center minus as ground and plus for positive . If leds still flicker or dim or some don't light than theres a short within the led strips .
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