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How To Bypass LCD TV Inverter Board Shutdown Backlight System

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Complaint- Hisense TLM46V66PK LCD TV With Display Shutdown Problem in Few Seconds

hisense LCD TV With Display Shutdown Problem in Few Seconds

When power on the LCD TV, after 1 second the display will cut off. But the TV sound was normal except no display only. The supply voltage to the inverter board was good and the inverter ON signal from mainboard was present too. That’s mean the problem was on the inverter board or CCFL lamps. After measured all suspected components on the inverter board, it seems that all components were good. Since I do not have the CCFL Lamp Tester at the moment, i have used a special method to test the CCFL lamps to check if they are working or not.

You need to be careful about this method because it could harm the components in the inverter board.  This method is called as “Bypass Inverter Protection” because it could let the inverter board to continue provides high voltage to the CCFL lamps. In other words, this method will let you know if either the CCFL lamps or the inverter board problem that had caused the LCD TV to shut down after a second. Further inspection I found that this LCD TV was using OZ9938 IC as inverter IC. If you refer to the pin 3 in the diagram below you could see the pin marking was “Timer” and the description as “timing capacitor to set striking time and shutdown delay time”. This means that external capacitor will make the decision to “ON” the CCFL lamps timing. If the voltage value reach on their setting voltages and the lamps still not start-up, then the OZ9938 will stop working and lock up.

OZ9938 ic

Now, when you short pin 3 of OZ9938 IC to ground, it will temporary cancel the backlight system cut off protection feature and the LCD TV can be turned on with good display without shutting down anymore. Look at the screen now to see if there are any black/dark bar area and make sure the brightness are even in all angles. If the display and brightness are good then this means the backlights are working fine and the problem is in the inverter board. If you could see dark/black area then this means one of the LCD backlight have problem. Do not turn on the LCD TV for too long for safety purposes.

Since I have checked all the components in the inverter board to be good so I powered on the LCD TV again and let it run for about 3 minutes (after shorting pin 3 of OZ9938 IC to ground). After 3 minutes, I turned off the LCVD TV and quickly touch on the 7 HV transformers and feel the temperature with my finger. I found one of the 7 HV transformers temperatures was quite hot. The suspected transformer was T914 and the winding get shorted when under load causing the over current protection ( CCFL Lamps Current Balancing Circuit) to kicks in thus shutting down the backlights.

Please refer to the picture below of the CCFL Lamps Current Balancing Circuit:

 CCFL Lamps Current Balancing Circuit

After replacement with a new HV transformer on T914, the LCD TV worked perfectly fine. Tested the LCD TV for more than 30 minutes and found the temperature of T914 to be normal again. Hope this tip helps.

 

 

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Published in our category of LCD Televisions on Monday June 13, 2011 with the keywords , , , , , , , , .
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13 Comments

  1. siebo
    Posted Tuesday June 14, 2011 at 3:04 pm | Permalink

    isn’t this a dangerous way to test. possible could do more damage?

  2. Jestine Yong
    Posted Wednesday June 15, 2011 at 12:47 am | Permalink

    HI Siebo,

    Do it with caution that’s why the article have mentioned "Do not turn on the LCD TV for too long for safety purposes.".

    Jestine

  3. Eklunge
    Posted Thursday June 16, 2011 at 2:11 am | Permalink

    This faulty was mostly caused by the protection circuit of the inverter board. Usually, I prefer grounded the pin 7 ( OVPT ) to see whether the there is backlight or not.

    - If backlight is observed, this means there are overvoltage/current in the circuit. It mostly caused by the H.V. transformer, loose connectors .

    - if no backlight is observed, this means the IC, power FIT or fuse have problem.

    This is my little experience in the LCD TV repairing.

    • Jestine Yong
      Posted Thursday June 16, 2011 at 3:19 am | Permalink

      Hi Eklunge,

      Thanks for the contribution of your good tip!

      Jestine

  4. Badea Mircea
    Posted Sunday June 19, 2011 at 4:06 pm | Permalink

    inverter transformers can be tested with ring tester

    • Jestine Yong
      Posted Monday June 20, 2011 at 1:18 am | Permalink

      Hi Badea,

      Yes it can. You also could compare the result of the each of the inverter transformer.

      Jestine

  5. Marc
    Posted Sunday August 28, 2011 at 6:29 pm | Permalink

    As a Newbie….how do I short pin 3?

  6. Posted Friday October 14, 2011 at 2:15 pm | Permalink

    just make a jumper that will only connect on pin 3 and to the ground, make sure that it is the configuration of the IC pin that you are going to short.

  7. Imran khan
    Posted Tuesday October 25, 2011 at 9:19 pm | Permalink

    Yes i repair many of LCDs in this problems and many time i got Dry sold at H.V side many times heat up parts like Transformers and out put FETs.

  8. Phil
    Posted Sunday May 6, 2012 at 10:28 pm | Permalink

    If the back lights come on briefly at startup with a picture (about 2 seconds) would this confirm the inverter is at fault, the green led is on and the tv is not in standby mode, this inverter makes a hissing sound as the ccfl lamps start up.

  9. jojo
    Posted Saturday September 1, 2012 at 11:29 am | Permalink

    Is voltage present in the CCFL connector port if the connector of the lcd panel disconnected when powering the lcd monitor?can i measure voltage there?My understanding is that if the connector of CCFL is disconnected or unplug the unit will shutdown because there’s no LCD panel or CCFL connected.How can i measure voltage in the high voltage port of the CCFL when the unit is shutting down?

  10. SUASASU
    Posted Friday September 21, 2012 at 8:08 am | Permalink

    Haier L24f6 Tv Power Indicator Continue On And Off 1s.when It Is Off,hear Tik.
    after I was change defective capacitor (22uf/50V). When it is on and sound is OK. But LCD is not display. When It is on, display is light up few millisecond and off. Why?

  11. Paris
    Posted Saturday October 6, 2012 at 4:07 pm | Permalink

    Hi, i have an LG RZ 30LZ50 and my problem is that the backlight comes on and off on its own while the tv is on standby. Any advice? Thank you

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