LCD Backlight(?) Problem

Discussion in 'General Electronics Chat' started by Lysol, Mar 12, 2008.

  1. Lysol

    Thread Starter New Member

    Mar 12, 2008
    4
    0
    Hello! I've obtained a broken 17" LCD Phillips 170S, and I'm trying to fix it.

    It's first symptoms were that the backlights, simply wouldn't light up (the LCD still displays an image). Once taking it apart, I found 4 blown capacitors on the power inverter, which I replaced with ones of equal voltage and capacitance.

    Having done so, no smoke (Hoo-ray!), and the backlights lit up, but only for a couple of seconds. (They will stay on for about 3 seconds every time I turn the power on.) However, the display on the screen is still shown, so I assume that's good news :).

    My next guess would be to find another 17" LCD of same proportions and switch out the backlights, but then I started thinking. Seeing as the backlights _DO_ light up, even for just that brief amount of time, it leads me to think that maybe something else in the power inverters circuitry went bad, and is only allowing for a short burst of electricity, which is needed to power the backlights.

    I suppose my question at this point is, what should I look at? I'm nowhere near being able to call myself a circuitry expert, or in an intermediate position for that matter, I'm a programmer, but I'm a fan of salvaging parts from dead components :).

    They only component I know how to check(Other than the universal signs of a fire) are capacitors, and noticing their nice little blown top. What other components should I look at, and how can I distinguish if they're good or bad?

    I thank you very much for your time and effort!

    ~Lysol
     
  2. Ohms Law

    Active Member

    Oct 14, 2007
    32
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    If i remeber correct lcd's have ac voltage on the display so if u need ac u had cap problems seems as if another cap is bad since caps block dc but pass ac just a guess
     
  3. Lysol

    Thread Starter New Member

    Mar 12, 2008
    4
    0
    So AC current can still pass through a blown capacitor? Interesting, well, it looks to me as though all the capacitors are fine... (I'll upload a picture of the power inverter right after dinner). The problem is with the backlights, not the display.

    Note: Click pictures for original images. Thumbnails scaled to 10% original size.
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  4. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    Did the backlight work as it should be during that 3 seconds (light intensity, colour, etc.)? If it did, try looking at the safety circuits in the inverter, such as over temperature or over voltage/current protections.

    I had one monitor with a failed temperature sensor about a year ago. It showed the same symptom as yours, after about 5 seconds the switching IC just stop dead because it thought it was running too hot.
     
  5. Lysol

    Thread Starter New Member

    Mar 12, 2008
    4
    0
    Thanks for the ideas!

    Yes, the backlight works perfectly for those few seconds, and will continue to do so when ever it comes on from the off position. Can you identify which component in the circuit would be checking for the heat :confused:, and how I could establish weather or not it is the cause of my problems?

    I also forgot to mention that I have checked the fuse, and found it to be fine.


    Thank you very much for your time :)
     
  6. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    I can't pinpoint the likely device from the pictures. Try looking for a switching controller, which is the most likely culprit. If there is no IC on the board, identify the transistors and look for possible current sense resistors or temperature sensors.

    First check that no component goes too hot during that 3 secs. If there was, then focus on that first.
     
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