Question about HP f2105 21" LCD Monitor

2 Answers

Monitor's backlight inverter shuts down.

After a minute or so the inverter shut's down, so no light, and no picture.
Turn off and on the monitor brings the inverter back to work for a minute or so.
I turned the small potentiometer on the inverterboard, there seems to be a little influence on the time.
Seems that there is a component overloading protection misadjusted or so.
Does anyone have a schematic on this board, or an idea how to save 90 bugs to buy a new inverter?

Many thanks.

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  • 2 more comments 
  • mkieffer Oct 31, 2008

    It's a computer monitor from HP.
    Model HP f 2105.
    model from HP HSTDN 2B05
    product number P8727AA
    As i opened everything, can see that all the internals are from BENQ
    The part number of the inverter seems to be 48.L1S04.A01
    I'm quite good in electronics, so you can put details in your help.
    Many thanks.

  • mkieffer Oct 31, 2008

    The video source can be excluded,as i have tried already different sources.
    Just touching the potentiometer on the backlight unit with the screwdriver, sometimes makes it shutdown already,depending of the position of the potentiometer.
    Looks like if the oszillator switches off.

  • mkieffer Oct 31, 2008

    On the backl.unit is a 9 pin connector for the power.
    Colors and voltages

    black 18.95V
    brown 18.95V

    red 18.95V

    yellow 0V
    orange 0V
    green 0V

    blue 3,27V
    violett 0V
    grey 0V for normal operation. 3,27V when backlight fails.

    So far for my analyze now.

  • mkieffer Oct 31, 2008

    More news....
    Just took off the grey wire from the connector i told you before ( 9pin backlinvert.)
    The voltage on the wire goes up to 3,28 volt, but the monitor continues working.
    So it seems that this wire comes from the control unit and tells the inverter to switch off.
    What purpose is it??
    The monitor continues working.


2 Answers

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  • Master
  • 6,966 Answers


Initially, visually inspect for any leaked or bulging capacitors. The problem may be heat related. In the absence of a schematic diagram, you may have to wing it. To verify if it is heat related is to have a can of compressed air or "instant freeze". Soon as the unit shuts down, spray suspected area. If unit resumes operation, you have pinpointed the area to start component check. If it is a major problem, the unit will not even start up. But since it works except shuts down, then again as initially posted heat related.

Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.

Good luck and kind regards. Thank you for using FixYa.

Posted on Oct 31, 2008

  • Louie  Role
    Louie Role Oct 31, 2008

    A couple of standard tests/ideas:

    • it is rare for all CCF (cold cathode florescent) lamps to go out at the same time, hence least likely;

    • to confirm if it is the backlight/inverter shutting down, shine a flashlight at an angle across the LCD display screen when the CCF lamps go out. You would still see a semblance of desktop except it would be very dark, if yes, then it is backlight/inverter issue;

    • if not then it would be a video interface or controller issue (again unlikely);

    • pls check voltages of the PSU, in most models, the voltages needed are 12~15, 5 and 3.3. They should remain constant even if the lamps shuts down.
    Again, no exact service manual could be located. But as you posted "good in electronics", then these might be of help:

    Hopefully the above would at least give you an idea.


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  • HP Master
  • 3,614 Answers

What is the exact model maker of the tv...? and also if you can provide the part # that would be great.!

Posted on Oct 31, 2008

  • 1 more comment 
  • micromaster
    micromaster Oct 31, 2008

    Do the following items to prepare for troubleshooting:

    • Turn off the computer.

    • Disconnect
      all connected devices from the computer except the mouse, and keyboard.
      This includes removing thumb drives and camera-type memory cards.

    • If the monitor has a detachable cable, obtain a second monitor cable for testing purposes.


    1. Turn on the computer and press the
      key repeatedly when the first startup screen appears.

    2. If you are using Windows Vista, select
      Enable low resolution video (640 x 480)
      , press
      , and wait until Windows opens. To set display settings that match the monitor, use the steps in the HP support document,

      Changing Display Settings, Background Image, and Screen Saver in Windows Vista


      If you are using Windows XP, select
      Enable VGA Mode
      , press
      , and wait until Windows opens. Continue using these steps to set display resolution.

      If you are using Windows 98 or ME, select
      Safe Mode
      and perform the following steps to enter into a standard VGA mode:

      1. Once Windows opens to the desktop in Safe Mode, click
        , and

      2. In the Open field type
        and press

        The MS Configuration Utility window appears.

      3. Click the
        button, and select
        VGA 640 x 480 x 16
        if it is available.

      4. Click
        and restart the computer.

      5. After Windows opens, click
        , and then click

      6. In the Open field type
        and press

      7. Click the
        button, and remove the checkmark next to
        VGA 640 x 480 x 16

      8. Click
        . Do NOT restart the computer.

    3. After Windows opens, right-click on an open area of the desktop and select
      (Windows XP, 98/Me) or
      (Windows Vista).

    4. In Windows Vista, Scroll down and click
      Display Properties

      In Windows XP, 98, or Me, click the

    5. In Windows Vista, select the native display resolution size using the Resolution slider. Set Colors to
      Highest (32 bit)

      In Windows XP, select the native display resolution size using the Screen resolution slider. Set Color Quality to
      Highest (32 bit)

      In Windows 98 or Me, select the native display resolution size using the Screen Area slider. Set Colors to
      True Color

    6. Click
      after you have made your adjustment.

      The screen should blank out temporarily and come back.

    7. If you are satisfied with the new display settings, click
      on the Monitor Settings windows. If you are not satisfied or if the
      screen just goes black, wait: Windows sets the screen back to the way
      it was before. Try another setting until you find a display
      configuration that is compatible with the monitor and meets your needs.


      the native resolution is not available, download and install updated
      video driver software for your computer. Updating the video driver
      software can add more resolutions (graphics modes).

    8. Restart the computer. The monitor should now display a full screen image in high quality. You are done.

  • micromaster
    micromaster Oct 31, 2008

    it seems that you already open the monitor. its better first check that you have that problem only on that monitor. try this;

    Using a different cable, connect the
    monitor to another computer that has video. Turn on the monitor and the
    newly connected computer. What happens?

    • If
      the monitor works, the video hardware on the original computer is
      likely to be the source of the problems. You can try connecting the
      original computer to another monitor that works to update its video

  • micromaster
    micromaster Oct 31, 2008

    Checking the back light unit


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