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Compaq Presario - odd backlight problem

Archived from groups: uk.comp.sys.laptops,comp.sys.laptops (More info?)

I have a Compaq Presario 705EA on which the backlight doesn't work. But if
I switch the display to another similar machine, the backlight does work.

So, the backlight tube and high-voltage module are OK. Anyone know of
anything on the motherboard or setup that might mean the backlight doesn't
work?

I've strippped the machine to its underwear (OK, down to extracting the
motherboard) and can't see any dry joints, obviously blown components.

Absolutely everything else works fine (including fine image on an external
monitor). Even the LCD display has a dark, ghostly image. So any ideas?
Alternatively, any idea how I could jerry-rig a supply to the backlight?

Cheers,
Paul DS

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11 answers Last reply
More about compaq presario backlight problem
  1. Archived from groups: uk.comp.sys.laptops,comp.sys.laptops (More info?)

    Paul D.Smith wrote:
    > I have a Compaq Presario 705EA on which the backlight doesn't work.
    > But if I switch the display to another similar machine, the backlight
    > does work.
    >
    > So, the backlight tube and high-voltage module are OK. Anyone know of
    > anything on the motherboard or setup that might mean the backlight
    > doesn't work?
    >
    > I've strippped the machine to its underwear (OK, down to extracting
    > the motherboard) and can't see any dry joints, obviously blown
    > components.
    >
    > Absolutely everything else works fine (including fine image on an
    > external monitor). Even the LCD display has a dark, ghostly image.
    > So any ideas? Alternatively, any idea how I could jerry-rig a supply
    > to the backlight?
    >
    > Cheers,
    > Paul DS

    Is the inverter board outside of or inside of the display? If it is on
    the mainboard or otherwise outside of the display and has failed, that
    would explain your observation.

    Q
  2. Archived from groups: comp.sys.laptops (More info?)

    The lid switch is stuck down.. (Not the one on the power bar the one on
    the board directly underneath this.. Common on this model
    www.ikenfixit.com
    ken@ikenfixit.com
  3. Archived from groups: comp.sys.laptops (More info?)

    The lid swich on the board is stuck down.. Pull the plastic top cap off,
    Pull the tin contact strip oput and retry. It will fire up now..
  4. Archived from groups: comp.sys.laptops (More info?)

    The lid swich on the board is stuck down.. Pull the plastic top cap off,
    Pull the tin contact strip oput and retry. It will fire up now..
  5. Archived from groups: uk.comp.sys.laptops,comp.sys.laptops (More info?)

    > Is the inverter board outside of or inside of the display? If it is on
    > the mainboard or otherwise outside of the display and has failed, that
    > would explain your observation.
    >
    > Q
    >
    The inverter board is a small board within the housing of the display. It
    gets swapped around when the display is moved. In short, it seems to be OK
    since it works on another PC. My suspicion is that for some reason, either
    the inverter is "switched off" or there is no power getting to it - can't
    see why this should be the case though.

    Paul DS.
  6. Archived from groups: comp.sys.laptops (More info?)

    "ikenfixit4u" <ken@ikenfixit.com> wrote in message
    news:54be65a967bb78ead2818a1d25f47c6e@localhost.talkaboutcomputing.com...
    > The lid swich on the board is stuck down.. Pull the plastic top cap off,
    > Pull the tin contact strip oput and retry. It will fire up now..
    >
    Is this the same switch that when I close the PC, might cause the PC to
    hibernate? A dodgy switch was something I wondered about but if I enable
    this feature, pressing the switch causes the machine to hibernate which
    seemed to tell me that the switch was working fine. Is it perhaps a double
    pole switch and one pole is working whilst the other isn't?

    Cheers,
    Paul DS
  7. Archived from groups: comp.sys.laptops (More info?)

    Sorry Paul.. See new post of switch.. Got mis-posted somehow..
  8. Archived from groups: uk.comp.sys.laptops,comp.sys.laptops (More info?)

    Paul D.Smith wrote:
    > I have a Compaq Presario 705EA on which the backlight doesn't work.
    > But if I switch the display to another similar machine, the backlight
    > does work.

    Check the switch that turns the display off when it's closed!

    Regards,

    James
  9. Archived from groups: comp.sys.laptops (More info?)

    Thanks for the tips. Unfortunately it seems that the switch was fine. I
    removed as suggested and no improvement, however I have now found the
    following.

    If the machine is left on for some time, it gets warm (a known issue with
    the model) but then the backlight starts to flicker and, sometimes, comes
    on. The backlight is still flakey but this suggests to me that there may be
    a dry joint somewhere on the motherboard that makes a contact when warmed
    up.

    Now I'm more than happy to take a soldering iron to the motherboard (heh, I
    can't make it worse than broken, and I've fixed motherboards before like
    this) but the question is, which are the main suspects? Anyone know the
    705EA motheboard and where the power components that supply the backlight
    are located?

    If all else fails, trial and error will be attempted, but a set of initial
    targets would be nice ;-).

    Thanks for any help that can be offered.

    Paul DS.
  10. Please visit the following link and scroll down to the picture of the lcd inverter.

    Basically what you have to do is trace the number 4 pin from the power side of the inverter and disconnect it near the mother board side and then feed it 3v + from some part of the motherboard. You want this 3 v source to be switched with the laptop so the screen powers on and off... I fixed mine by soldering to a 3.27v source near the sound card ports, however I now find my screen backlight turns off whenever I plug in either my headphones or microphone. The build in speakers do not have any problem though. My solution is to get one of those cheap usb plug in adapters for sound. However, if you follow the post in the following link all you have to do is connect the wire from pin 4 of the power side of the inverter to pin 1 of the u20 ic, or if you prefer pins 2 or 3 either of these pins should work to power your backlight. The reason I did not use ic u20 to power my backlight is this ic is located on the bottom side of the motherboard near the button cell cmos battery and I did not feel comfortable routing a wire from this location or soldering onto a small ic pin and risking more damage then to where I did tap into the power.

    http://www.techsupportforum.com/forums/f108/compaq-c700-screen-problem-295670.html

    after you visit that link there should be an external link to the following schematic:

    http://www.eserviceinfo.com/downloadsm/56126/hp%20compaq_presario%20C700.html

    Do a search for u20 in the resulting pdf and you will see the pin out for ic20.

    My other thought was to tap into 5v usb from one of the usb headers however I was not sure if this would provide too much power to the inverter to signal it to turn on. Thus I have not tried applying more then 3.27vdc to the wire I mentioned above.

    The proper way to fix the backlight not turning on issue is to have the GPU professionally re-soldered.
    BTW I did need to get a new inverter as the original got fried when the problem with the backlight first occurred. I also had to pull the fabric tape off the ribbon cable when I accessed the wires for the backlight power plug. I retaped these with electrical tape when reinstalling and also had to solder a short jumper wire onto the pin 4 wire. I taped these joints a well. The resulting bundle of wire did not reinstall as cleanly as the original but it got it the computer going again with minimal modification.
    c751nr
  11. This topic has been closed by Brett928S2
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