Laptop screen problem

Discussion in 'General Electronics Chat' started by mentaaal, Jul 24, 2009.

  1. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    Hey guys, I have a rather strange problem concerning a laptop screen

    Brief history:
    A friend gave me a laptop to repair, saying that the screen will only turn on for a few seconds.

    I have verified this by using the fn and a key used to change between the LCD and an external monitor. every time I change from external monitor to LCD, the screen lights for just over a second.

    I have connected and external monitor to the laptop which works perfectly and when the LCD is use, the screen is FAINTLY visible.

    After googling this problem i found numerous people with this problem, the general concensus was that the inverter(which transforms a dc voltage in to a high frequency, high voltage signal which powers the screen) was damaged. Before getting a replacement i checked the voltages from the motherboard and they corresponded to the voltages on the other end of the cable which rules out the cable between the motherboard and the lcd from being a problem.

    I have since replaced the inverter and the problem hasnt changed!!

    The cable I have been referring to a 6 pin cable.
    The voltages I have measured on these pins are as follows:
    pin 1: 0V (for lcd and external monitor)
    pin 2: 0V(for lcd and external monitor)
    pin 3: nearly 3V, around 2.7 If i recall (when on lcd otherwise 0 when on external)
    pin 4: 5 volts (when on lcd, 0 when on external)
    pin 5: 19.6 (when on lcd and external)
    pin 6: 19.6 (when on lcd and external)

    I dont think the problem is with the LCD panel itself as I have used a multimeter to carefully tell when the inverter is powered, and when the screen is not lit, the inverter is not outputting high voltage
    (before any of you mention it, i merely moved the probe of the multimeter near the inverter, i did not make electrical contact with it)

    I do not what the correct voltages of these pins are supposed to be, so could anyone tell me are these correct?
    If they are not then I am really stumped as to what the problem could be...

    its extremely unlikely but i could have just replaced the inverter with a faulty one! :)

    Help!
     
  2. millwood

    Guest

    sounds like the backlight is gone.

    it could be as simple as a connection problem, or a bad converter. in the 2nd case, you can built a new converter and reuse the existing led array.

    if it is ccfl, it is tougher.
     
  3. millwood

    Guest

    on a 2nd read, are you saying that you see 19.6v when lcd / external monitor is tured on? what would pin5/6's voltage be if you just turned on lcd, or just external monitor?

    if it is 19.6v with just lcd on, and 0 with just external monitor, the inverter is good but the led array (or the connection) has gone bad.
     
  4. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    well i am pretty sure that pins 5 and 6 were 19.6 volts all the time.
    pin 3 was at near 3 volts.. is this normal?

    if the backlight is lighting for the same duration of tiime just after the lcd is selected can this really indicate a problem with the backlight?
    it isnt random is what i am saying its very much regular. i have also checked for the corresponding voltages on the other side of the power cable which plugs into the inverter and all the signals are there.

    if you have any suggestions as to what i can do to further narrow down the source of the problem millwood, i am all ears
     
  5. millwood

    Guest

    I missed that piece of info as well (that the backlight came on briefly) - that would suggest that the backlight is good but for some reason it just isn't on longer.

    can you monitor voltage on the pins when the backlight is on vs. when the backlight goes off? that way, you presumably can figure out which pin is controlling the backlight. my guess is that somehow that voltage on the control pin isn't holding.
     
  6. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    Thats what I was thinking. As mentioned in first post, the only pins that appear to change voltage are pins 3 and 4. pin 3 is at nearly 3 volts and pin 4 is at 5 volts when the lcd is on and both are at 0 when the external monitor is selected. As said its a very strange problem indeed!
     
  7. millwood

    Guest

    that may not be the problem. most laptops (all that I have used) would allow three modes: internal lcd only, external lcd only and internal / external.

    so those signals may not indicate a problem.

    what you need is to turn the laptop on internal lcd only and watch the pins when the lcd goes off.

    there are two possibilities: 1) the control signal for some reason is faulty; 2) the lcd has internal sensors that suggests a faulty "fault" - like overcurrent or over temperature when in fact there is nothing wrong. the lcd controller receives that faulty fault signal and turns the lcd off to protect it from further damages.

    if so, the fix is simply to find a way to keep the control signal in a position to fool the controller (like a permanent pull-up or pull-down resistor).

    but you will need to find out which pins change voltages when the backlight goes out.
     
  8. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    Really? No kidding! My laptop has the exact same problem. I don't understand why it would be the backlight, it lights up for a few seconds and works great but then it shuts down and I have to use an external screen. Sometimes the screen is even a reddish color.
     
  9. parawizard

    Member

    Jul 23, 2009
    10
    0
    What chipset is in the laptop? And what graphic chip? As far as I know LCD screens don't actually check if the backlight is working or not. In DIY projection there is no need to bypass this on the controllers so I don't see it being a usual issue? Mind you correct me if I am wrong.

    If you strip the LCD panel out of the housing and shine a light through it do you see the correct screen output? As you can say you see it very faintly. This sounds like a bad CFL bulb or ballast. As you have replaced the inverter maybe it is the bulb itself.
     
  10. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    Well, i am pretty sure that when i was checking the signal levels, pins 5 and 6 where on 19.6 volts all the time no matter what output was selected.
    Also, I dont think that its a problem with the LCD itself, because if it was, I would still be able to detect the inverter outputting power to the panel wouldnt I?

    As mentioned in the first post, when the LCD panel's light goes out, the inverter stops powering. (Again, I checked this with a multimeter)

    in answer to your question, I am not home at present but I will update with chipset as soon as I can
     
  11. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    Well i have narrowed it down to a problem with the LCD panel as the inverter works fine. I eventually, reluctantly look a multimeter to it and it was outputting 250 V AC (approx) at 35 kHz so that was fine.

    I am going to try replacing the ccfl bulb (yes it looks like a pain in the ass to do :)

    thanks for all your help guys
     
  12. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    It is usually the ccfl bulb. It somehow draws more current, which the inverter can't sustain, and the voltage at the inverter output drops below the holding voltage for the bulb thus it goes off.

    Just make sure you are replacing it with the same type bulb. They come with various striking and holding voltages/current.
     
  13. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    Cool, thanks for the tip, I will keep it in mind when ordering the replacement!
     
  14. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    Yep, I can faintly see the screen image on the internal screen and it is identical to the projected image on the external screen.
     
  15. loosewire

    AAC Fanatic!

    Apr 25, 2008
    1,584
    435
    With 50,000 members I will be disapointed If no picture's are posted and model
    numbers and a sucessful repair of this laptop.This seem simple If the member has the money to follow direction's. There has to be people that know laptops inside out. If It worked on an external screen.The member should not be expected to do all the posting.Come on guys repair this laptop,get the Info you need and go do It.
     
    Last edited: Jul 28, 2009
  16. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
  17. loosewire

    AAC Fanatic!

    Apr 25, 2008
    1,584
    435
    Good luck,I hope the guy's get with you and see this throught.
     
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