laptop screen (Compaq Evo) backlight failure

Discussion in 'General Electronics Chat' started by hobby16, Sep 15, 2010.

  1. hobby16

    Thread Starter Active Member

    Aug 30, 2010
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    Hi Rifaa,
    I like you!

    Just OT, I've got a laptop screen (Compaq Evo) which doesn't have backlight anymore (well it lits up 2s at reboot then goes off). The LCD is perfectly OK (under certain angle, I can read the screen but it's understandably hard to work in such inhumane conditions, lol) except the high voltage fluorescent tube backlight.

    At first sights, do you think it's repairable and worth it (I have a digital scope, solder, no dead brain, yet) ?
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. SgtWookie

    Expert

    Jul 17, 2007
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    It's likely the CCFL (cold cathode fluorescent) driver circuit that's failed.

    It's low current, but high voltage. CCFL's usually require somewhere between 600v-1.2kv to light initially, and then maybe 250v-450v to stay illuminated.

    You would need a high-quality meter like a Fluke with a high voltage probe to troubleshoot it, and probably an oscilloscope as well. If you are not trained in working with high voltage, have a professional repair it. It is not worth the risk for an untrained person to attempt repairs on a high voltage circuit.
     
  4. tom66

    Senior Member

    May 9, 2009
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    The output of these CCFL inverters is less than 20mA at high voltage AC (around 300V.) However, this current can still present a shock hazard. It will be difficult to repair the module, usually because of the small components (think surface mount transformers about ~5mm high sometimes.)

    However, you can replace the module, here are several options (most are under £20 each):

    http://shop.ebay.co.uk/i.html?_nkw=...paq+evo&_osacat=0&_trksid=p3286.c0.m270.l1313

    I've actually seen that exact same inverter in an Acer laptop, and the backlight failed on it too. So it's not uncommon for these to go after a while. The laptop was about 6 years old.
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Keep the solder and direct connections well away from the laptop.

    You probably need a new inverter such as this one.

    http://www.nylaptopparts.com/index.php?main_page=product_info&products_id=29

    however it is important to get the correct EVO model no, as you see from the reference.

    If your scope, or other meter, has a digital frequency readout in the 100kHz range you can safely test the inverter by laying a sheet of paper over it and then passing the probes along the upper surface of the paper, holding them 1mm apart. A working capacitive pickup will show a good oscillation at 30 - 60 Khz.
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    How did I ever missed a post specially for me.. :mad:

    Hello. Glad that you like me, even though I donno how.

    Fixing an inverter is no biggie, provided you can get the components.
    Other members have suggested ways to get a new one. It is cheaper I think in ur area. Not for me.

    But if you like to tackle it for the heck of it...sure, I can throw in some pointers any day.:D
     
  7. hobby16

    Thread Starter Active Member

    Aug 30, 2010
    30
    4
    Because you help and get things fixed ;)

    My compaq is an Evo N1005. In fact, I'm not interested at all in buying a new inverter for several reasons
    - I'm used to high voltage inverters, I've designed electronics with LCD display planes based on CCFL backligth years ago and many other high voltage stuffs, so the risk to toast is manageable.
    - I routinely open my Compaq laptops to undust the fan because after about 1 years, it's systematically make huge noise because of the accumulated dirt, so accessing the inverter seems daunting but no big deal.
    - I'm not in the necessity to repair the Evo, because I've changed for a Compaq Presario. But I like the old Evo WinXP, its // and serial ports, its silence (my new laptop tends to noisier because much more powerfull), I'm currently using it in my WK electronic workshop with an external CRT monitor.
    - I secretly hope the problem is well known and someone would tell me it's just an old electrolytic to change.

    Anyway, thanks to all who have replied. If you have some more information, I'm all ears.
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Nope,. I do not think it's a cap. these tiny suckers rarely have those.

    Show me a PIC, that way my gears starts turning
     
  9. tom66

    Senior Member

    May 9, 2009
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    Most likely a shorted winding on the transformer. This will stop the circuit oscillating. The transformer is small, and prone to failure, as it is under lots of stress. It will be difficult to find a replacement transformer, but if you can, give it a go. The second most likely parts to fail would be the power transistor(s).
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Of course if one of the leads between the mainboard and the inverter or the inverter and the CCFL tube or the screen has become damaged, this will also present with the same symptoms.

    I am just fixing an aspire with this problem, since being dropped on its corner.

    edit update

    the screen is overheating at one end, just burnt my finger.
     
    Last edited: Sep 16, 2010
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I like to see the effect of this symptom
     
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