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Weird behaviour of LCD monitor CCFLs

Discussion in 'Troubleshooting and Repair' started by wiru, Mar 6, 2015.

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  1. wiru

    wiru

    16
    4
    Nov 27, 2010
    I have a monitor Samsung P2470HD that I fixed, but I have no idea why my fix worked. Can anyone explain why did it work?

    The monitor would power up and enable backlight only for a few seconds. Then it would continue to work and display image but the backlights would turn off.

    I found a defective inverter transformer with one primary, two secondary windings. The secondaries measured ~1k and ~1.5k. I ordered a new one both secondaries of which measured ~1,07k.

    However that fixed the issue for exactly one turn on. After I powered it off and on it would have the original fault. The transformer tested OK. I did several other tests, re-soldered a few CCFL connections etc to no avail.

    The inverter seemed to light other known-good CCFLs just fine. The "defective" CCFLs would work with an inverter from another monitor (Samsung 19") fine as well. So... what could be the problem?!

    Back to working only with the defective monitor. I then tried swapping CCFL connections in random patterns. The combination that I found "fixed" the monitor but left me puzzled as to why it works.

    Here's what I've done:
    DSCF3971 fix.JPG

    1 & 2 connect to bottom CCFLs
    3 & 4 connect to top CCFLs

    If I interchange 1 and 2 - backlights shut off
    If I swap between 3 and 4 - backlights turn off still

    However, if I swap 1 and 3 (or 2 and 4) - CCFLs start up and work fine.

    The question is - why would this work?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,412
    2,780
    Jan 21, 2010
    Perhaps one of more of the ccfls is drawing more current than it should. Your wiring may be a fortuitous combination that distributes the load in such a way as to not overload the inverter.
     
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