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LA40N71B -SAMSUNG - LAMP OFF AFTER 3 OR MORE HOURS

abou jaafar 06 Gru 2012 07:51 1599 7
  • #1 06 Gru 2012 07:51
    abou jaafar
    Poziom 14  

    hello my freinds

    as the topic , the sound still good but the backlight gone after 3 and more maybe 5 hours ..and the data still ok to ,just the lamps turn off , and when you turn off the tv and turn on the lamp work good ..
    i check the power ( i/p ) seem good anyone have the service manual or any help ,thank you .

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  • Relpol
  • #2 06 Gru 2012 10:24
    conan02

    Poziom 29  

    Hello, Samsung was common problem with power unit, certainly weak capacitors. If You are familiar with base electronic skills, try disassemble TV and inspect capacitors in power module, if caps are punched out then need replace it. Use Low ESR (low impedance) capacitors for using in PSU systems. Backlight problems is first sign weak powering.

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  • Relpol
  • #3 06 Gru 2012 11:50
    abou jaafar
    Poziom 14  

    thank you for replay

    yes i'm electronic tech .. and all capacitors was good ..but i saw some zener diod was cuting number on board zm803 i dont know if is from company or ..


    LA40N71B -SAMSUNG - LAMP OFF AFTER 3 OR MORE HOURS

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  • #4 06 Gru 2012 14:39
    conan02

    Poziom 29  

    One more question: after power off/on cycle, working time is same (next few hours), or significantly shorter?
    If time is similiar long as first run, then maybe problem with CCFL tubes - invertor circuit have overvoltage protection, if CCFL are near end of life then voltage raising and shuts down backlight, after power reset things are back to work.
    If second run is very short, the problem maybe in overheating something up to protection level.
    Next point - inspect soldering pools, cool-soldering have similiar effects, randomly malfunctioning is typical.
    On PCB silkscreen is printed table with voltages, use it to examine PSU under load.
    Remark: yellow marked diode on photo is clamp element in protection power MOSFET.

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  • #5 06 Gru 2012 15:23
    zbyszekskibinski
    Poziom 32  

    You will not be able to localise an intermittent fault like that. The best option is to fit in a low ohm resistor between Vcc, 12V pin and LD (after disconnecting LD from the balancer board) and observing if the fault re-appears.

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  • #6 06 Gru 2012 16:38
    abou jaafar
    Poziom 14  

    mr conan02
    when the lamp turn off i puch bottom the power and puch it again then its work well , theris no dry solid , thank you for your help .
    mr zbyszekskibinski : you want me to cut wir from board and contect with resistor with 12 vcc this is the photo .


    LA40N71B -SAMSUNG - LAMP OFF AFTER 3 OR MORE HOURS

    Dodano po 9 [minuty]:

    pc : when test ld pin its 6.7 volt

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  • Pomocny post
    #7 06 Gru 2012 17:00
    zbyszekskibinski
    Poziom 32  

    Yes, disconect the wire from the LD pin and connect a resistor from 12V Vcc line to this point on PSU board. You should select a right resistor to obtain about 7V there but this value is not critical, I've seen Samsungs with 12V and 10V on this pin. If the backlight doesn't go off anymore this will tell you a CCFL is at fault.

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  • #8 08 Gru 2012 14:13
    abou jaafar
    Poziom 14  

    dear zbyszekskibinski : thank you for your helped , the tv still on more then 10 hours , and i saw something , the customer was putting the brightness and the sharpness on the maximum level , then i minimized them to the medium level .

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