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dim LCD laptop screen

Discussion in 'Troubleshooting and Repair' started by pharaon, Mar 13, 2019.

  1. pharaon

    pharaon

    222
    3
    Oct 28, 2014
    i have picture but no back light , like the brightness of the screen is set to 1 or 0
    i upload the picture
    i found 3 capacitors that both sides are shorted to ground ...when i removed from the board i test it and it was working fine giving me 4+ for capacitors measure...but it's place on the board both sides are still shorted to the ground.
    the B1 and B4 both sides are shorted to each other but not to the ground. dunno if it's fuse or what
    any idea what would be the failure that cause the shorted of capacitor's connectors to the ground

    [​IMG]

    [​IMG]
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Another component parallel to the capacitors.
    That component could be e.g. a zener diode (used for overvoltage protection), a rectifier diode from a switch mode regulator, the lighting fixture itself.
     
  3. pharaon

    pharaon

    222
    3
    Oct 28, 2014
    that made sense
    on the picture could you show me what or where to check and how to define smd zener and smd rectifier diode
     
  4. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    Sir pharaon . . . . .

    **** How many brands of laptops are there X( TIMES)X How many different models of laptops are there X (TIMES) X how many different years have they been made . . . . . and you expect us to work with what is probably only the units unpopulated / raw circuit board number of . . . LTN140KT04_201_V0_1 HF visible .
    And maybe ? a fancy LG logo to the left .
    Come on now . . . . .! . . . . . with decent information forthcoming.
    Especially BRAND and MODEL . . . .

    **** Closest estimation . . . .97,613 ± 2 %.

    Working . . . . . totally in the blind . . . .

    Looks like power comes in at the bottom right connector as ? CON 10 ?, then passes to the left to a sub power supply with its 24 pin IC1 ? 7 ? and its associated 6.8 uh ringing inductor WHICH should have its associated ultrafast ringing rectifier placed very NEARBY.
    So that might be the very close by, BLACK ? D1? just below the IC numbering.

    If that is shorted that would account for your RED CIRCLED E filter just below the 6.8 unit reading as being shorted also.
    What is its marked capacitance and voltage rating ? . . . . . considering it to be an aluninum mini canned filter mounted on the opposite side of the board.
    There are two intermediate pot cored inductors located between the second E filter to the left, give that e-caps specs also.

    Thaaaaaaaaaaaassssssit . . . . .


    73's de Edd . . . . .


    What happens when you get scared half to death . . . . .the next time? . . . . . is it retroactive ?

     
    Last edited: Mar 14, 2019
  5. pharaon

    pharaon

    222
    3
    Oct 28, 2014
    d1,d2,d3 none of them is shorted
    i attach better pic and another two pic under microscope for better view hopefully

    [​IMG]
    [​IMG] [​IMG]
     
  6. 73's de Edd

    73's de Edd

    2,558
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    Aug 21, 2015
    Sir pharaon . . . . .

    The camera procedure /settings / lighting is optimal for the top photo.
    On the two lower ones there is a shift to being at a lower F-stop and a resultant POOR depth of field .

    PUH-LEEZE . . . now . . . . . the brand and model of that laptop.

    73's de Edd . . . . .


    Copy and pin to workshop / lab wall . . . . .

    Cycles to Hertz conversion Chart:

    https://www.kb6nu.com/wp-content/uploads/2012/04/cps-hertz-chart.png
     
  7. pharaon

    pharaon

    222
    3
    Oct 28, 2014
    they are microscope's pictures that's why only the center got excellent resolution, microscope can only show small part of the board..but it's good to check very tiny things like IC number and etc

    those are all the numbers and text on the PCB board

    [​IMG]
     
  8. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    But . . . .BUT . . .BUT . . . .BUT . . . . WE want the BRAND and MODEL of the computer that it goes into !
     
    davenn likes this.
  9. pharaon

    pharaon

    222
    3
    Oct 28, 2014
    dell latitude e6420
     
  10. bushtech

    bushtech

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    135
    Sep 13, 2016
    Eureka!
     
    davenn likes this.
  11. pharaon

    pharaon

    222
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    Oct 28, 2014
    what do you mean
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

    8,994
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    Nov 17, 2011
    After 9 posts we finally got some vital information...
     
  13. 73's de Edd

    73's de Edd

    2,558
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    Aug 21, 2015
    Sir pharaon . . . . .


    I then TOOK that ALL important brand and model number and related to all of its manual and an additional 42 pages of schematic connection flow paths .
    Ending up at the point where the display was connected into the system.
    Guess what ? . . . that whole display is being a drop in from Samsung under that LTN140KT04 number that I initially referred to.
    So Dell is providing ZERO info relatable to the display unit. I am now hunting up Samsung info and can initially see that the principal power for the display comes in from a 40 pin connector at the center of the board, which your pic only shows an edge, as being visible .

    Your pic shows two sets of ceramic block filter caps being pulled and off the board,which you show in your added RED mark ups.
    NOW what you want to do is check across their pads for a short STILL being present and if so, then carefully disconnect the 40 pin connection so that we can then see if the short is ON the display board or OFF in the circuitry associated with the other side of the 40 pin connector.

    73's de Edd . . . . .

    I had to stop driving my car for a while . . . . . the tires got dizzy.
     
  14. pharaon

    pharaon

    222
    3
    Oct 28, 2014
    that's what i exactly did before posting this topic and the short is on the display board

    now what i'm really looking for is what other component that might cause that short

    i also watched this

    and also this one


    they test component in diode/continuously mode
    but my issue the shorted is in the board not in the capacitor it self
    need to find what cause that short
     
    Last edited: Mar 17, 2019
  15. pharaon

    pharaon

    222
    3
    Oct 28, 2014
    does anyone know what B1,B2,B4 are?
    their both sides are shorted to each other but not to the ground. dunno if it's fuse or what

    [​IMG]
    [​IMG]
    [​IMG]
     
  16. Harald Kapp

    Harald Kapp Moderator Moderator

    8,994
    1,785
    Nov 17, 2011
    Your photos are really pepper to the eyes, sorry :(.

    A fuse is usually labeled Fxxx. Bxxx doesn't fit in the standard nomenclature (Where B would be used for a battery).
    These could be SMD jumpers (also called bridges, which would explain the "B") or 0 Ω resistors which are often used to configure circuits for different features or chips using one and the same PCB. These measuring 0 Ω would be perfectly fine then.
     
  17. davenn

    davenn Moderator

    13,139
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    Sep 5, 2009
    those are impossible to see any info
    post some decent sized images 800 x 600 and sharp focus
     
  18. Bluejets

    Bluejets

    3,288
    652
    Oct 5, 2014
    Look up the chip number for the specs, look for what options are available on those pins and it might give some insight.
    Suspect a bridge though as previously quoted.
    Photo is out of focus so cannot do here.
     
    philadelphia1 and davenn like this.
  19. davenn

    davenn Moderator

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    1,704
    Sep 5, 2009
    well figured out that this is the chip number .... LAU5211X08-E1

    but I cannot find a datasheet for it :(
     
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