repairing LCD

Discussion in 'The Projects Forum' started by Bear_2759, Jan 11, 2012.

  1. Bear_2759

    Thread Starter Active Member

    May 23, 2008
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    Hi Guys,

    so a while back (2yrs) we had a power surge at work, 42inch LCD killed. I thought I'd take it home and try and fix it rather than chuck it in the bin. over the last 2 years I've gradually tried to fix it and I'm sooooo close now. after opening the back of the unit there was a whole lot of popped caps on the power supply. first idea was to replace the popped caps. this didn't make a difference. unit still had no power. months later I searched on good ol fleebay for the power supply unit (by model number). found stacks of them so just bought one. now when I put it in, I noticed that even though the 2 power supplies had the same model# they were different !??!!?!?!?!. the new one I bought were missing 3 pins for the output plug. 2x +5v and a +12v. but with this one the unit at least had power but wouldn't turn the screen on.

    so today, I checked the front/back of both boards. on the old one, the extra pins are directly connected to the others (of the same value). no other components inbetween. so I thought I'd try connecting a few wires between them on the back side of the plug. hey presto it worked!!! tv powers on, display on screen only problem now is there's a constant buzzzzzzzzzzz from the built in speakers. now I dont know if this is caused by my "fix" or if it's because of the power surge back when the TV was first killed. I had thought I'd just disconnect them and run speakers through the aux output. but what do you know there's no aux out. so if I can't get rid of the buzzzz it means I cant use the built in HD tuner.

    so far though I've got a working full HD 42" LCD that I can use any of the other inputs on, just not the built in HD tuner :D

    can anyone suggest ways that I can test/isolate the issue that's causing the buzz through the speakers? the buzz doesnt increase/decrease with the volume control if that helps...
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Show me what you got.
     
  3. Bear_2759

    Thread Starter Active Member

    May 23, 2008
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    I'll take some high res pics and put those plus model numbers etc up as soon as I get home.
     
  4. Bear_2759

    Thread Starter Active Member

    May 23, 2008
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    ok so here's some pics.

    [​IMG]
    the back of the unit. (everything)

    [​IMG]
    the new PSU

    [​IMG]
    the board with all the outputs. dont know what it should be called

    [​IMG]
    assuming HD tuner. but that could also be on the other board

    [​IMG]
    the old PSU.
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    U missed a cap in the supply.

    Try renewing all the electrolytic in the secondary stage.

    There will be a 33V regulator. Basically a Zener regulated used as the VT of the tuner. Try to find that area and replace any caps there. Mostly would be between 10u to 100uf ones
     
  6. Bear_2759

    Thread Starter Active Member

    May 23, 2008
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    which one do you think I missed?
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Depends on from circuit to circuit. To be exact I need to trace the Tuner Supply pins. The Vcc, or Vdd. and VT.

    Depending on the tuner it might use a 12V line and 5V line. But the VT will always be there. As far as I have seen it is there.
     
  8. Bear_2759

    Thread Starter Active Member

    May 23, 2008
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    excuse my noobness, VT? I may know it as something else but haven't heard VT before.
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Tuning Voltage. labelled VT..which is 33V max

    Any ripple in the tuner supply or VT will be injected into the video...

    My first assumption regarding ur problem.
     
  10. Bear_2759

    Thread Starter Active Member

    May 23, 2008
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    but this is the new replacement PSU. not the one that was fried so there shouldnt (touch wood) be an issue with power, and couldn't I just disconnect the power from the tuner card to isolate it and see if that is the issue?
     
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    without a schema or close ups I can't tell.
     
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