TFT Monitor Flickering - Repair

Discussion in 'General Electronics Chat' started by Georacer, Sep 18, 2013.

  1. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    My Samsung monitor on the PC is acting up, after 6 and a half years of use. I guess that its backlight LED strips are dying out and is flickering. I may or may not go for an LG next time.

    On the CRTs: Decent and big ones weigh a ton! You would need a special piece of furniture to hold the 40kg that it would weigh. I don't see a reason to dedicate so much space in a room for a CRT TV nowadays.

    EDIT: Split from here: http://forum.allaboutcircuits.com/showthread.php?t=89456
     
    Last edited: Sep 22, 2013
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  2. killivolt

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    Jan 10, 2010
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    No, typically it's the 1000mic caps on the secondary, check their ESR or just replace them. It's power supply issue Samsung uses cheap Capacitors with the wrong type of dielectric combinations.

    I fix them all the time.

    Regards,
    kv
     
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  3. GopherT

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    Nov 23, 2012
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    Samsung and Sony make their panels in a joint venture facility run by Samsung in Korea. The panels made before 2008 had fluorescent tubes for backlights. By 2011, all were LED. Lower energy consumption, cooler running, thinner and brighter.

    Newer concept is edge lighting instead of back lighting. They use more powerful LEDs and have no issues of irregular LED colors or intensities (bins) because the edge lighting averages what gets projected to the etched front surface. I don't know the cut-off when a panel becomes too big that edge lighting is less effective than back lighting.

    All that being said, I would go with an edge light Samsung for 42 and below. Samsung back lit for bigger panels. No reason to pay for a Sony - the panels are the same.
     
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  4. killivolt

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    FYI, on the smaller monitors, sometimes the video card gets taken out.

    I don't know why, but flickering of the video, is the power supply. If Dead and no video, could be a video card or power supply. Putting in the caps, with no result, is the video card. I have to remember the back light issue's. I'll post those later, as an edit.

    Edit: Bad inverters could be an issue, monitors with no backlight. Just smell it for a burnout. Or check the windings and look re-solder if either look like they could have a cold solder joint.

    kv
     
    Last edited: Sep 20, 2013
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  5. Georacer

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    Maybe I'll open mine today, change the caps and tell you how it went.
     
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  6. GopherT

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    Like KV said, if you have an older LCD, (especially if it is the thicker type - 5cm or so), the backlights could be fluorescent tubes. These do start flickering. Surprisingly, replacing caps on the power supply can extend the life of weak tubes and replacing weak tubes can extend the life of weak caps.

    I would try the the caps first, generally cheaper and easy enough for someone who knows how to solder.
     
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  7. Georacer

    Thread Starter Moderator

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    The screen is a Samsung 2032BW.

    Just changed the caps. They where 470μF, 25V, CapXon brand. They where bulged indeed.

    Didn't check the backlight. It was a much more involved teardown.

    The screen seems to work fine now. Thanks killivolt and GopherT!

    Pics will be edited in the post in a short while; Win8 are freaking out right now.

    EDIT: PICS!
     
    Last edited: Sep 20, 2013
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  8. Metalmann

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    Dec 8, 2012
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    I can see one capacitor had already split. Good thing you switched them out.:cool:
     
  9. killivolt

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    Over here those same caps are 1000uf 25v. Must the the frequency and voltage difference.

    I'm glad you saved another monitor from going to the dump.

    I edited the above for the Backlight issues.

    Cheers,
    kv
     
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  10. sheldons

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    Oct 26, 2011
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    use 100 degree hi temp ones and ask yourself why the voltages rose in the first place to cause the damage in order to prevent it happening again.....this is one of the main reasons of failure with these power supplys....to give you a clue check the condition of the capacitor sitting next to the TIL431 and the TIL431 itself by replacement and the condition of the opto....the regulation voltage wise depends on the correct operation of the feedback side of things or voltages will not be stable and you will have the same thing happen again ....bush vestel powersupplys although a bit more complex than this particular one suffer from the same failure ....
     
  11. Georacer

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    I guess I am the intended recipient of this message?

    I did use 105oC capacitors, just like the ones that where in there before. I don't think I can source higher temperature ones.

    As for the rest of your post, I don't quite get it. Can you please use better punctuation?
     
  12. killivolt

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    I think maybe he's trying or meaning to say, if you don't replace with the correct caps, the feedback voltage of the Opt-coupler, while taking in considering the overall design by vetal power supply's, which cause Temp rise in the caps, may create failure in the future.

    Although, over here across the pond, it's designed slightly different. After the secondary is pushing voltage to the Inverter section of the board, I have only 2 caps or 1 can't remember. You have 3 that failed in that section by the inverters, plus an additional 3 in the secondary.

    I have only had failure in the secondary, plus there are only 2 1000uf 25v caps that go bad, instead of your 3 470uf.

    It's really cool to see the different designs needed to accommodate the difference in Voltage and Frequency. 220v 50Hz vs. 110v 60Hz.

    Of course, I'm only guessing your voltage and frequency.
     
  13. Georacer

    Thread Starter Moderator

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    Greece (and Europe in general) has moved onto 230-240 (ish) @ 50Hz, some time ago. But, yeah, I get the point.

    I used the same temperature, capacitance and voltage rating capacitors. Not the same brand, nor measured the ESR.

    I can't do the analysis off the top of my head to verify the circuit operation, though.

    I think I'll split this now.
     
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  14. sheldons

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    Oct 26, 2011
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    sorry for that i was due clocking off for the day to get to bed when i posted that but.....yes .....inthe pic you posted,there is what looks like a TIL431 -not far from one of the caps you have changed which needs replacing.Also there is a small electrolytic which needs changing too which isnt far from the TIL431 and is just to the right hand side of it.
    The opto also needs checking/changing too as well.Due to the poor design of these power supplies and the manufactureres not fitting parts that are up to the job the fault you have with split capacitors due to the supply rails rising too high ,is a regular occurrence.
    Most of these supplies have very poor protection built in to safely shut the set down in an overvolts condition-not like the crt counterparts of which i can name many and have serviced many in the past which have protection built in which prevents operation of the smps when you have this fault(oh they where the good old days ....easy sets to work on and many are still working now).
    I find if you dont tackle the root cause of the failure you are just wasting your time as it will fail again.
     
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  15. Georacer

    Thread Starter Moderator

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    Interesting. What makes you believe that those components are at fault? Experience on similar rectifier circuits?

    Is it a poor part selection, bad part sizing or a bad schematic altogether?

    How would I go about checking the TO-92? I won't take it apart to look for the exact model; it's a pain, actual pain on the fingers, to release the clips holding the screen together.
    Similarly for the electrolytic capacitor and the opto-coupler.
     
  16. sheldons

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    i would just replace the TIL431,opto and that last electrolytic as a matter of course when servicing these supplies because any one which may be at fault will affect the feedback and overall regulation of the power supply. I have found due to manufacturers fitting the wrong voltage electrolytics and poor components in general to the supply rail which is being used as a reference to set the feedback and voltage of the supply rails as a whole all sorts of failures will occur, when the supplies go too high(some lg and phillips models use 10v rated electrolytics on 12v supply rails which although doesn't seem much rating wise, but as the esr increases and the feedback has to compensate for it its not long before things go wrong).
    The bush vestel pw series lcd power supply are not forgiving when they fail in this way and the amount of parts that need to be replaced sometimes with this particular type of supply is a pain.Overall ,the reliability and build quality of these supplies being made to a price isnt good at the best of times
     
  17. THE_RB

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    Feb 11, 2008
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    I agree re replacing that small electro next to the heatsink and opto. You should definitely do that one.

    And personally I would replace the small electro on the other side of the opto too, (the "cold" side).

    And check with a microscope for dry joints, circle fractures other heat issues etc around the yellow transformer, there are at least two parts there look heat damaged (diode and resistor).
     
  18. sheldons

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    it seems to be a major design failing with these power supplys regarding layout and placing components right next door to hot running heatsinks...im sure a bit more thought about the layout would improve things
     
  19. THE_RB

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    It's deliberate. They "design in" a short lifespan of the product.

    They also use plastic enclosures where the assembly tabs are internally hidden (so they can't be pulled apart without nasty cosmetic damage).

    Most of the smaller stuff now has ultrasonic welded enclosures (like small PSUs) and they are even starting to do that on some larger appliances. To get them apart you have to hacksaw it open!
     
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  20. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    Tell me about it. I lost half the skin of my fingertips prying open a dozen plastic clips. Luckilly, I found a service manual with nice images that showed me how.
     
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