Help with HF buzz on plasma

Discussion in 'General Electronics Chat' started by fabchef, Oct 13, 2014.

  1. fabchef

    Thread Starter New Member

    Oct 13, 2014
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    Hi everyone, this is my first post here. Recently my plasma tv has started making a high pitch buzz when brighter pictures are displayed and quiet on darker ones. I tightened all screws inside and changed out power supply board (with a used one). I thought maybe the noise filter box(where the power cord plugs into tv) is defective? Any help would be great. Thanks. Fab
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Does the TV have its own internal sound system that might be making the noise? A plasma generates a ton of RF interference, similar to a light dimmer.
     
  3. fabchef

    Thread Starter New Member

    Oct 13, 2014
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    Hi Wehne... Yes it has internal speakers but still does it when they are switched off. My second pana plasma doesn't do this noise. As i said it happens more with the intensity of brightness on the screen. Fab
     
  4. wayneh

    Expert

    Sep 9, 2010
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    The RF interference I noted also varies with the lightness of the display.

    I'm no expert, but I believe the standard suggestion is to start replacing electrolytic capacitors. They age and frequently go bad. As they fail, larger ripples fail to get filtered out properly. A schematic and service manual can be very helpful in this.
     
  5. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Also remember plasma TV put out a lot of heat and that's what kills caps really quick.. I most times try to isolate the board from where its coming from..
     
  6. fabchef

    Thread Starter New Member

    Oct 13, 2014
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    Hi gang, you must forgive me, i'm not an electronics pro. If someone can tell me how i could check the caps (that are still on the board),with a basic, digital multimeter, that would be great. It seems that the biggest cap has a tiny, slight bulg to it, its not flat. Thanks
    Fabs
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Show us! That could be a big clue.
    Checking the caps on-board can be tough. First rule is, don't do ANYthing until it's been unplugged for a while. And then check any cap for voltage first. There could be a high voltage there for even weeks after it is turned off.
    If you find a voltage, dissipate it through a resistor or a lightbulb - not by shorting it.
    Finally, you can look for a short with a multimeter. I believe fancier ESR meters might be able to test in-circuit, but there's not a lot you can do without removing the cap from the board. Hoping to learn otherwise.
     
  8. fabchef

    Thread Starter New Member

    Oct 13, 2014
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    Hey, i managed to take it off the board, it's been unplugged for about 6 days now. The problem is that my mulitmeter doesn't have the "capacitance" setting on it. Is there i can test it another way with the meter? it's a 230V 1000uf(m) 105 degrees cap.
    thanks
    fab
     
  9. Lestraveled

    Well-Known Member

    May 19, 2014
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    To test a capacitor with multimeter is kind of a guessing game.

    - Set your multimeter to "ohms" on the highest scale.
    - Short the cap leads together for a few seconds.
    - Connect the plus to plus, negative to negative.
    - Observe the reading.

    If the ohms reading stays at a low value, the cap is bad.
    If the reading instantly goes to over-range, the cap is bad.
    If the reading slowly climbs and after many seconds reaches over-range, the cap is probably good.
    If the cap never reaches over-range, the cap is probably bad.

    The speed that the reading increases is proportional to the uF rating of the cap.

    If the cap shows physical deformation (bulging) it is bad or will be bad soon. Replace it.
     
  10. fabchef

    Thread Starter New Member

    Oct 13, 2014
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    Hi guys, well i changed that big cap but still the same. I was thinking that somehow the noise filter (where power plug goes into), might have caught a beating...anyway to find out if its that? thanks
    fab
     
  11. fabchef

    Thread Starter New Member

    Oct 13, 2014
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    The odd thing is that, the power outlet that my power bar is hooked up to, has another power bad that operates my stereo. When i turned on the tv and it started to buzz, i also noticed that my CD player from the stereo was also making a slight hum, when it was turned off. It's odd that BOTH pieces started to make noise at the same time. I also tried different outlets in the house and the tv buzzes.
    fab
     
  12. wayneh

    Expert

    Sep 9, 2010
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    It appears your TV is placing interference onto your power lines. That may be a clue that the source of the problem is very early in the power supply.

    No schematic or service manual yet?
     
  13. fabchef

    Thread Starter New Member

    Oct 13, 2014
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    No schematics yet... Will try to find something. Can the plug (filter) that i mention have something to do with it? I don't know if these filters ever go bad? The readon being that the buzz is heard through out all the boards inside the tv.
    Fab
     
  14. fabchef

    Thread Starter New Member

    Oct 13, 2014
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    managed to find some schematics..now...if only i can read them! hahaha
     
  15. wayneh

    Expert

    Sep 9, 2010
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    Find the power supply section and post it. Folks here might make sense of it. Post the whole thing if it's not to big.
     
  16. fabchef

    Thread Starter New Member

    Oct 13, 2014
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    having a hard time trying to upload it...sorry
     
  17. fabchef

    Thread Starter New Member

    Oct 13, 2014
    11
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    here is a pic of the board itself...
    [​IMG]
     
  18. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    To make audible noise there must be some kind of physical transducing going on. Locating the physical source of the noise should point you toward the electronic source of the noise. Possible sources:

    the speakers - muting them removes the program audio, but they still are connected to the output amplifiers. If the noise is coming from them, then the sources are the power supply rails, radiated noise pickup by the input circuits, etc.

    the coils - the wires in inductors and transformers move. They are wound tightly, taped, etc., but still the wires physically rattle around a very small amount every time the current changes. As things age, they can rattle enough to be heard, either because they are more loose or because the waveform through them is larger (capacitor aging mentioned above). A short length of garden hose stuck in your ear can act as a super-directional microphone.

    ak
     
  19. fabchef

    Thread Starter New Member

    Oct 13, 2014
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    AK.thanks for the reply…i know that on the original board, if i pushed a tad on the transformer with the butt of a screwdriver it slightly quieted the sound. The tech that came over said it was the power supply board, can it be the second used board also suffered the same fate? The thing is that this buzz is heard through all the boards not just the power supply. If the PS is buzzing can it also make the other boards buzz? I spoke to another TV repair guy who said that chances are, the board will always buzz does to age and not much can be done. After 4 years and very little use, i find this a hard pill to swallow. Am i wrong???thanks
    fab
     
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