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Bad Caps

Discussion in 'Troubleshooting and Repair' started by gearmate, Jul 27, 2010.

  1. gearmate

    gearmate

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    Jul 27, 2010
    Please help, this cap tested .33uF with capacitance meter but 43 ohms with ESR meter.
    Does anyone knows if it is bad, because if feel the esr reading is too hig and if so which nearest match can i replace it with. i can't get the exact one.
     

    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    It's not an electrolytic capacitor, so the classic "bad cap" thing is not going on here.

    The ESR seems high, but check that your ESR meter can accurately measure the ESR of such a low valued capacitor. The risk is that the reactance of the capacitor at the frequency the tool is using is affecting the reading.

    I'm pretty sure I've gotten sensible readings out of my ESR meter for quite low capacitances, so I'm not totally disregarding the reading. Have you checked similar capacitors to see what ESR they give? (Preferably check a new component of the same composition, capacitance and voltage rating).
     
  3. gearmate

    gearmate

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    Jul 27, 2010
    Thanks Steve, I am actually using the latest Bob Parker design 'Blue ESR Meter'
    I have tested similar with higher value (not new) and got low ESR reading.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    You really need to compare it with the same type, value, and voltage rating.

    What is the minimum capacitance recommended for testing with this meter? My reading of the specs suggests 1uF.

    Whilst you can make comparative measurements with smaller capacitors, it's not actually measuring ESR (or rather the reading may not be totally dominated by the ESR). If you are using it for smaller capacitors, you should compare the reading with a known good unit.
     
  5. gearmate

    gearmate

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    Jul 27, 2010
    Yes it is 1uF minimum capacitance.
    Ok i will compare and let you know of the result
    Thanks
     
  6. davenn

    davenn Moderator

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    Sep 5, 2009

    BTW ... 333 is .033uf not .33uf
    334 = .33uF :)

    cheers
    Dave
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,178
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    Jan 21, 2010
    And that low capacitance would pretty much ensure the ESR meter saw it as a high resistance.

    ESR meters rely on the dv/dt across the capacitor under test being very low for each pulse duration. Due to that, the voltage is determined by the ESR and the programmed current. With a small capacitor the pulse significantly charges the capacitor resulting in a higher dv/dt that may swamp the actual ESR.

    In theory the ESR meter could correct for this, but it would need to be able to sample the voltage extremely quickly. In practice they suggest a lower limit for capacitance so that the ESR reported remains fairly accurate.

    0.033uF is rather too low. Although, having said that, you should see reproducible values from similar capacitors (the values are not ESR though).
     
  8. gearmate

    gearmate

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    Jul 27, 2010
    Thanks Davenn,
    So do I consider .33uF too high capacitance compared to .003uF and that the cap is faulty.
     
  9. davenn

    davenn Moderator

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    you misunderstand :) forget about the .33uF, it doesnt exist, the capacitor you showed us is a .033uF you had misread the value. see my explanation in the other posting.

    Now as Steve stated you are not likely to get a reliable ESR reading on that capacitor as its value is too low for an ESR meter to give a correct reading.

    the cap doesnt look physically damaged and chances are its probably ok
    What made you suspect this cap initially that made you remove it from the circuit board ?

    cheers
    Dave
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    you misunderstand :) forget about the .33uF, it doesnt exist, the capacitor you showed us is a .033uF you had misread the value. see my explanation in the other posting.

    edit...
    ummmm if your capacitance meter actually read .33uF, that would be a bit suprising
    I would recheck that make sure its "Zero'ed" correctly etc see if you have some other caps around that you can read the values of and see if they come close to the value written on those caps.... just as a test of the cap meter's accuracy :)

    Now as Steve stated you are not likely to get a reliable ESR reading on that capacitor as its value is too low for an ESR meter to give a correct reading.

    the cap doesnt look physically damaged and chances are its probably ok
    What made you suspect this cap initially that made you remove it from the circuit board ?

    cheers
    Dave
     
  11. davenn

    davenn Moderator

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    Sep 5, 2009
    ummmm if your capacitance meter actually read .33uF, that would be a bit suprising
    I would recheck that make sure its "Zero'ed" correctly etc see if you have some other caps around that you can read the values of and see if they come close to the value written on those caps.... just as a test of the cap meter's accuracy :) I would expect a capacitor like that to read approx. somewhere between .028 and .038, give ot take a little bit

    Now as Steve stated you are not likely to get a reliable ESR reading on that capacitor as its value is too low for an ESR meter to give a correct reading.

    the cap doesnt look significantly physically damaged and chances are its probably ok
    What made you suspect this cap initially that made you remove it from the circuit board ?

    cheers
    Dave
     
    Last edited: Jul 29, 2010
  12. gearmate

    gearmate

    11
    0
    Jul 27, 2010
    Hi Davenn
    Sorry, my mistake, I wrongly read it as .33uF when it should have been read as 33nF and your are right.
    I remove it while probing the Horizontal circuit of a SONY KVG21Q1 CRT TV which has the picture ok but lack of brightness for about 30 minutes then the picture would shift to the left half of the screen but not stable.
    Any Idea though
    Thank for willing to help
     
  13. davenn

    davenn Moderator

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    Sep 5, 2009
    no probs :)

    OK many TV sets have a sub-bright trimpot on the cct board maybe this one does ?
    see if it makes any difference. I am not familiar with that particular TV set. do you have any cct info for ... a diagram? gosh havent done much TV servicing in recent yrs well not the ol' crt type ones anyway, mainly work on LCD panel displays these days.
    try a google search on that model # and see if you can find any service data for it, ya never know you luck. how old is the TV ?
    If you do find a sub-bright adjustment ... have a look around that area and see if there are any obvious suspect components

    cheers
    Dave
     
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