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Capacitor Value

Discussion in 'General Electronics Discussion' started by Pecten, Nov 16, 2013.

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  1. Pecten

    Pecten

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    Nov 16, 2013
    I'm fixing my tektronic 7704A oscilloscope and i found a blown capacitor. I do not know the capacitor value. It has K100 +20V. I'm assuming that 100 is the code, K is the tolerance (K=10%) and 20 is the voltage. Can somebody tell me if it is a 100pf or 10pf capacitor and how to know if it is a tantulum, film or mica capacitor?
     

    Attached Files:

  2. TorontoBob

    TorontoBob

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    Oct 6, 2013
    I would think that is an old 100uF 20V tantalum capacitor at that size.

    I have never seen a 20volt film capacitor and they sure don't make micas at that voltage, nor would they have a polarity.

    But a tantalum would get hot and blow in the way that one did.
     
  3. TorontoBob

    TorontoBob

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    Oct 6, 2013
    Hey its late and I am old. 30 minutes after I think about something the memory comes back to me, which is really inconvient when you need to remember a name.

    Yes, this is what early epoxy dipped tantalum capacitors looked like.
     
  4. Pecten

    Pecten

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    Nov 16, 2013
    Thanks for your reply Bob, yes it has to be very old, this oscilloscope is from 1969. I'm been looking for a replacement, but like you said, no 20V tantulum, film or mica capacitors online. Do you have any suggestions for a replacement?
    Thanks again.
     
  5. TorontoBob

    TorontoBob

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    Oct 6, 2013
  6. Pecten

    Pecten

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    Nov 16, 2013
    $8.00 for a cap is a little high price, but is the best option I have. It does not belong to the power supply, is part of the PBC that connects the plug-in modules, Thanks Bob for your help, i'm appreciate a lot.
     
  7. Pecten

    Pecten

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    Nov 16, 2013
    my last question, does the polarity matters?, because the one you sent me does not have it.
     
  8. TorontoBob

    TorontoBob

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    Oct 6, 2013
    Yes, polarity is extremely important, or the capacitor will fry just like your old one. Tantalums are very sensitive to reverse polarity.

    If you are talking about the picture on the digikey site, under the picture it says, "mage shown is a representation only."

    Remember that the fried tantalum could be failure with age, or it could be just a symptom of another problem that caused the capacitor to fail. If the capacitor is just a symptom of another problem/failure, replacing the cap without solving the real cause of the problem will mean you will fry your replacement cap too.

    Here is a picture of a couple of 100uf 20V tantalum caps pulled from old PCB's.
    [​IMG]
     
  9. Pecten

    Pecten

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    Nov 16, 2013
    Got it. With out the oscilloscope is difficult to check the rest of the PCB board. Thanks again.
     
  10. TorontoBob

    TorontoBob

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    Oct 6, 2013
    You might want to check your private messages too.

    I have been presuming you have the service manual for the Tektronix 7704a.

    If you don't you can get it here
    http://elektrotanya.com/tektronix_7704a.pdf/download.html
    just wait for the link to show up with a timed delay at the end of the line under the picture of the manual, "This file is downloadable free of charge" and this is followed after the delay with "Get Manual"
     
  11. Pecten

    Pecten

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    Nov 16, 2013
    You are the man. I need to get the capacitors any way, but i will have time to check the rest of the board. Thanks a lot for your help.
     
  12. Pecten

    Pecten

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    Nov 16, 2013
    Hi Bob, the schematics helped a lot. You are right with the cap value. The capacitors are located on the A20 Main Interface (page 155) and they are C81 and C85 (or C2081 and C2085 on page 5-3) The value for this one is Capacitor , FiXED, Electrolytic 100uf 20%, 20V. Do I have to use tantalum, or i can get a electrolytic one? I ask because every tantalum 100uf 20V i found is 10% tolerance, but i found some 20% tolerance electrolytic one. Thanks again
     
  13. BobK

    BobK

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    1,686
    Jan 5, 2010
    You can always replace a 20% tolerance capactor with a 10% tolerance.

    Bob
     
  14. Pecten

    Pecten

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    Nov 16, 2013
    Sorry to insist bob, but do you know the difference installing a tantalum cap vs a electrolytic cap?
    Pecten
     
  15. davenn

    davenn Moderator

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    1,891
    Sep 5, 2009
    Tantalums are generally not so leaky as electrolytics
    for most general applications either will work well

    Dave
     
  16. Pecten

    Pecten

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    Nov 16, 2013
    Thanks for the information Dave, i will get tantalums.
     
  17. DLD

    DLD

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    Nov 5, 2013
    Definitely it’s tantalum capacitors. Tantalum capacitors are
    relatively expensive, Mostly in the mil-spec
    grades
     
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