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Capacitor Polarity

Discussion in 'Electronic Repair' started by Paul, Jan 8, 2007.

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  1. Paul

    Paul Guest

    I was having trouble with CDs skipping on the factory radio in my 94
    Trans Am. I pulled the radio out of the car and took it apart,
    intending to clean the lens and adjust the tracking (if needed). While
    it was apart I decided to check ESR on the electrolytic capacitors. On
    the (Sony) CD main control board there are half a dozen 4.7uF @ 6.3v
    cap's. The ESR on all of those measured quite high, so I want to
    replace them. The problem is that I can not tell which lead is + and
    which is -. There is a polarity stripe on one end, but both leads come
    out on that end. These look like a black plastic surface mount
    tantalum chip except that instead of having a wraparound terminal on
    each end, there are 2 wire leads coming out of one end. They are
    actually made up of a radial lead aluminum can inserted into a hole
    drilled into the cube shaped plastic body. I pulled the can out of the
    body on one of them, but there are no markings on the can to indicate
    polarity. How can I determine the polarity of the capacitors? The
    level of disassembly needed to get at these parts does not lend itself
    to powering the unit up and measuring voltage accross the cap's.

    Also,
    Are there any adjustments on this particular CD transport? I do not
    see any pots.

    TIA,
    Paul
     
  2. Morse

    Morse Guest

    At least some if not all of those caps should have the negative leads at
    ground potential, so use your multimeter on Ohms or continuity setting and
    find out which lead is negative that way, no need to power the unit up. Or
    simply visually trace the cap lead to the ground plane on the PCB.

    Morse
     
  3. Paul

    Paul Guest

    ....How can I determine the polarity of the capacitors? The
    Thanks,
    But none of the 6 caps of this type seem to be grounded on either side.
    I guess I'll try to find non-polar caps small enough to fit (3mm x
    5mm), unless I get other suggestions.

    Paul
     
  4. Arfa Daily

    Arfa Daily Guest

    What do you consider to be a "high" value for ESR on these caps ? 4u7 at
    6.3v will likely have a correct ESR value of 15 ohms or more. You should
    also be careful when replacing small electrolytics like this, that you only
    replace with exact like for like. On many Sony HiFi CD players from a few
    years ago, the CD servo board had several small caps of very similar
    description to the ones you have there. One in particular used to leak. If
    you tried to replace these caps with anything other than the original low
    voltage surface mount types, the servos would never work again.

    Arfa
     
  5. Should not matter. Measure each lead, using ground as a reference. The
    more positive lead is the positive side. Or, the more negative lead is
    negative side. If one is pos. and one neg., then pos. is pos.

    Hope that helps.
     
  6. Paul

    Paul Guest

    The ESR measures 50 ohms on my meter.
    Typically fresh 4.7uF caps measure about 2 ohms on my meter.

    Thanks for the warning that generic replacements might cause trouble.
    So far, no luck finding original type replacements from Mouser, etc.
    Don't suppose you can point me to a source???

    Paul
     
  7. Arfa Daily

    Arfa Daily Guest

    Arfa
     
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