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How to clean my tuning capacitor?

Discussion in 'Electronic Basics' started by Bill Bowden, Jun 1, 2013.

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  1. Bill Bowden

    Bill Bowden Guest

    It's an old (1960s AM radio) air dielectric, planetary drive with ball
    bearing 6X reduction). It's a very small mechanical cap measuring
    about 9/8 inch square by 11/16 height with closely spaced plates
    (150pF and 100pF). Problem is, the oscillator section jumps about 5KHz
    out of 1Mhz or maybe 0.5% with small adjustments to the cap. It
    appears it may be losing a ground connection (or high resistance) to
    the moving plates, or possibly some dust between the plates. I assume
    the ground connection is made through the ball bearings. I tried
    spraying some contact cleaner on the ends of the shaft but it didn't
    help. What would you suggest to restore the capacitor to it's original
    condition? I thought about soaking it in alcohol, but maybe that's not
    a good idea.

  2. amdx

    amdx Guest

    Search the archives of the crystal radio section here,
    These guys have cleaned a lot of air caps.
  3. Phil Allison

    Phil Allison Guest

    "Bill Bowden"
    ** What did you use - Freon ?

    A few drops of WD40 on the bearings at each end of the moving plates usually
    fixes it. Do not spray, use a syringe or the tip of a small screwdriver to
    apply the liquid.


    A customer decided to fix his FM tuner ( Quad FM3) with the same problem.

    He sprayed WD40 all over the gang and nearby PCB - the oil layer left
    behind offset the local oscillator frequency by about 30 MHz, so right off
    the FM band completely.

    After a lot of rinsing with turps and drying with hot air, I got it back.

    .... Phil
  4. amdx

    amdx Guest

    Don't know if you tried my link, but here's more.

    If you use the dishwasher method, I would check the detergent on
    an aluminum pie plate to make sure it doesn't react badly.
    My wife changed detergent when she found the one I said got a best
    value rating in Consumer Reports was eating away her pie plates.
    These were the thin pie tins you get when you buy a crust.

  5. Bill Bowden

    Bill Bowden Guest

    Nice pictures of ultrasonic cleaning results. Look like new.

  6. Bill Bowden

    Bill Bowden Guest

    Yes, there is a spring blade contact at the opposite end of the shaft
    as the ball bearings. Didn't see any silver, the blade appears copper
    or brass. I soaked the cap in alcohol for a day and moved the plates
    many times. Then I dried it the sun and added a few drops of WD40 to
    both ends as Phil suggested. I removed the mica insulator and cleaned
    it separetely. Cap works much better now, but still a little eratic
    near the bottom of the band where the plates are near fully closed.
    Might need some more WD40, or maybe some thin sewing machine oil?

  7. Bill Bowden

    Bill Bowden Guest

  8. Jamie

    Jamie Guest

    You created a coating that increased the dialectic constant for the
    plate that tune's the antenna..

    THen again, could be the differences between G and E line effects..

  9. Phil Allison

    Phil Allison Guest

    "Michael A. Terrell = Trolling Psycho"

    ** The OP needed a contact cleaner FIRST.

    WD40 is that and then it leaves a thin, oily film the lubricates and
    protects against moisture.

    ** Much the same as WD40.

    ** ROTFLMAO !!

    Using any sort of grease is never going to help a bad contact.


    ** Not unless there is a LOT of heat involved and no moving parts.

    ** Which makes it a good surface cleaner AND totally contradicts everything
    you just said.


    FOAD you stinking TROLL !!!!!!!
  10. Phil Allison

    Phil Allison Guest

    "Bill Bowden"

    The cap got more erratic today, so I applied more WD40 and then lost
    all signals except one 50KW station about 7 miles away. Everthing
    appeared normal, oscillator and antenna circuit both on correct
    frequency. It appeared the Q of the antenna circuit degraded
    drastically due to WD40 which may have spilled on the capacitor

    ** More likely rust or dirt particles from the bearings made it between the
    plates - carried by the EXCESS of WD40 you applied.

    I have used WD40 successfully on intermittent tuning gangs in FM receivers
    many times, never any problem.

    BUT I used it sparingly and carefully !!!

    You may need to use an evaporative cleaner on the plates now - ie stuff
    that smells nice and disappears in seconds if you spray it on your hand.

    .... Phil
  11. Bill Bowden

    Bill Bowden Guest

    Yes, I think you are right. I disconnected the cap and connected an
    ohmmeter across the plates and read many points of low resistance. So,
    I soaked the cap in a cup of isopropy rubbing alcohol and rotated the
    plates every few minutes and dried it out in the sun and then applied
    a few drops of sewing machine oil and it seems ok now. I found
    someone who has a ultrasonic cleaner to loan and will try it out next

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