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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.

    -Bill
     
  2. amdx

    amdx Guest

    Search the archives of the crystal radio section here,
    http://theradioboard.com/rb/
    These guys have cleaned a lot of air caps.
    Mikek
     
  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.

    Anecdote:

    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.

    http://antiqueradios.com/forums/viewtopic.php?t=73596
    and
    http://theradioboard.com/rb/viewtopic.php?f=2&t=66&hilit=cleaning+air+variables


    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.
    Mikek

    Mikek
     
  5. Bill Bowden

    Bill Bowden Guest

    Nice pictures of ultrasonic cleaning results. Look like new.

    -Bill
     
  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?

    -Bill
     
  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..

    Jamie
     
  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.

    YOU FUCKWIT, RABID NUT CASE !!!!!!!!!!!



    ** 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.

    BTW:

    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
    plates.


    ** 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
    week.

    -Bill
     
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