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Crystal Radio Tuning Capacitor

Discussion in 'Electronic Basics' started by Dave.H, Dec 12, 2007.

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  1. Dave.H

    Dave.H Guest

    I'm planning to build a crystal radio similar to the one linked to at
    the bottom of this post, but the specs call for a 30-160 pf tuning
    capacitor, all I can find is a 60-160 pf cap. Will this still work?
  2. Dave.H

    Dave.H Guest

    Sorry forgot the link
  3. Dave.H

    Dave.H Guest

    I have a variable capacitor in a junker 5 valve radio from the '60's.
    This is marked 2 1215 What value would this be, how would I be able to
    measure it, there's 4 terminals two have got green wires, one with
    two, haven't followed them to see where they go, another terminal has
    a ceramic cap to chassis ground, the other isn't used.
  4. Mike

    Mike Guest

    The 60-120pf will work. It will just tune a narrower range of frequencies.
    Most of the old tuning caps were 2 section units. One used for the oscillator
    and one for input tuning. The input tuning section was larger and should be in the
    20-365pf range. I don't remember the typical values for the oscillator section,
    but it was lower. Some had three sections and used 2 sections for input tuning.
    The easiest way to measure it is to borrow a capacitance meter. Otherwise
    a you'll need a calibrated signal generator (or generator and frequency counter)
    and an oscilloscope to measure the tuning range, or maybe a grid dip meter.
    If you can find an old transistor radio just take both the ferrite antenna and
    cap from it and use them together. Don't expect too much from it and be sure you
    have a good ground. Make sure the earphone is right type. A low impedance type
    will not work well if at all.

    When truth is absent politics will fill the gap.
  5. Dave.H

    Dave.H Guest

    I don't want it to tune a narrower range of frequencies, maybe I
    should try the old capacitor I have
  6. Phil Allison

    Phil Allison Guest


    ** The AM band goes from 550 kHz to 1650 kHz = 3:1 range.

    So, the RF tuning capacitor needs at least a 9:1 range - since frequency
    is inversely proportional to the square root of the capacitance value.

    Typical numbers are 15 to 200 pF for miniature types and 30 to 450 pF for
    larger ones.

    Coil design is important so there is not too much C included.

    ....... Phil
  7. Dystopia

    Dystopia Guest

    In practice I think you will find that only 198KHz, Radio 4 signal
    will come through.
  8. ehsjr

    ehsjr Guest

    Yes, with a narrower tuning range. But you can add
    switched fixed value capacitors to extend the range,
    if you want. This is a kludgy arrangement, but will
    get you the full tuning range:

    S1 S2
    --------+-----o o---+---o o---+
    | \ | \ |
    [C1] [C2] [C3]
    | | |

    C1 60-160 pf variable
    C2 56 pf fixed
    C3 30 pf fixed

    Range(pf) S1 S2
    29-41 Closed Open
    36-68 Closed Closed
    60-160 Open Open

    The site you posted sells a variable cap for use in
    their radios - that may be a better choice than
    using the switch capacitor kludge.

  9. Dave.H

    Dave.H Guest

    Thanks for all your help, I've found a better crystal radio diagram on
    the internet, where I get to wind my own coil, something I wouldn't
    mind trying, and I've found a source for the parts.
  10. Dave.H

    Dave.H Guest

    How many turns of wire would I need to get a value of 230 mH using a a
    carboard tube from a toilet or paper towel roll? The site I've linked
    to in this message has some calculations, but I never was good at
  11. Dave.H

    Dave.H Guest

    Sorry that should be 230 uH.
  12. Mike

    Mike Guest

    Somewhere around 100T of #24.

    or google for an inductor calculator if you don't like that one.

    Your best bet would be to wind maybe 130T turns and addd taps
    every 10T starting at about 70T, or unwind turns until you get
    the right tuning range.

    If you add more taps, who knows, you might pickup some shortwave
    depending on antenna, ground, and receiving conditions in your area.

    When truth is absent politics will fill the gap.
  13. Guest

    You might get some ideas from my simple coil winding Webpage. My coil
    can use a fine tuner brass screw.

    Also gutting a cheap $5 battery AM Radios found in discount stores
    (BigLots)will give you a variable capacitor and ferrite rod coil.

    Merry Christmas,

    * * *

    Temecula CA.USA
  14. When I went to school, the values of capacitors in parallel added, so
    the ranges would be (bottom up, on your table) 60 - 160pF, 116 -
    216pF, and 146 - 246pF.
  15. ehsjr

    ehsjr Guest

    Yup, me too. I drew it wrong. The intent was to have
    the variable cap switched in series with the C2 & C3
    combination, where C2 & C3 are switched in parallel.
    Like this:

    S1 S2
    +-----o o---+---o o---+
    | \ | \ |
    [C1] o [C2] [C3]
    | | | |
    A-------+ +----+----------+----B

    The effective capacitance is across A and B
    The switches are shown in the open position.

  16. Rich Grise

    Rich Grise Guest

    When I was a Boy Scout, we would make variable inductors. Take a TP roll,
    and wind some manageable magnet wire around it, maybe #32-#36; close-
    wound. Leave about 1/4"-1/2" of naked roll at the ends. Secure the
    wire, do whatever you need to do to the leads, and fasten it down
    to your board. I mean board, as in wood. Then, mount a post a few inches
    away from it, that's as tall as the diameter of the coil. Get a strip
    of tin, like from a coffee can lid or so, put a hole in one end, and
    bend the other end into a "v" shape, lengthwise, as if it were a little
    scoop. Make the 'v' long enough to accommodate the next step. With some
    fine sandpaper, clean the enamel off the wire at the top of the coil.
    Mount the tin strip with a screw through its hole into the top of the
    post, and the 'v' on top of the coil, making contact.

    Voila! Variable inductor!

    If you've got the patience, you could make your own capacitor with
    aluminum foil and Saran wrap. ;-)

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