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Substituting Variable Capacitor with Varicap Diodes

Discussion in 'Electronic Basics' started by Richard, Jan 10, 2006.

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  1. Richard

    Richard Guest

    If I were to try to replace the 2-gang variable capacitor in the a.m. bcb
    radio shown on the webpage below, roughly what varicap would I need to get,
    and how would I connect them? I'm not really so bothered about trying to
    employ a 2 gang potentiometer. TIA.


    http://www.electronics-tutorials.com/receivers/receiver-basics.htm
     
  2. Richard

    Richard Guest


    Would I remove V.C. and do this?:

    Coil---------C 0.01uF-------------Resistor 100K------ +ve
    |
    Varicap 300 odd pF.
    |
    Coil-------------------------------------------------------------- -ve
    |
    \\\\\\\\\\\\ Chassis


    For AE coil and Osc coil.
     
  3. Rich Grise

    Rich Grise Guest

    Probably, pretty much, yeah. It'll still be parallel-resonant, and the .01
    will have neglible reactance - if I was feeling tight-assed, I might
    put a ferrite bead at the left lead of the 100K, but where in the
    world do you get a 300 pF varactor? (That's the word that one of my tech
    school teachers said was the "proper" term, instead of "varicap". Sorry. ;-) )

    Thanks,
    Rich
     
  4. Richard

    Richard Guest

    ----- Original Message -----
    From: "Rich Grise" <>
    Newsgroups: sci.electronics.basics
    Sent: Wednesday, January 11, 2006 12:56 AM
    Subject: Re: Substituting Variable Capacitor with Varicap Diodes
    Hmm, I thought that some (modern) BC band RX's were/could be tuned
    by varacters. Maybe I got things wrong? :c)
     
  5. Richard

    Richard Guest


    I'd have to use special varacters like Phiips BB1122, BB212 and similar.
    Made especially for AM radios.
     
  6. Richard

    Richard Guest

    I'd have to use Philips BB112, BB212 varactors. Special AM radio varactors.
     
  7. Digikey carries a 360 pf 20 V KV1520 and a 467 pf 16 V KV1560 by Toko.
    Well, they don't actually stock them, but they do list them. Less
    than $2 each, if you are willing to wait while they order some for you.
     
  8. Richard

    Richard Guest


    Thanks for the info. I actually paid a small fortune tonight on Ebay for a
    pair of (fairly rare) Philips BB212. Three pin devices with two varactors in
    them. I'll take apart, then rebuild on my own PCB, a generic 6 transistor
    "pocket radio" that I have. Luckily it had a tiny circuit stuck on the back
    plate. It will be interesting making things work. I don't think tracking
    will be a problem with a dual potentiometer. Tracking I think is a matter of
    padding capitors. Not sure though if each gang of these 2 gang variable
    capacitors you see in these small pocket radios are identical or not or
    specially devised to achieve tracking without padding etc. Mmm, I wonder if
    TOKO publishes data sheets on these things. I'll take a look.
     
  9. Don Bowey

    Don Bowey Guest

    The variable caps in the gang are most certainly different values, in order
    to track over the different frequency ranges of the local oscillator and the
    radio frequency; the local oscillator is the lesser of the two.

    Don
     
  10. Richard

    Richard Guest

    Noted. No data about either on those miniature variable capacitors that were
    (invariably) Japanese. I've searched using TOKO, TSURU, MITSUI (?), SANESU
    (from junkbox) - but no dice.

    My circuit does not give the tuning capacitor values, but as you say I think
    if it did, each gang would have a different value. No doubt someone's
    shematic of a "pocket radio" or similar would confirm. Trying to acieve
    tracking is going to be ineresting. I cannot find a schematic though on
    the WWW of an AM radio tuned by varactors.


    As to varactors, TOKO Europe has some data:

    http://www.toko.co.jp/products/index_product_e.htm

    One looks at: "Variable capacitance diodes for AM tuning"
     
  11. Richard

    Richard Guest

    Noted. No data about either on those miniature variable capacitors that were
    (invariably) Japanese. I've searched using TOKO, TSURU, MITSUI (?), SANESU
    (from junkbox) - but no dice.

    My circuit does not give the tuning capacitor values, but as you say I think
    if it did, each gang would have a different value. No doubt someone's
    shematic of a "pocket radio" or similar would confirm. Trying to acieve
    tracking is going to be ineresting. I cannot find a schematic though on
    the WWW of an AM radio tuned by varactors.


    As to varactors, TOKO Europe has some data:

    http://www.toko.co.jp/products/index_product_e.htm

    One looks at: "Variable capacitance diodes for AM tuning"
     
  12. Richard

    Richard Guest

    http://www.6v6.co.uk/candypress/scripts/prodView.asp?idproduct=1217

    Yep, here the values are:

    Antenna caacitance: 3.0 - 141.6pF

    Oscillator capacitance: 4.0 - 59.2pF

    Tolerance ± 1.0pF +1.5% (at full rotation)

    http://www.oselectronics.com/ose_p98.htm


    Miniature 2 Gang Poly-film Variable Tuning Capacitor For Broadcast Band

    Maximum capacity: Antenna section. 15 - 140PF

    Oscillator section, 10 - 60PF.

    Trimmer capacity: variable to over 12PF
    ----------------------------------------------------------------------------
    ----

    Miniature 1 Gang Poly-film Variable Tuning Capacitor For Broadcast Band

    Maximum capacity: Single section tunes from 10 - 280pf
    ---------------------------------------------------------------------------
    -----

    Miniature Poly-film Variable Tuning Capacitor For AM & FM Bands

    Antenna section AM 21 - 152PF

    Oscillator Section AM, 10 - 74PF
     
  13. Jasen Betts

    Jasen Betts Guest

    Electronics australia did one in the earlt 80s it used a zn414 IIRC
    and a simple tuned circuit.

    mk484 seems to be a similar device in current manufacture
    you might try getting the data sheet.

    Bye.
    Jasen
     
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