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Variable capacitor

Discussion in 'General Electronics Discussion' started by ecollen, Dec 2, 2015.

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  1. ecollen

    ecollen

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    Jul 31, 2015
    Help for a newbie, please. I’m planning to build a regen receiver (schematic attached), but I can’t seem to get hold of the required variable capacitor. Is there any way to adapt the circuit so that I can get away without the variable capacitor, or any other way around the problem?

    Also it seems strange that variable capacitors are so difficult to find. Surely they're still used in many small transistor radios nowadays.

    All assistance will be much appreciated.

    Errol
     

    Attached Files:

  2. dorke

    dorke

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    Jun 20, 2015
    If you need to tune your receiver to different frequencies you need the variable capacitor.
    Instead of the mechanical air type you can use a varicap.

    The variable air cap. your circuits uses 365pf has huge physical size.
    This kind is quit obsolete now days.
     
  3. shrtrnd

    shrtrnd

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    Yeah, I don't think anybody makes the old air/fin variable caps anymore.
    They were commonly used in old radios for tuning.
    Any old electronic scrapyards near you? Some place that would have a scrap heap of old discarded electronics?
    Amateur radio clubs in the area might be able to find you one.
     
  4. ecollen

    ecollen

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    Jul 31, 2015
    Hi dorke

    Thanks for your reply. Yes, I need the variable capacitor for tuning different frequencies, and I can't see how to make the radio without one. It's strange that they've become obsolete. I wonder what the manufacturers use in small transistor radios nowadays.
    Would one be able to use a varicap instead?
     
  5. ecollen

    ecollen

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    Jul 31, 2015
    Hi shrtrnd
    Thanks for your reply. I don't think I'll have any luck in that direction. I found a site online though where there is a description of how to make an air variable capacitor. I'm thinking of having a go at that as a last resort. It seems strange though that there's no way around the problem. I must have seen thousands of schematics that require a variable capacitor, and now they're all redundant. Strange.
     
  6. Minder

    Minder

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    The other way is to make the inductor variable, move the core through the coil for tuning and use a fixed cap.
    Older car radios used this method, btw.
    M.
     
    davenn likes this.
  7. dorke

    dorke

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    Jun 20, 2015
    It isn't strange at all they are obsolete:
    They are very big,expansive,mechanical, and... No body needs them(almost)...;).
    In small analog transistor radios they are still used,but the capacitance is much smaller about 30pF.
    There are many ways around:
    1.Use a varicap.
    2.Change the design to use a smaller variable air-cap of 30pF.

    BTW,
    What is the frequency range your aiming at?
     
    Last edited: Dec 2, 2015
  8. ecollen

    ecollen

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    Jul 31, 2015
    Hi Minder
    Many thanks for your reply and the good advice. That makes a lot of sense. It's a very good option. I think I should try that.
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    FYI, the small transistor radios of the 60's to...?? didn't use air dielectric variable caps. The tuning caps employed in those radios were about 1/4 the size of the typical air gap type of equal capacitance. They used a solid paper thin, flexible dielectric between the plates. This allowed for the stator and rotor plates to be spaced much closer than the air gap designs, without fear of the plates shorting. The stator and rotor plates were also very thin and very flexible.

    Chris
     
    duke37 likes this.
  10. Y2KEDDIE

    Y2KEDDIE

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    I feel your pain. I have a few. If you lived closer I'd trade for different parts you might have. People have called me a hoarder ( and even worse) but after45+ years of collecting junk electronics, I have a pretty good size junk box of materials.

    Difficult to find, but not impossible. Just Google air variables and you will find what you need. Expensive, but it's out there.

    When I was in 7th grade I built a crystal diode pocket radio as my science project. (1971). I used a ferrite antennae rod from an old transistor radio in parallel with a Mica padder / trimmer capacitor off a larger variable. The padder capacitor had a smaller range but was enough that I could tune a couple stations in the broadcast band.

    Seek out and find others with your interests. Trade and barter for parts.

    Good Luck!
     
  11. Martaine2005

    Martaine2005

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    There are plenty on Ebay now!!
    Large and pricey, but are out there!.

    Martin
     
  12. ecollen

    ecollen

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    Jul 31, 2015
    Hi Dorke
    As for the frequency, the instructions say:

    A 220uHy will tune the lower part of the AM band from about 570kHz to 1.15Mhz, a 5.6uHy will tune from about 3.5MHz to 7.5MHz and a 2.2uH will tune from about 5.6MHz to near 11.6MHz.

    I well remember playing with those big old capacitors in years gone by ... they were beautiful!

    Could you perhaps give more details about 1 and 2? How would I use a varicap? It's not variable like a variable capacitor, is it? Even finding smaller capacitors is a problem out here (South Africa), but I would have a better chance with that. So how would one change the design to use a 30 pF instead? If I knew that I'd be smiling.
     
  13. ecollen

    ecollen

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    Jul 31, 2015
    Hi Chris
    Thanks. That's interesting. So would they be the 30 pF ones Dorke mentioned above.?
     
  14. Bluejets

    Bluejets

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  15. Martaine2005

    Martaine2005

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    Bluejets?
    It's this here.:D

    Martin
     
  16. ecollen

    ecollen

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    Jul 31, 2015
    Hi Martin
    Thanks for your reply. The last time I looked I found only one on the whole of Ebay - but I see there are more now. But when I enquired from the guy (in Canada) about the postage he didn't even reply. Unfortunately I have another problem though. Our postal system here in South Africa is collapsing. I have ordered numerous items from overseas recently (in the past it worked fine), but have received nothing at all for months. Perhaps I will try ordering one of those from say Hong Kong again, but I won't hold my breath. It gets a bit expensive though, ordering these things and paying for them and then they never get here, but we live in hope! Surely something will sneak through!
     
  17. Colin Mitchell

    Colin Mitchell

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    "So would they be the 30 pF ones Dorke mentioned above.?"

    No.They are about 120p or higher. The mylar dielectric gave them the high capacitance.
    On talkingelectronics.com under Crystal Set I show how to make a variable cap with cooking foil
     
    ecollen and Martaine2005 like this.
  18. ecollen

    ecollen

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    Jul 31, 2015
  19. ecollen

    ecollen

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    Jul 31, 2015
    Hi Martin
    Thanks for that. But again did you see what they say? "Doesn't post to South Africa." Irritating, isn't it? Maybe one day I'll get them to post it to you and then you can post it to me! That'll fox 'em! However, with the cost involved I'll have to save up quite a bit of our worthless currency first.
     
  20. Martaine2005

    Martaine2005

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    You could always be-friend a good S African man I know called Martin J Lorton.
    A very trusted guy in the electronics world.
    You can contact him via youtube 'mjlorton' and ask if he can order for you and re-direct within S Africa?
    You have nothing to lose!!

    Martin
     
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