How to attach a knob to a screw variable capacitor?

Discussion in 'General Electronics Chat' started by agargara, Jul 2, 2015.

  1. agargara

    Thread Starter New Member

    Jul 2, 2015
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    I built a video transmitter, and it works great! However, I'd like to attach a knob to the variable capacitor which tunes the frequency of the transmitter.

    The capacitor looks like this:
    http://i.imgur.com/ufHRjgP.jpg
    (Jameco part 32855)

    I'm not sure how to attach a knob to a screw. I thought about soldering a shaft of metal to the screw and attaching a knob to that. I know solder isn't meant to be used as glue, but I'm happy with any solution as long as it works. The screw doesn't take much torque to turn (you can turn it with your fingernails) so I don't think the solder joint would snap off easily. Any better ideas?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    That is a screwdriver adjustable trimmer capacitor. You need a variable with a shaft, like these.
     
  3. agargara

    Thread Starter New Member

    Jul 2, 2015
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    Thanks, I know that using a cap with a shaft is the most proper solution, but I don't want to rebuild my circuit if I can find a workaround.
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You could just epoxy a plastic shaft to it, but keep in mind if you bump it you could easily break the cap.
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    I apologize for the mistake to locked the thread.
    There is nothing wrong for the contents of this thread, when I moved the thread from project forum to the General Electronics Chat, after that time my computer stuck, so probably happened something I didn't know, I apologize for your inconvenience.

    Thanks HP(Hypatia's Protege) to reported this.
    Now is reopen.
     
  6. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Please be advised that trimmers are not designed for frequent adjustment -- even should you succeed in attachment of a 'knob', the Cap. won't last long under frequent 'tuning'...

    Best regards
    HP
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    For a temporary solution, JBWeld (epoxy for metal) should work. Mix well, then mix that much more. As soon as you feel it get thick, apply only the amount you need.

    The bigger diameter disk (knob) you attach, the better your fine tuning will be. Again, not intended for years of use but 90% of radios play the same station day in and day out so I think you will be fine in most cases (unless you are one of those 10% who can't keep their fingers off the radio tuner and never ride in my car a second time.
     
    Last edited: Jul 3, 2015
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  8. Ancel UnfetteredOne

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    Jul 3, 2015
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    That looks like a metal screw, which prob. means you can solder a metal (brass/copper/zinc/galvanized iron) adjustment (gizmo) onto it.
     
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The metal is pretty big so it acts like a heat sink when soldering. The whole thing will get pretty hot by the time solder melts. Therefore you risk damaging the capacitor. Also, once solder starts flowing, you can't really determine where it will end up - you also risk soldering the rotating part of the cap to the chassis of the part and suddenly the variable part of the variable capacitor is gone.
     
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  10. Ancel UnfetteredOne

    Member

    Jul 3, 2015
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    Well, it requires the use of the reflow soldering approach. You tin the screw and the adjustment fixture then mate them and apply the iron and a little solder for a good result. I have done this often.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    You could try soldering a bit of brass strip into the slot - but the heat would probably destroy the silver plate vane.

    The oscillator section of a transistor radio polyvaricon is usually about 60pF - you can attach a small spacer to the stub spindle with a centre screw in the normal way, then a regular knob will fit the spacer outside diameter.

    MW radio tuning capacitors are getting rare as rocking horse manure - the SW ones that might be nearer what you need, are even more unobtaneum. Try your local Ham radio suppliers - if you get really stuck, Spectrum Communications in the UK carry this sort of stock.

    They also had a load of clones manufactured of selected Toko 10mm coils. Mostly as appropriate for Ham bands, but some still match common IFT frequencies.
     
  12. GopherT

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    Nov 23, 2012
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    Agreed but I am feeling mixed signals about the skill level of the thread starter. On one hand, he says he built a video transmitter while on the other hand, he is asking how to attach a knob to a Varicap. I wanted to give an easy solution that didn't take too much skill - nor too much explanation or rabbit holes he could go down. I fully Agee that a solder option can work if you guide home down the path.
     
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do you have the schematic of the video transmitter?
    Perhaps you can change the capacitor for a varicap diode.

    Bertus
     
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  14. ian field

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    Why go to the expense of a varicap - pretty much all diodes vary in capacitance as reverse bias widens the depletion region, I've even seen people use LEDs.

    A MW loop antenna published in a magazine used a pair of 16A Shottky-barrier rectifiers in TO220 style package.

    In a Hi-Fi separates tuner; you'll usually find an inverse series pair of varicaps strung across the tank coil, this is because the inductor has an alternating voltage that would alternately add or subtract from the tuning voltage on a single diode - with the inverse series pair it alternately adds to one and subtracts from the other, so they cancel the error.

    Having said that - millions of TV tuners everywhere use single ended varicaps, and don't seem to be having any great difficulty.
     
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  15. Hypatia's Protege

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    Mar 1, 2015
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    Should you indeed opt for the 'knob route', please bewared that a metallic 'knob' will make for rather annoying tuning -- unless the 'rotor' is at common...

    Best regards
    HP
     
  16. ian field

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    Oct 27, 2012
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    A Band II FM polyvaricon may have a low enough capacitance oscillator section - if its still too high, just calculate a series capacitance to reduce it.

    The moving vanes are ground, a short spacer would need to be fixed to the stub shaft to put a knob onto - you can get plastic insulating spacers.
     
  17. agargara

    Thread Starter New Member

    Jul 2, 2015
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    Thank you everybody for all your kind help and advice!

    Thank you, that sounds like a pretty easy and safe solution!

    You're correct that my skill level is pretty low. I just took one basic electronics class, so there's much I still don't understand. I have soldered with a makeshift heat sink (an alligator clip) before to avoid damaging a transistor. What does it mean to "guide home down the path"?

    I'm not sure what a varicap diode is, but here's the guide I followed. It's not so much of a schematic as a tutorial with pictures.
    http://crackedraytube.com/pdfs/building_a_diy_transmitter.pdf

    I have noticed the annoying fact that tuning the screw changes the tuning frequency while the metal screwdriver is touching the screw. What does a rotor being at common mean?
     
  18. ian field

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    The moving vanes are usually attached to the spindle, the convention was always that the spindle regular ball bearings that were also in contact with the earthed frame. I can't offhand think of a variable capacitor type that the spindle isn't ground.

    The varicap option is one or two diodes, if you apply a reverse bias the depletion region gets bigger - that's the same as the distance parameter between the plates of a capacitor, the higher the reverse bias - the lower the capacitance. I referred to this in an earlier post that you can refer back to - so I'm not going to type it all in again.
     
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