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  • I was building a crystal set for my son, but it came to a halt. When i found outthat i have no variable capacitor in my pile of junk.Scavenging one from an old radio was not an option. Since most of new radiosuses analog tuning. And the ones with air variable capacitors are very rare, andare collectors item.I have read an article once about building an air variable capacitor. So i decidedto build my own from scraps of aluminum sheets and from things that are easilyfound around the house.If you have a drill, a scissors, a file and some sandpaper. You can easily buildthis one. It doesn't require much skills.:-)I made mine from aluminum sheets 1.5 and 2mm thick. The housing is from analuminum heatsink, i also manage to get 3 pieces of bolts with a few nuts frommy junk box. The plastic bushing which also acts as an insulator is from a plasticpen and some plastic jar caps.

    (http://cdn.instructables.com/FWD/IOE6/FYTCOV8U/FWDIOE6FYTCOV8U.LARGE.jpg)

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    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    Air variable capacitor from scrap aluminum sheets http://www.instructables.com/id/air-variable-capacitor-from-scrap-alum...

    1 de 22 09/03/2014 12:39 a.m.

  • I also made my own washers/spacers from the 2mm thick aluminum sheet.The contacts of the rotor which also acts as a tensioner. Were from a brokenwashingmachine timer.You can experiment with the number of plates and with the size op gaps byadding more plates and washer, in the rotor and stator.

    Step 1: Marking the center for pilot holes

    On this step we will do the measurements.Lay the aluminum sheet flat on your work bench. From on end draw a squaremeasuring 5cm x 5cm. Using a sharp object like a nail or file.Get the center and punch a hole in it using a small nail. Punch a hole for every 5centimeters along the straight line as shown on the photo.

    Step 2: Draw some circles

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    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    2 de 22 09/03/2014 12:39 a.m.

  • Here we will draw circles on the sheet. We can draw circles perfectly by using acompass.If you dont have a compass.You can improvise, by driving two small nails on asmall piece of wood.Insert one nail on the hole that you made on the sheet and turn it. It is importantthat the circles doesn't overlap. So that we will have enough space when we cutthem out later.You can make as many circles as you like, if you have enough aluminum sheet.It is better to have a spare you if you made a wrong cut. Than to repeat thesame process if we you ran out of pieces at the middle of your project.

    Step 3: Marking lines for the stator plates

    (http://cdn.instructables.com/FXH/TUDL/FYTCOWXA/FXHTUDLFYTCOWXA.LARGE.jpg)

    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    3 de 22 09/03/2014 12:39 a.m.

  • Using a tri-square, draw a line from the edge of the sheet up to the center of thearc. (The one that is pointed by a pen on the photo)It is important to do this, because we need each part to be uniform in size. Andthis also serves as a guide when we cut them out.

    Step 4: Cutting

    With a big scissors and a strong grip. You can easily cut out all of the pieces.And then flatten each piece with a rubber mallet.

    (http://cdn.instructables.com/FK8/88S9/FYTCOXC9/FK888S9FYTCOXC9.LARGE.jpg)

    (http://cdn.instructables.com/FAE/G22Z/FYTCOXS6/FAEG22ZFYTCOXS6.LARGE.jpg)

    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    4 de 22 09/03/2014 12:39 a.m.

  • You can also use a hammer. But be very gentle.

    Step 5: Marking the holes for the stator

    Get a piece and draw a line from the center. Going to the left corner.Get the centerline (from corner to arc) and punch a hole in it.Mark this piece as pattern.Put it on top of another piece. And puch a hole on the second piece under it.Using the hole on the first piece as a template. Flip the pattern and punch thesecond hole.We do this because we will drill all the rotor and stator plates one at a time. Idont recommend stacking all the pieces and drill them all at one time. The drillbit always tend to bend. Or you may if you have a drill press

    Step 6: Marking the inner circle

    (http://cdn.instructables.com/FMS/V77A/FYTCOXXT/FMSV77AFYTCOXXT.LARGE.jpg)

    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    5 de 22 09/03/2014 12:39 a.m.

  • Again improvise a compass to draw the inner circle.Using a scissors cut it from both end, up to the arc of the inner circle. Be verycarefull not to cut it all the way to the center. :-)when all of the pieces are cut. Flatten them again with a rubber mallet.

    Step 7: Cutting the arc

    (http://cdn.instructables.com/FTN/DYDM/FYTCOXXQ/FTNDYDMFYTCOXXQ.LARGE.jpg)

    (http://cdn.instructables.com/FRB/QPHN/FYTCOY0V/FRBQPHNFYTCOY0V.LARGE.jpg)

    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    6 de 22 09/03/2014 12:39 a.m.

  • Here we will cut the arc that still connects the rotor and the stator plates. Youcan always cut it with a scissors.But it's easier and faster if you use a curved chisel. (the one's that wood carversuse)I have one but i don't want to ruin it.:-)So i made one from a small pipe. Sharpen it at one end with a file. And removedhalf of it. So that it will be just like a curved chisel.Dont get confused when cutting. The rotor must have the part with a hole in it.(This is where the spare pieces come in handy:-)

    Step 8: Drilling

    (http://cdn.instructables.com/FRB/QPHN/FYTCOY0V/FRBQPHNFYTCOY0V.LARGE.jpg)

    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    7 de 22 09/03/2014 12:39 a.m.

  • Now that we have our rotor and stator. It is time to make the holes bigger. Withthe use of a drill. Find bolts (3pcs.) with nuts ( i used 12 pcs. Of nuts because iremoved the head of each bolt so i can open my varicap on both ends.) i used a4mm. drill bit. Find bolts that will fit the holes perfectly, specially for the rotor

    Step 9: Turning with a drill

    At this stage we will make our rotor plates perfect arcs. And to remove burrs andsharp edges. With the help of a drill and a coarse file and some sand paper.

    (http://cdn.instructables.com/F7S/SUPW/FYTCOY0P/F7SSUPWFYTCOY0P.LARGE.jpg)

    (http://cdn.instructables.com/F2G/QRCQ/FYTCOY0O/F2GQRCQFYTCOY0O.LARGE.jpg)

    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    8 de 22 09/03/2014 12:39 a.m.

  • Stack all of the rotor plates facing each other forming a circle. (we do this toavoid vibrations when we turn them with a drill) Insert the bolt and tighten thenut.Insert the end of the bolt on the chuck of the drill. Just like a normal drill bit. Turnon the drill and use the file to smoothen the sides of the rotor plates. Be verycarefull set the drill at low speed. Do not press the drill swith for a long time. (thenut may loosen) if this happens reverse the direction of the drill.Finish with a fine sand paper

    Step 10: Shaping the stator plates

    We will shape the stator plates at this part of my instructable.Like what we did with our rotor plates. We will also stack all of the stator plates.Insert the bolts on each hole, and tighten the nuts. If you have a bench vise somuch the better.If you can see the photo clearly. I colored the parts to be remove with a blackpen. Or just simply follow the arc and avoid the head of the bolt and the nut. Bepatient you can do this with a coarse file. Remember aluminum is not that hardto work with. :-)

    Step 11: From this to that

    (http://cdn.instructables.com/FHM/O7Q4/FYTCOY5D/FHMO7Q4FYTCOY5D.LARGE.jpg)

    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    9 de 22 09/03/2014 12:39 a.m.

  • At this stage, your pieces must look like the 3rd piece on this photo. Well done.At this point you can also remove the paint. If they have paint on. And checkagain for curves or if ever they are crooked.

    Step 12: Making your own washers

    Make your own washers/spacers. From a heavier guage or from the samesheet. But you will need 2 pcs. Of washers/spacers for each gap. I recommendusing the same guage as the stator and rotor plates.Lighter guage is much

    (http://cdn.instructables.com/FUN/OWJO/FYTCOY5C/FUNOWJOFYTCOY5C.LARGE.jpg)

    (http://cdn.instructables.com/F9I/KMWN/FYTCOY5A/F9IKMWNFYTCOY5A.LARGE.jpg)

    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    10 de 22 09/03/2014 12:39 a.m.

  • easier to cut.Cut a strip of aluminum about 1cm. wide. Drill holes in it using the same drill bityou used for the stator and rotor plates. Cut the strip into squares. Making surethat the hole you drilled is on the center. Insert the bolt with its head on top.Then cut the corners using the head of the bolt as a guide, as seen on thephoto.

    Step 13: Finishing your washers

    As what you did with the rotor plates. Hammer them gently to make them flat.Stack them up, insert the bolt, tighten the nut. And again with the use of a drillturn them and smothen with a file and finish with a fine sand paper.

    Step 14: Rotor and stator assembly

    (http://cdn.instructables.com/F5U/F1PP/FYTCOY59/F5UF1PPFYTCOY59.LARGE.jpg)

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    11 de 22 09/03/2014 12:39 a.m.

  • Remove the head of each bolt. Put one nut on one end of eah bolt. Turn the nutuntil 5cm. of the bolt protrudes from the nut. Get one of the bolts, insert on onestator plate followed by two washers. And again a stator plate and two washers.Repeat this step until you finish. Leave enough space for 2 bolts and twowashers and dont forget the shaft on where to put a knob

    Step 15: The housing

    I made the housing from an aluminum heatsink salvaged from a television

    (http://cdn.instructables.com/FWQ/A1TF/FYTCOY5B/FWQA1TFFYTCOY5B.LARGE.jpg)

    (http://cdn.instructables.com/FPR/KAL5/FYTCOY6X/FPRKAL5FYTCOY6X.LARGE.jpg)

    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    12 de 22 09/03/2014 12:39 a.m.

  • chassis. It is more or less 2mm. thick.I placed the rotor and stator on top of it. leaving a distance of 1mm. Between therotor and stator. Marked the 3 holes to be drilled using the rotor and stator holesas template. Then drilled all the holes using the same drill bit used for the rotorand stator plate. Then i just cut it in a triangular shape and rounded all of thecorners.

    Step 16: The insulator

    Look for something that will insulate the rotor's axel from the end plate/chassis.Like for example a rubber hose from a fuel line of an automobile. Since i don'town a car. I just used a plastic pen for a bushing. Note: look for the right penbefore you make the top hole bigger. Don not drill until you have your bushing:-)

    Step 17: Plastic jar caps

    (http://cdn.instructables.com/F0A/G32S/FYTCOY6U/F0AG32SFYTCOY6U.LARGE.jpg)

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    13 de 22 09/03/2014 12:39 a.m.

  • Cut two triangular shaped plastic from jar caps. Or any plastic of your choice.This plastic will insulate the rotor contact from the aluminum housing. I just gluedmy contact temporarily. I will later change it with a copper connector. The onesthat look like washers like the ones used for ground connection.Now you just put the two triangular plastic on both sides. Followed by the twotriangular shaped aluminum and you are done.Remember if you have two stator plates, you must have three rotor plates. 4/5,5/6 and so on.I hope that this instructable will be of much help. Thanks :-)

    Step 18: Finish

    (http://cdn.instructables.com/FI0/6CKI/FYTCOY6T/FI06CKIFYTCOY6T.LARGE.jpg)

    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    14 de 22 09/03/2014 12:39 a.m.

  • Make Comment

    (/member/scraptopower/)

    2 years ago Reply (CCEL6Q1GR9SXOAH)

    (/member/Hinesdarrel/)

    22 days ago Reply (CZ3DWDGHRKR1JOX)

    (/member/dditlya/)

    3 months ago Reply (CVR6V0LHOPQDV9U)

    Here is the finished variable capacitor. Installed on a crystal shortwave receiver.

    scraptopower (/member/scraptopower/)

    Whoa! This is incredible, true DIY!

    Hinesdarrel (/member/Hinesdarrel/)

    Hey it is really amazing to make a capacitor for just scrap aluminium. Its very gooduse of wasted aluminum. Appreciate you a lot. Thank you for sharing this idea.

    http://www.analogtechnologies.com/capacitorkits.html

    dditlya (/member/dditlya/)

    Or you can get a variety of variable capacitors from us - spraguegoodman.com

    Tachyon (/member/Tachyon/)

    This is a really great Instructable. I especially like how you did everything usingsimple tools and scrap.

    (http://cdn.instructables.com/FWB/4P7P/FYTCPCTT/FWB4P7PFYTCPCTT.LARGE.jpg)

    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    15 de 22 09/03/2014 12:39 a.m.

  • 7 months ago Reply (C96EK08HJGDL0FS)

    (/member/Tachyon/)

    (/member/Dr.Bill/)

    7 months ago Reply (C0B115MHJKC48K6)

    (/member/Jezan/)

    4 years ago Reply (CH9ZRJ1FYTCPTLU)

    (/member/Drakekay/)

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    (/member/Dr.Bill/)

    2 years ago Reply (CXSIUE7GRBTNMIU)

    (/member/raypsi/)

    1 year ago Reply (CBYSLY7H2BMNUYN)

    Every crystal radio buff outthere is going to love you forthis as those darn variablecaps are hard to source.Nice work!

    Dr.Bill (/member/Dr.Bill/) Tachyon

    http://www.midnightscience.com (http://www.midnightscience.com) SourceAir Variables

    Jezan (/member/Jezan/) (author)

    Here is the finished radio. All the wirings are done under the plastic base. I willpublish an ible of this crystal set soon.

    Drakekay (/member/Drakekay/) Jezan

    How would you measure this type of Capacitor? I'm guessing it would havea resistance range? Do you think the number of plates would add variabilityto those values? Perhaps adding in a few extra plates at the top end only.You could do this by using 1/4 and 1/8 of a circle for a few of the rotorplates instead of the 1/2 circle rotor plates. I'm not sure this will translateinto capacitance the way I'm image it. Varying the degree of the rotor plateto achieve a larger range?

    (http://cdn.instructables.com/FGU/1CXC/GLWQVDOU/FGU1CXCGLWQVDOU.LARGE.jpg)

    Dr.Bill (/member/Dr.Bill/) Drakekay

    You need a meter that measures Farads. An Air Variable like this forBroadcast band should go from 0-365 pF.Air Variables can be had from MidnightScience.com for about $12 +/- US.Adding or subtracting plates will increase or decrease the caps value.

    If you can make caps like you suggest then you should have a Ham RadioLicense and build your own Transmitter and Receiver to operate.

    raypsi (/member/raypsi/) Dr.Bill

    A little FYI goes a long way:the equation for a air plate capacitor is:

    C= {k*0.2248*A}/d

    this is in inches:C is the capacitance of 2 platesk is dielectric constant air and vacuum are 1A is the area of one plate in square inchesd is the distance between the plates in inches

    The maximum capacitance of a multi plate variable capacitor would be:

    Air variable capacitor from scrap aluminum sheets by Jezan (/member/Jezan/)

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    16 de 22 09/03/2014 12:39 a.m.

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    1 year ago Reply (CT1VOGWH2WEX6WL)

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    7 months ago Reply (CWCDBS9HJKC48HP)

    Ct= {n-1}*C

    Where:Ct is the total maximum capacitancen is the number of platesand C is the capacitance in pico farads of just 2 plates

    If you do like the Japanese do and put a dielectric between the plates it canmake the capacitor smaller no real advantage in a crystal set. but if youwere shipping tens of thousands of them it would save you sum money.

    Say you take a 3 inch diameter circle the area fro the caqpacitor would ber^2*PI= Aor about 7 square inches then you only will be using half of the circle somaking A=3.5 inches for the capacitor equation.

    say you make the distance .04 inches between the platesso:{1*3.5*.2248}/.04=79.4 pf for 2 plates

    so 365/79.4=4.59 so 5 plates total will get you close

    {5-1}*79.4=317.6pf6 plates would be{6-1}*79.4=397pf

    it is perfectly ok to have even about of stator and rotor platesor you can tweak the diameter with a spread sheet to get an odd number ofplates

    73de N8ZU

    Dr.Bill (/member/Dr.Bill/) raypsi

    I just stick ah meter on it.

    73'sde WH7WP

    zwheel (/member/zwheel/) Dr.Bill

    Or.. for the purpose of building a crystal set... why bother with the meter.Just hook it up and see what frequencies you get. That could be part of thefun!

    Dr.Bill (/member/Dr.Bill/) zwheel

    When I build an X-Tal set I tend to use what I have in the junk box.I have BCB air variables from the tube era that I use that are not near365pF but they usually have trimmer caps on their sides. If that do the trickI have been known to adjust the capacitance with fixed caps and microswitches. My last X-Tal set I had set up under the transmitting antenna, at aBoy Scout Jamboree, of a 100 watt HF transmitter. My long wire was lowerand parallel to the antenna on the transmitter. With the switched caps onmy air variable I was able to have complete separation of frequency's. HFradio did not bleed over to the BC X-Tal set. I had 2 meters on the thing todetect null spots for each tank circuit on the X-Tal set. I had a Push Pull coiland a spider web coil all working together with a matched pair of 1N34detector diodes. No "Taps" on any of the coils. The results I had were

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    astonishing to me. I was able to tune from below 53kHz AM all the way intothe SW Bands to around 40 meters somewhere.The separation was great. The 2 Meters I used as tuning aids. Best X-TalSet I ever had. Next time I use 600/44 Litz wire for the coils.

    73'sWH7WP de

    zwheel (/member/zwheel/) Dr.Bill

    That capacitor doesn't look like it would go as high as 365pf. But.. he didn'tsay Broadcast Band, he said Shortwave. That wouldn't require as large acapacitor so it makes sense.

    If you are going to build something like this shortwave might be more funanyway. AM broadcast radios are still so common. I'm sure we all haveseveral even if we never use them. It's fun to build anything, just to be ableto say you built it yourself but better yet when it is something you don'talready have!

    micchow (/member/micchow/)

    Just got this Instructable in my e-mail digest. I see that the plates are made fromaluminum. Would it be possible to make the plates from the aluminum from an olddrink can?

    Thanks for the wonderful build.

    The Green Gentleman (/member/The+Green+Gentleman/)

    Late to the party, but this is really cool!

    tlynch1 (/member/tlynch1/)

    wow neatmy grandfather built a bunch of those old radios theirs even a couple sitting in mydads basement

    Dr.Bill (/member/Dr.Bill/)

    In the begining of Amateur Radio operators built All the parts needed to assembleTransmitters and Receivers.

    wirecutter (/member/wirecutter/)

    Wow! First class! A True instructable and a real make your own radio. Will it be amake your own crystal next.?This has taken me back to the 50s when I use to go around the Second handshops, and scrounge 'Old Faulty Radios' for a few pence of my pocket money, andeither make them work or use the parts. I did find a box of Carborundum crystalsbut the best sensitivity was far less than an OA90 diode.I had thought that no one was doing crystal sets any more, as in Europe T.P.T.B.(the powers that be) have sold off for "Data" use most of the AM and FM .frequencies and pushing for every one to listen to D.A.B. ( and I am not saying theother use of those initials!) You try building a receiver for digital out of just sixsimple items, and explain how it works to a youngster, if three of them are littleblack multi-legged chips!Oh yes, for those that wanted to know how to find out the value of of 'C' the mid

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    range of digital meters have an "F" or "C" range. Once again what a greatinstructable.

    joliwana (/member/joliwana/)

    way to go Jezan! keep it up!

    MACKattacksnipe (/member/MACKattacksnipe/)

    Awesome

    genesis_tan (/member/genesis_tan/)

    nice!! great job tito!!

    Re-design (/member/Re-design/)

    You take DIY seriously don't you!

    Jezan (/member/Jezan/) (author) Re-design

    It's habit forming. lols

    Jezan (/member/Jezan/) (author)

    Thanks a lot for all the nice comments.

    parker216 (/member/parker216/)

    Jezan, I really like what you did here. Your instructions were exact and precise. Ilove this because I try to build EVERY component of a crystal set myself. I wouldlike to suggest the book "The Voice Of The Crystal". There are a lot of diy crystalradio projects inside. You might be able to find it on amazon, butmidnightscience.com has it in stock and they are a great crystal radio club withmany books and parts. Thanks for this instructable!

    scottm_113 (/member/scottm_113/)

    Being an ex-radio operator (that made Most of his toys) I have to say that this hasGOT to be one of the BEST Instructables that I've seen to date! Well Done!

    seandogue (/member/seandogue/)

    Ok, I'm officially impressed. That's ingenuity and gumption

    Jezan (/member/Jezan/) (author) seandogue

    Thank you Sean.

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    Nucleus (/member/Nucleus/)

    where should I hook up the terminals using this capacitor?

    Jezan (/member/Jezan/) (author) Nucleus

    Hook up terminals on the rotor usingthe thin copper sheet ( photo step17.) for the stator part you can hookup wires on any of the four nuts thatholds the housing. Or you can put aterminal lug on each nut if you are tomount it on a PCB. These are likewashers that have a portion to crimpa wire in place

    gmoon (/member/gmoon/)

    You did an absolutely fantastic job--really! (but radio-type variable capacitors arefairly cheap and easy to find...)

    Jezan (/member/Jezan/) (author) gmoon

    Yeah right, cheap and easy to find.Im in the middle of the desertnowhere to find and nowhere to buy.:-) thanks..

    gmoon (/member/gmoon/) Jezan

    Oh, sorry, you've got a delivery / shipping problem? If I'd had a way to get itto you, I'd have sent you one...

    Jezan (/member/Jezan/) (author) gmoon

    And so do you, lols

    gmoon (/member/gmoon/) Jezan

    Not so much--on this end. ;-) Good luck. Looking forward to seeing your thefinished radio.

    Stuffses (/member/Stuffses/)

    how much can the capacitor hold

    Jezan (/member/Jezan/) (author) Stuffses

    I'm sorry i dont know how much capacitance it holds. I have seen some 25

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    Make Comment

    rotor plates/24 statorplates at 450 fF. Mine is6 rotor plates/5 statorplates. Maybe its about100 fF im not reallysure.

    Stuffses (/member/Stuffses/) Stuffses

    i forgot to add a {?}

    lemonie (/member/lemonie/)

    Very, very good. We'll be seeing the full set soon I hope? L

    Jezan (/member/Jezan/) (author) lemonie

    I'm sorry i never had the chance toanswer. When you asked me aboutthe sites that i have already visited.I was already building it then. Imade a version of the mysteryshortwave receiver.

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