Crystal Radio Tuning Capacitor

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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.

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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.

Mike When truth is absent politics will fill the gap.

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

"Dave.H"

** 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

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

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] | | | --------+------------+----------+

Values: 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.

Ed

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.

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 maths.

Sorry that should be 230 uH.

Somewhere around 100T of #24.

Try

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.

Mike When truth is absent politics will fill the gap.

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,

• * * Christopher

Temecula CA.USA

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.

--
Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

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.

Ed

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. ;-)

Cheers! Rich