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J. B. McCreath Crystal Set

This page was uploaded on August 25, 2009.
Crystal Radio Project

Description of My Crystal Radio System

This setup is my first serious attempt at building a crystal radio. I decided to go with a modular approach, which would allow maximum flexiblity. An antenna matching unit, the radio set, and an audio matching device make up the three modules. This allows for making changes without having to do a complete rebuild.

After much reading on various websites, I came up with an overall plan for the system. The antenna matching unit (AMU) is a series tuned arrangement using a basket-weave coil, a variable capacitor, and a fixed capacitor. It tunes the entire AM broadcast band in two ranges, selected with a toggle switch, which can also bypass the AMU entirely. A 3:1 vernier drive is used for smooth tuning of my 200 foot longwire antenna.

The radio set is based on the KISS priciple. The antenna/ground connection is to a 6 turn link coil which is coupled to the tank coil. Both are basket-weave design and are 4 inches in diameter. They are mounted in line on a pair of hardwood dowels allowing a change of coupling from tight to loose. The tuning capacitor is a unit made in England, and has ceramic insulators for the stator plates and the rotor supports. A National "Velvet Vernier" gives a smooth touch to the tuning. The detector diodes are held in a pair of mini alligator clips allowing for quick and easy changes.

Crystal Radio Project

To match the high impedance output of the detector diode to that of the relatively low impedance headphones I used a Hammond model 832 transformer with dual primaries and secondaries. Having the primaries in series and the secondaries in parallel, I achieved a 16:1 impedance step-down which along with a "Benny" allows for a good match to a pair of hi-Z headphones. A second jack, of the switching variety, is wired to the detector so that an outboard matching circuit/device can also be used with the set.

For the audio matching module I built "The Speco Match" based on the BT-Ultimatch circuit from Ben Tongue's site, using a Hammond model 145D and a Speco 70 volt speaker transformer. It allows a wide range of settings for matching headphones to the detector. The 8 ohm winding of the Speco is wired to a stereo phone jack, so "Walkman" types can be used as well.

My headphones are a pair of "ear defenders" from my shooting days with a pair of receiver elements removed from telephone handsets fitted into the foam liners. The elements are connected in series which gives about 400 ohms impedance, based on info about them I found on websites.

Crystal Radio Project

Crystal Radio Project

Parameters and Parts List:

   1) The Antenna Matching Unit

      Tuning capacitor Cmin =  50 pF. (measured)
                       Cmax = 225 pF. (measured)

      Fixed capacitor = 130 pF.

      Loading Coil L = 260 uH. (measured)

   2) The Radio Set
         
      Tuning capacitor Cmin =  32 pF. (measured)
                       Cmax = 560 pF. (measured)

      L1 Tank coil = 198 uH. basketweave (measured)
                     
      L2 Coupling coil = 14  uH. basketweave (measured)
                      
      Fmin =  500 kHz. (calculated)
      Fmax = 2100 kHz. (calculated)

      Diode = 1N34, IN70, and other unmarked ones

      Audio = 80k:5k transformer, 470 pF. cap, a "Benny", and a jack
              for direct diode output for external audio matching.

   3) Audio Matching Device

      Input  Z = 208 k ohms
      Output Z = 8, 500, 1k, and 2k
      "Benny"  = 1 meg and 0.1 uF.


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