Selective Frequency Crystal Radio

  
The coil on the left is coil#1, and has 17 taps. The other coil is coil #3. The vertical strips are the lath material, screwed to the 1 foot long base. Holes drilled in the ends of the lath hold the dowel rod; it can be removed to change coils. The bottle caps are the knobs of the two polyfilm rf tuner capacitors, which are stuck to the base with sticky tape. On the right of the base are screwed 3 fahnestock clips. The detector is connected between the rear clip and the right front clip. Leads with alligator clips are connected to the right rear and front left fahnestock clip, and leads and alligator clips are also connected to each capacitor. The two front fahnestock clips are for phones.
Selective Frequency Crystal Radio - schematic

The capacitors I used have three connections. The center one is the base, and the outside ones are for the rotors of the two sections. You need to connect the two outside ones, and connect them to one lead, about 8 inches long, and connect the center connection to another lead. If you look on the back of the capacitor, you will notice two small trimmer capacitors with a screwdriver adjustment. Adjust both these to minimum capacitance ( you will see a full circle instead of a half-moon shape); this will give you full BC band coverage when used with the coils you will wind next. Cut holes in the center of the two plastic bottle caps ,and shape them with a knife to fit snugly over the tuning shaft of the capacitors, and then hold them in place with a small washer and screw. Use the double sided sticky tape to mount the capacitors on the base, and fasten alligator clips to the ends of the leads. To avoid working the delicate capacitor leads, I used an ordinary stapler to staple the leads to the base close to the caps. Construction of coil #1: This takes about 72 feet of #24 wire. Using a pin, punch two holes about 1/4 inch apart close to and parallel to one end of your coil form. Thread about an inch of wire into one hole and out the other to anchor your coil. Now wind about 152 turns of wire in a single, closely wound layer along the coil; at every 10 turns (yes 10), make a small loop that sticks above the...



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