Cigar Box Radio — Crystal Set #29
Now don't e-mail me and tell me that I ruined a cigar box worth $500.00. That would just wreck my day! This is one of those fun type radios, folks. Nothing too serious here. The tuning range is lousy and this set is about as sensitive as a Marine drill sergeant.
A good friend gave me this cigar box. He saw that I liked building radios in just about anything. He had also seen my #27 radio. I had some copper that I use for my tube sets, and thought I would see about using this material to make a capacitor or two.
The material I used is some very thin (.017 inch)copper circuit board. This is important to use a similar material for the capacitors on the left as the board is double sided with a thin glass epoxy board sandwiched in between. The large variable capacitor can be a plain copper sheet. Make sure that the top plate is the grounded end, or you will have lots of hand capacitance problems!
The variable capacitor is made up of a bottom plate (shown installed in the radio pictured below) and a top plate. The top plate is held at one end with three screws with a nut spacer under the top plate. At the other end of the top plate, I soldered a brass nut. Then I used a grinder to remove the top part of the screw head. The screw is then threaded through the nut and a wire nut serves as a knob. BTW, I do this without drawings or any other pre-planning. It kind of shows, huh?
The antenna is connected through a capacitor made from a small square pc board. The value is around 150pf. A quarter inch hole was drilled through the center of this square copper board. The hole was made larger so that the screw wouldn't short the board. This technique was used with the rf bypass capacitor that is next to the antenna capacitor.
The rf bypass capacitor is mounted with two screws. The bottom one has a metal washer on the top of the board, and the top screw has a metal washer on the bottom and a nylon washer on top. That way each screw connects to one side of the board. I used a 68k ohm resistor as a DC load for the crystal earphone.
The coil is wound around four styrene pegs placed in four holes that I carefully drilled in the top of the box. The 40/44 litz wire is wound around and held with Q-Dope. I was able to get 15 turns on the coil, so I stopped there. As it turned out, that was enough coil. That combined with the variable capacitor to allow me to tune the middle portion of the broadcast band. I can receive one signal on 820 khz with the capacitor screw at mid position. Good enough! It won't win the crystal set contest, but could win a beauty contest.
I didn't spend a lot of time making this radio. I just had fun with it. Best wishes and good DX. Dave - N2DS