Crystal Radio #67, "Michael's Radio
Hey gang! Thanks for coming back. I know my last radio description was very long winded. I promise to make this one short and sweet. During the spring and summer of 07, I seem to be in a building frenzy again. That's fine with me.
This is my favorite one coil circuit. It looks like the antenna tuner of some of my two coil sets, but in this case, the detector circuit connects directly to the first (and only) tuning coil.
The small variable capacitor and 27 mh choke are part of my little Selectivity Enhancement Circuit hookup. I now dislike tapping coils, but if I don't do something, the selectivity goes to pot because of the loading of the tank coil. You can lose some volume doing this, but in the end, it is a good trade off. The small capacitor adjusts the selectivity vs. sensitivity tug of war. Decreasing the capacitor improves the selectivity at the expense of sensitivity, and so on. Get the picture?
The default earphone I chose for this radio is a crystal or piezo earphone. To make the radio work, there has to be a dc path for the detector to work. These earphones have an extremely high resistance of around 20 megohms (about 10 - 15k ohm impedance though). I used a 39k ohm (not critical) resistor so the dc current can flow though the diode properly. DC flow is also the reason for the 27 mh choke too.
If a magnetic earphone or a transformer is to be used with this radio, the resistor should be removed from the circuit. I provided a convenient way to do this by using a fork solder lug to connect the resistor to the earphone connection. It is easy with a screwdriver to loosen the screw and remove one side of the resistor.
My radio is built in a wood box that I bought at
Michael's Store. Besides being a good place to buy
beads, picture frames, and chat with the women, they also have a good selection of
wood boxes. I was there last week, looking over the situation and came across their much
improved box selection. I purchased the box that you see that has become my #67 radio.
The box is 9.5 inches wide (24,1 cm) by 5.5 inches deep (14 cm) and is 5.5 inches tall. A very nice size to build a radio.
I used two sticks, 1/2 inch (12 mm) by 3/4 inch (18 mm) to provide a way to mount the black Garolite® panel. These pieces should be a soft wood, such as pine. These two sticks are recessed by 1/2 inch from the top cabinet lip. The space in the lid is not very deep and I wanted to make sure that a variety of knobs would fit when the lid as shut. To hold the panel down, I use some T-Nuts, as shown in the picture near the bottom of this page.
With all the parts mounted, I started the wiring. I used 20 gauge bus wire everywhere I could. This makes the radio look nicer when someone takes it apart for a look.
I have to tell you, friends and neighbors, that the coil darned near didn't fit. I was able to do a little grinding on the edge and it fit. If it didn't, I would have had to use a smaller litz wire. The coil form is low loss HDPE, or high density polyethylene material that I have been using as coil material for the last year or so. I used a 3 gang capacitor but only two gangs are needed. By using a smaller capacitor, you will have more breathing room for your coil as it can be mounted on the back of the variable capacitor.
BTW, the knob is an old timer that really puts a nice finishing touch on my little radio. The labels are made with a Brother P-Touch labeler that is connected to my computer. This time I decided to label this set in German.
Connect a wire antenna, a ground and your choice of high impedance earphones. Then twiddle the one dial until the station comes in. This set works better than the run of the mill sets that are around. This is due mostly to the larger size litz wire coil that I used. Other coil shapes and sizes can be used, as well as other types of wire in your project. The inductance required is about 150 µh.
This was a great summertime project that didn't take a long time to build. You are welcome to build this circuit. I'm sure you will be happy with this moderate performing set.
73 and good crystal DX. Dave - N2DS