Dave's #14 Crystal Radio.
Let's see.... I finish a radio, then write a web page about it. Ok got it! It has been a couple weeks since my last crystal set and I had to make sure I hadn't forgotten the routine.
I am using the same cheap basswood box that I had used for some of my other sets.But with that MinWax mahogany stain, this box does look nice. Golden oak stain used to be my favorite then it was red oak and now mahogany. I better check out the rest of their stains. I also used the same 1/8 inch Garolite® but I did change how I cut it. I bought some cheap (what else) forstner bits on ebay. These gave me a better cut and less jamming and pulling than regular drill bits. BTW, I did buy some tan Garolite® so don't be surprised if you see a "blond" radio. Remember that furniture from the 50's?
Ok. There is a picture below on how I fasten the top panel to the main box. I cut some 5/8 inch by 3/4 inch pieces of soft wood and fastened them to the side of the box with #6 x 1 inch flathead wood screws. Then I set the panel on top of the box, using masking tape to hold the top to the box. I then drill (after measuring) the 4 holes. Since I am using #8 screws on this project, the drill is 11/64 inches. As I drill each, I drop a #8 screw down just to make sure that the top remains steady. After the last hole is drilled, I then take out the screws and take off the tape. I then drill only the wood holes to 3/16 inches to accept the t-nuts.
This set centers around two major themes. First the coil is a honeycomb coil. This is a coil wound on a form to provide as much air space between each winding. This gives it maximum "Q". The higher this Q, the better the coil. A high Q coil will provide maximum selectivity. It has been over 30 years since I have studied Q, but even though I can't explain it, I know a good one when I see it. This coil was in a box of coils that I bought at a hamfest. The inductance measured about 320 micro henries. I carefully took off enough turns to make a 240-250 micro henry coil. There was a another winding of a few turns mixed in with the coil. I left this winding open.
The honeycomb coil is mounted to the panel with plastic tyraps and two pieces of Garolite® just over 2 inches long and 3/4 inches wide. I put in some notches at the top so that the tyraps would stay in position. The brackets are held to the front panel with 4 little metal brackets and screws.
The other bright spot of this radio is it's moveable coil. The closer the coil is in line with the other one, the higher the energy transfer. But you also ruin the selectivity. Having an adjustable coupling coil has its advantages. As an experiment, I took my octocap and placed it in series with the antenna connection. Placing the coil at maximum and changing the capacitance had about the same effect as having no capacitor and just moving the coil. This was done in the daytime with just one trial, so I will be doing more research.
The antenna coil is a 2 inch mailing tube 1-3/4 inches long. It is wound with 45 turns of 40/44ga litz wire. Why this size, etc? Because that is what was laying around. The 45 turns is what would fit on the mailing tube. Not too scientific. I like using the litz wire as the leads are very flexible and works well when one part is moving and the termination is fixed. I cut a 1/4 inch dowel 3-1/2 inches long and drilled a small hole in one end. I then drilled a 1/4 inch hole in the mailing tube and a small hole opposite the first. The dowel fits through and is fastened to the mailing tube with a little wood screw. Be careful not to split the dowel, like I did. A quarter inch bushing is put through the panel and a bar knob is attached to the other end. The bar knob tells me what position the movable coils is in at all times.
I found that the coil movement was just too loose, so a change I made was to put a compression spring and a flat washer between the top of the coil and the bushing. The coil now has some tension and tuning is a little tighter.
Another addition is the toggle switch at the top. The switch adds a fixed capacitor in series with the main tuning capacitor to reduce it's overall range and thus allowing better fine tuning at the high end of the broadcast band. This feature is optional and has not been well tested as of this writing. I just thought it would be a good idea as the top end of the broadcast band is crunched up in a very small portion of the variable capacitor.
How does it work? Don't know, been writing this web page. Well, it seems to work ok. It operates somewhat differently from my other sets. I can tell when I am over coupled and I can see the selectivity change as the coupling changes. Building these crystal sets is a very good way to learn electronics and physics. I would recommend all dads work with their kids making these sets. Your son or daughter will never forget the experience!
Best wishes from -- Dave N2DS