Everything But The kitchen Sink
Dave's #15 crystal radio
#15 is finished and it isn't even Christmas. How about that. So here is another one of my bright ideas brought to the world wide web. This set expands on my last set. I brought the idea of the variable coupling coil and honeycomb coils and put it all together with my successful designs of past sets.
This homemade radio uses a basswood box which are fairly cheap, but are very nice to build radios in. The box is slightly larger than the one in my sixth set. A lot of internal height was needed to give the honeycomb coils room to breathe. I once again used Min-Wax mahogany stain. A couple of coats of gloss deft followed. I used the thicker 3/16 inch Garolite® placed on the top of the box. This works very well as long as your capacitor shafts are long enough that the knob can be tightened down on the shaft. Some of these old time knobs need a long shaft. Speaking of knobs, look at the ones I found. They were made by Premier in the 1920's. I bought this pair sight unseen and I wasn't at all disappointed. Of all the knobs I bought this time, I knew that these were for this set.
I had planned on using a couple new style rotary switches, but at the last minute decided that I would not use them on this set. Those new switches destroy the old look that I desired in this radio. The #8 screw, brass thumbnuts and brass links makes an excellent old time switch. The links don't have to be changed too often, so this works fine. The link on the left will put in two more gangs of capacitance for the antenna tuning section. This is nice for the low end of the broadcast band. By opening the link, only one gang is in the circuit allowing for easy tuning of the high end of the band. The link on the right shorts out a 100pf capacitor that is in series with the tuning capacitor. When tuning the high end, remove the link for a better tuning range. Both of these links aren't necessary but make operation a little nicer. The link in the center is the diode coil tap selector. Placing the link to the right gives the highest volume but the worst selectivity. Placing the link to the left gives you the opposite. The center position is the compromise.
The coil arrangement is based on my last set. I bought a bunch of these coils at a hamfest this year and I am having a lot of fun with them. This time I used two of them. The movable coil measured around 250 microhenries but I found this was a little high so I took some turns off it. Now the coil measures about 200 microhenries. I used some litz wire to connect the coil to the rest of the radio. This gave me good flexibility in the wire. The connections should last for years. I used a 1/4 inch dowel rod connected to a little piece of wood to attach the movable coil. This turns very nicely and allows for loose or tight coupling to the detector coil. Generally the looser the coupling the lower the volume but the selectivity increases. If the coil is over coupled, the sensitivity and selectivity is ruined. I do like building these mechanical devices.
The fixed coil is the detector tuning circuit. This is much like the rest of my crystal radios except that this coil has 3 taps for the diode. As described above, the sensitivity and volume can be adjusted to meet the conditions. You might have noticed a couple of turns of green wire mixed in with the brown. This was an extra winding. I elected to leave it where it was so as not to damage the fragile coil. I did remove turns to bring the inductance down to 240 microhenries. This is the proper inductance to use with the variable capacitors to resonate in the broadcast band. The coil is attached to a piece of Garolite® strip with 4 cable ties. The Garolite® strip is then attached to the main panel with two little metal brackets. You might not be able to duplicate my arrangement as these coils are not available, but you can work out something with a couple of spider coils.
I think I will let you in on a little secret. Several months ago I purchased a cool little device that has helped me with my radio designs. It is a digital inductance and capacitance meter. I bought it from AADE. It is not real expensive and it works well. This is a kit (or you can get it wired). If you aren't familiar with putting little stuff together, you might want to buy it wired. If you build it, read the instructions very carefully and double check before soldering. With this L/C meter, I can check the inductance of my coils, take off a few turns and check it again. This really fine tunes the circuit. I also have a lot of those old buzzard square capacitors. The values are hard to read but with this meter, it is a snap. I can use these in my radios and keep that antique look that I desire.
It is kind of early to tell the performance. I don't see a lot of performance differences in my crystal sets except to say that the more tuning circuits and switches, the better the set operates. This is due to the better antenna matching capabilities. This is probably the closest I have to a dx receiver in my collection. I hope to enter the crystal set contest coming up in January.
I have more sets on the horizon, so please bookmark my main radio page. If you have questions or comments (yes, someone told me to get my head examined and to get a life) please use the e-mail link at the bottom of this page.
Best wishes from -- Dave N2DS