This is a nice little solderless starter crystal radio, sold by Transtronics . Cost is $6.75 plus s&h, and the downside here is that their minimum order on the web is $20.00, plus shipping and handling.. This wasn't a problem for me, since I got a mess of them for my students to build, and was able to spread the shipping cost around. For a single buyer, you might look at some of the other goodies Transtronics offers, such as the companion speaker and amplifier.
Okay, what do you get with this
1N60 type germanium diode
Plastic chassis with schematic overlay, and 5 springs for quick contacts plus mounting hardware for the coil and capacitor
High impedance crystal earphone
Variable capacitor - this is a 5-59/5-141 pF 2-section capacitor with both sections hooked together. Each section also has a 10 pF trimmer, giving you 200 pF of tuning range - as low as 17-217 pF up to 34-236 pF depending on how you set the trimmers. Note: The one I checked had the trimmers fully engaged, cutting out the top of the band, so I opened them up, and got it back - I did the same thing with all the sets my students built as well.
Ferrite core coil - 400 uH, with taps at about 6 uH (long antenna), and 135 uH (diode). Uses a 5/16 inch diameter by 1 3/4 inch long core.
Assembly was a snap, and the instructions were clear. Oh yes, they give you the usual 10 feet of wire for antenna/ground as desired, but you will still need to put up a real antenna and ground unless you live very near everything you want to hear or just don't get out much. You will need a small Phillips head screwdriver to install the coil mount and the capacitor, and a pair of needle nose pliers was useful in making the spring connections. A small flathead screwdriver will be needed to adjust the variable capacitor trimmers on the back; just open them up all the way (minimum capacitance).
So how does it work? About as well as expected for a basic crystal set. Which antenna connection works best depends on the length of your antenna. With just the antenna connected to the top of the tank circuit (not one of the recommended antenna connections), it picked up my four local stations between 790 and 1490- classic "rocket radio" performance, or maybe a little better. With only a 50 foot "student" antenna and an electric outlet cover plate screw as ground, it picked up several more stations about 90 miles distant. By adding an antenna tuning coil as described below, it worked even better. With at least one of the sets my students assembled, the audio was weak and distorted in the earplug until I added a 47k resistor across the phone terminals. Now let's improve on the original a bit:
Try adding an antenna loading coil. This is done by winding
about 15 to 20 turns of small (#24 -#32) magnet wire on a paper core that
can be slipped over one end of the ferrite core. Leave about 3 inches
of wire at each end. You might find winding this a small chore, but
don't hesitate to use glue, tape, or whatever to hold the turns on.
I started the coil with a half-hitch, held the loose end of the wire down
with a piece of tape on my "former" (a drill bit slightly larger than the
ferrite core), and wound on the turns, finishing off with another half
hitch. Then I spread white glue over the coils, and then put tape
over the whole mess. One end of this coil goes to the ground connection
of the set (or you can put it directly to your earth ground connection),
and the other goes on the spring connection marked "long antenna" after
disconnecting the set's coil from this connection. This gives you
a little more coupling than the original long antenna connection, and you
can move it away from the main tuning coil to improve selectivity a bit.
b. Add an antenna tuning coil. Take a toilet paper core and close wind 100 to 150 turns of enameled wire; I used #24, putting a tap at one end and every 10 turns (more taps are better, if you have the patience). Leave about 4 inches free at the end of the coil. The free end connects to the antenna end of the loading coil, or you can just connect it to the "long antenna" tap (set tuning coil connected there too). Use an alligator clip to connect your antenna to the antenna tuning coil tap that gives you the best signal. There is room at the top right corner of the chassis to mount this coil - I just used masking tape. With just about any antenna, you can tune it to a near-resonant electrical length by moving the tap.
As you will quickly realize, these two simple modifications, particularly the second, give you more flexibility than the original set; you can mix and match taps etc. with the original set and play around with what works best for you. If you are trying to separate a bunch of strong, close stations, a loading coil with fewer turns might be better. The antenna tuning coil will improve your selectivity and sensitivity over the band as well. If you have another variable cap on hand, try putting it in series with the antenna coil and antenna, or in parallel with the antenna coil and then find the best antenna tap.
1. It works
2. It is a good starter kit, with decent quality parts.
3. Yes, you need an antenna and ground. For my students, I used the green wire provided with the set and connected a small soldering lug to one end; they put this under an electrical outlet cover plate screw for a ground. A good earth ground is important, and you should pursue it if you can.
4. It can be used as the basis for more sophisticated radios. Use it with different earphones or headsets.
In my opinion, the major limitation of these kits is the ferrite core tuning coil. It has a small core and small wire (not litz), which limits the selectivity and sensitivity of the set. The coils I use in my project radio with the same capacitor, while not optimum, work better.
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