Da Bandbusta — #50 Crystal Set
Hello again radio friends. Since this is my #50 radio, I thought I would make it a big one. It is much like my #35 radio that I built last year. I liked that one so much that I decided to build another. I can't think of a better crystal set design for DX. It is kind of a "$X" set too (Dollars Unknown).
This radio turned out very well. Plans? Plans? I have no stinkin plans! I wasn't even sure of all the parts I was going to use until the last moment. I had these very expensive capacitors I wanted to use in a project. So now I present to you my #50 high performance DX crystal receiver.
My DXceiver is built in a modular style. That is, the antenna tuner is on one board and the detector is on another. This is so I can separate the coils for the best coupling. The base is 6 x 15 inches (15 x 38 cm). This gives me plenty of room to construct the radio.
The antenna tuner uses a four gang, 250 pf per gang variable capacitor. This is a high quality, low loss type with ceramic insulators. Using such a capacitor insures that the Q, or tuning sharpness gained by the large litz wire coil is not destroyed. Mounting the capacitor on ceramic or styrene standoffs is a must. Also an insulated shaft coupling is also essential. If your shaft is metal, then when you place your hand near the knob, the receiver will detune. This is called the "hand capacitance effect".
If you don't have a four gang capacitor, just use a dual like I did on my #35 set. I added the link switch to be able to switch one gang either across the tank circuit or across the antenna capacitor. The switch can be left open for better tuning at the high end of the band. However, I found there was not a lot of advantage in this arrangement, so if you want to leave it out, you won't miss much.
The coil is made with 660 strand, 46 gauge litz wire. The inductance is about 150 micro henries. I made a styrene spider coil form 6-1/2 inches (16.5 cm) wide. The bottom has holes to attach to the base with a small piece of wood. Styrene is a very good rf insulator, and your coil will be very efficient. Careful dressing of the wires helps the efficiency too. The terminal block/link switch is made with styrene too.
The detector board also has a 660/46 litz coil, but this coil is larger. The width is 7.75 inches (20 cm). The inductance is a little over 250 micro henries. A very high quality 440 pf tuning capacitor is used. The tuning range is very wide (about 480 khz - 1.8 mhz) due to the very low distributed coil capacitance. A 365 capacitor will most likely give you the full band coverage.
With such a sharply tuning system, using vernier dial drives is essential. If you don't, the tuning will be difficult and you will miss a lot of stations. The white styrene dials are attached to the vernier drives with two small screws. I made the dial 4 inches in diameter (10 cm). This is large enough to put good scale markings on the dial. I found that putting frequency markings on the dial is best for quickly finding stations. You have to be a little careful putting these markings on the antenna tuning unit. Changing the antenna, will shift the dial somewhat. However, when I recently doubled the length of my antenna, I found it made very little difference. The detector unit will not shift because of a different antenna.
Moving right along. . . , the selectivity is controlled by a differential capacitor. This is the "Hobbydyne" circuit recently developed by Jim Frederick in Florida. The Hobbydyne allows for a very light coupling to the tank circuit by the detector diode. This increases the circuit Q, thus making a very sharp tuned radio. And folks, this does tune sharp. If you don't have a differential capacitor, you can use a regular small value variable capacitor between the tank and the diode switch.
I have been putting in diode selecting switches in all my high performance sets. This gives me a good way to test and evaluate different diodes. My favorite diode overall is the FO-215 diode made 40 years ago or so by ITT. At the top end of the band, I like to use a schottky diode as better selectivity can be achieved with this diode. I also have a 1N34A as my benchmark diode.
After the diode switch (which should be a low loss type), there is a resistor with a capacitor in parallel. This is called a "Benny". This circuit provides the same DC load to the diode as the impedance that the transformer provides (100,000 ohms). This resistor value isn't real critical. The capacitor offers a low impedance to audio frequencies, and that energy gets transferred to the transformer.
The audio matching transformer I used is a surplus 100k ohm to 1500 ohms. This gives me a good match to my sound powered phones. This part will be difficult to get, so I would recommend that you use a Bogen T725 auto transformer instead. For the price, it's the best thing going. The transformer is used to match the fairly high detector impedance to headphones. This is another one of those essential parts.
A feature I added was a method to disconnect the internal transformer so I can use external types. If you aren't going to be doing that kind of experiment, you can wire the radio output directly to the transformer input (O to X terminals).
If you are familiar with crystal set reception but have never used a large litz type set, you are in for a treat. This baby does tune very sharp. I hear a station on every frequency! At night, the frequencies between 1230 and 1490 are filled with a lot of low powered stations. Once you tune to a frequency, you have to sit a while until one station comes up out of the mud. It is a lot of fun to DX with a crystal set.
Ok, that's about it for this radio. I had a lot of fun building this one. I have fun building them all! My #51 set is already to be started. This one will not be a solo effort. I will have a little help. It should be ready in about a month. If you have questions about the sets I have built, please use the contact link at the bottom of this page.
73 and good crystal DX. Dave - N2DS