Dave's #45 Crystal Set — aka 'Roids R' Us
This is my first toroidal crystal radio. I was lucky to receive some ready wound toroids in the mail the other day and I couldn't wait to build one of these sets. The large crystal radio group in Birmingham, Alabama have been building toroid sets for a while now. I thought that this trend should move north. My first 'roid set is a modest kind of radio offering decent performance.
The box is the same type I used to make my #26 radio. Although the box is a plain basswood variety box, it turns out the be a silk purse after the stain and poly is applied. I really like working with these boxes. On each end is a piece of 5/8 (15mm) x 3/4 (18mm)pieces of soft wood that supports the panel. I used the T-nuts as I have in many of my boxed sets. It is easy and works very well. Can't ask for more than that. The front panel is made from 1/8 (3 mm) thick styrene. Garolite® may also be used.
The circuit uses three 365 pf variable capacitors. Two of them form the input circuit. By juggling the adjustments back and forth a good sensitivity vs selectivity compromise can be reached. The input circuit is then transformer coupled to the detector circuit. Toroids make it very easy to do this without the need for special shielding.
The detector circuit is the now popular Hobbydyne circuit. This circuit was recently reintroduced by Jim Frederick improving on an old Australian circuit from 75 years ago. I came up with some 27 mh chokes that make this circuit work well.
The choke maintains the dc circuit that was broken by the 8 pf coupling capacitor. That 8 pf can be experimented with. If you want higher selectivity, this value can be reduced. A small trimmer may be substituted.
The output works the best if used with an audio matching transformer. Many use the inexpensive Bogen T725 speaker matching transformer. Darryl Boyd has a lot of audio transformer information on crystalradio.net. If you don't have a transformer, this set will also work with a crystal earphone if you place a 47k ohm resistor across the radio output.
This radio can be built as a high performance set if these changes are made. Change the capacitors to low loss ceramic stator insulators. Then add vernier tuning to all the capacitors and you move up to the big contest sets.
Last night after midnight and after my computering was all done, I put on the headphones and heard a bunch of stations with this set. I was able to separate a local on 1490 (10 miles away, 400 watts) from WTOP on 1500 in Washington. It wasn't a complete separation but I clearly heard the DC station. My other observation is that the detector tuning is unaffected by the adjustment of the antenna tuning capacitors. The two antenna tuning capacitors did interact, but juggling two is a lot better than 3 adjustments. Steve Bringhurst introduced me to these fascinating little doughnuts and I thank him for that.
73 and good crystal DX. Dave - N2DS