The Gold Mine Crystal Set
Lawrence A Pizzella
a.k.a. Loose Coupler
Since I have started building Crystal Radios I have made most of them on a piece of wood or cardboard. The size of the radio has varied from "table-top", as for my competition rig to a couple of cute ferrite tuned rigs on small scraps of wood trim. The one common feature they all have is that the "guts" are always exposed. Perusing the ebay listings, and admiring the radios that Scott and Roger build in wooden boxes has made me yearn to make a radio that fits entirely in a wooden box. I figure that maybe a small radio that is fitted into a handsome wooden box might have a better chance of surviving me, and perhaps wind up on the top shelf of one of my kids closets, to be taken down now and then to be shown to the grandkids, and maybe even great-grandkids with the words. "This is something your Grampa made when he was alive." The Building Contest provided the impetus to get cracking, and make such a radio.
The Gold Mine Crystal Set
I call this radio The Goldmine Crystal Set because the salient parts come from The Electronics Goldmine. I have dozens of poly caps and ferrite bars that were ordered from The Goldmine for the Crystal Radio Workshop that I teach at the museum. I decided to make as high performance radio as possible with these parts. The design evolved over several months of occasional weekend and evening experiments with different configurations of these ferrite bars and poly caps. Fine Litz wire (7x44) was tight wound, loose wound, and space wound, in the center, or near the end of ferrite bars in an attempt to find a configuration that provides the highest Q. I tried High L with the 220 pF max poly Caps and lower L with more capacitance switched in. What I found to work best overall was larger Litz, (21x44) lower L, and larger C. I found I could accommodate a wide range of antenna types if I allowed for selection of Tuggle or classic series tuned antenna coil configurations. Switching in additional C for the lower frequencies allows the high frequency end of the dial to spread out more conveniently too. I also wanted maximum flexibility in earphone selection so I wired in a 50k:1k transformer, a 2k:8 ohm transformer, as well as providing for selection of no transformer at all. Figuring out how to wire the switches for these options using only DPDT toggle switches was a little tricky.
A unique feature of this radio is the ability to adjust the coupling between the antenna coil and detector coil, and between the absorption trap and detector coil. The gray bars are used to move the coils and mimic their position. This feature is quite useful for optimizing the set for different listening conditions.
I decided that if I wanted maximum flexibility in earphone selection that I would have to tap the detector coil. I had a nice miniature rotary switch for this, so that aided the decision. I know the purists abhor taps, but these coils are not super high Q anyway, and being able to tap down on the detector coil, specially when listening to the strong locals provides a convenient volume control and clean clear audio into low impedance walkman phones, which are far superior for fidelity and comfort, and are likely to still be around in 50 years.
I chose cherry wood for the enclosure. It has the hardwood properties I was looking for, as well as a nice color and grain. I didn't want to stain or varnish this box. A light application of wood oil vigorously rubbed in was all. I had in mind a "monolithic" look, so the box would have to be constructed with as much precision as I could manage. I also wanted the corners to be dovetailed because it looks cool when done well. Considering I had no table saw, routers, tool fixtures, or anything else that I now know you really should have to do the job right, I confidently forged ahead. I figured I could grind, file, and sand to the precision I needed, so long as I was careful, took my time, and was willing to scrap what I was doing and start over if things weren't working out. I was about to learn a big lesson, and that is it is too hard to start over, especially when you're running late, and the deadline is approaching.
I didn't want anything to protrude from the box. This meant no external latches, hinges or hooks. I know there are latches that can be inserted into the wood and that the hinges for a lid can be recessed, but I have no idea where to get these things, or how I would neatly mount or insert them. I also wanted the Panel near the surface when the box was open. So the clamshell design was adopted. The brass pegs mounted in the lid fig snugly into recessed brass tubing in the base when the halves of the box are pressed together.
The panel is made of 1/8" black Plexiglas. The frosted matt side looks almost like Bakelite, takes rub-on transfer letters well, and doesn't show fingerprints or scratches. The layout and parts placement diagram was worked out with a CAD drawing program, pasted to the Plexiglas, and used as a drilling template. When you drill Plexiglas you must use a special type of bit to prevent chipping the hole as it exits the material. Planning the layout proved a good exercise. Everything looked good and fit right. The transfer letters look as close to "engine engraving" as I could get. I could have made labels using my CAD drawing program, and I tried to do so, but nothing came out looking the way I wanted. Eventually I would like to mount one of those engraved brass labels near the lower left corner.
I am not really very happy with the schematic. I simply do not have the artistic flair that some of the other diagrams and labels that I have seen evidence. Perhaps some artist out there will embellish my schematic and send me a copy. Make it 4x7 inches, so it will fit in the lid.
After working with high Q air core Litz coil radios in the DX contest, this radios performance seems pretty dull. But, I can clearly hear all SF Bay area stations distinctly using the 1N34 and the low impedance earbuds. Detector coil tuning seems a bit mushy (perhaps it's the taps) but the trap and antenna coils tune sharply, and do not require any tap change. I have not had a chance to try the set with SP phones, Schottky diode, matching transformer and all, or to listen for DX. I expect I should hear some.
I had so much fun making the wooden box that I plan to make another for a QRP transceiver that I have built but never put in a proper case. It should be about the same size with a compartment for earbuds, and antenna wire, but NO internal battery, --no way. Since I have learned after the fact how one REALLY should make grooves and tabs for dovetailed corners using a table saw, router, guides and fixtures, I am sorely temped to get these things. I am always in awe when I watch The New Yankee Workshop.