Homebrew Galena Crystal Set
Ever since my youth, I have been attracted to crystal radios I had seen pictured in old books. There is a special appeal about a device, however primitive, that can pull in radio voices and music through the air from many miles away without batteries or electrical power of any kind, just an antenna and a ground.
That explains my immediate attraction to this little homebrew crystal radio with galena detector found at a swap meet. It was built into a deep cigar box using a heavy paper roll as a coil form. The coil form measures 4 1/4 inches long with 3 3/4 inches outside diameter. The coil itself consists of 99 turns of solid double-cotton covered wire tapped every 9 turns. Those taps are fed to ten smooth-head screws that make up the tap selector switch contacts.
As with all older electronics, the first repair is a good cleaning, making sure the solder joints are in good shape, and applying a bit of deoxit to all moving electrical contacts, in this case the ten-position tap switch and the crystal holder.
The top of the cigar box had been removed and apparently lost over the years. The cover had been fastened with a small nail which was still on the box. I made a top cover from another old cigar box lid.
The galena crystal was potted in Wood's metal that had separated into two pieces. Quick work with a soldering iron solved that problem. I cleaned the galena surface with rubbing alcohol as recommended in older articles. I then measured forward and back resistance ratios of the galena-and-cats-whisker diode. While there is a signifcant difference between forward and back resistance readings on an ohmmeter, the ratio was nowhere close to that of a modern germanium diode, thus limiting the efficiency of the crystal.
A 1920s experience
I hooked up the set to my 80 meter dipole antenna and listened to the two local AM stations with adequate volume using my equally old but very efficient Baldwin headphones. I could find sweet spots with the "cat's whisker" on the galena crystal that worked reasonably well. However, connecting a 1N34A germanium diode in series with the headphone lead to the galena crystal made a major difference. I could see why the introduction of fixed germanium diodes quickly ended the era of the galena and cat's whisker detector.
The internet has lots of good sources on building crystal radios with modern parts. Here are a few:
"Build A Crystal Radio" from The Stay Tuned Website
"Simple Crystal Radio" from techlib.com
"Building a crystal radio out of household items" from scitoys
"Quaker Oat Box Radio Project" from The Xtal Set Society