Crystal
Radio
This was the second version designed in CAD. I added the drawer and a
log scale to the radio. It is not build to any scale.... just a "rough
sketch" to try different configurations.
Rather than use the typical antenna tuner, which is a brass strip on a
pivot, I decided to run my tuner contact on two brass rods, with a
crank and screw to drive it across the antenna coil.
The cat wisker rod is 1/8" brass, with a solid brass ball I drilled
through, then soldered to the rod. I did away with the CAD drawing's
bail and spring arrangement, and in practice found a simple formed
spring was simplier.
Here
is the radio apart and stripped in preparation for finishing. You
can see the turned cherry posts for supporting the capacitor in the
drawer. To the left of them are the knobs for the tuner crank and the
wisker rod.
I plan on finding either real or good fake bone to make a log scale for
the white oak "bar"... so finding the proper place on the coil for a
given antenna will be easier.
On the threaded rod can be seen the brass crank, minus it's knob. The
crank is cut from 3/16" brass plate, threaded 1/4-20, then soldered to
the rod. There is a nut in
the dark wood tuning block, which this rod threads into. So... turning
the crank moves the rod: 20 turns=one inch.
I was lucky to find an classic (from the 40's)
high impedence earphone which fits in this drawer... head piece, cloth
wire, connectors and all!
The
radio almost done. It is all shellacked, and I
polished all the brass
with White Diamond on my Unimat. I still need to add feet, a log scale
to the oak bar above the antenna tuner, and make a plexi case for it. I
also will make up an antenna, ground wire with small stake, and
instruction booklet to keep in the drawer.
And it works! In the early evening I was getting a local station, WLNA,
Peekskill 1420. I also got two other stations I could not identify...
they are pretty faint. Imagine my surprise when I tried it again after
dark, and heard Arabic... loud! So I played around, and heard Chinese.
It was in Chinese, with Chinese music... I did not know, nor think it
was possible, that these radio picked up shortwave. I listened for a
bit, and sure enough the announcer said in English: "CRI", for China
Radio International. Then more Chinese, then "...cri.com...". So unless
they use a repeater closer to me, I just heard the other side of the
world on this job.
This was with a 100' long wire antenna, and a 1/4" copper ground stake
for a ground. I later heard French, two different Spanish stations, and
various other faint and garbled stations. I had no idea this was even
possible on a crystal set. I've since read that it is common to receive
shortwave on such a setup... and that time of day is a major factor in
what you will recieve when you have no tuning capacitor. I tried again
just after dark and heard French, two Asian, two Spanish and one German
station. I also could clearly hear (and understand!) an American
evangalist... I think he was one from Indiana.
I've left room on the chassis for a tuning capacitor I have been
designing. Rather than rotating, it will mesh rectangular sheets of
aluminum flashing on a flat plane. The sheets will stand up, and be
meshed with a screw crank similar to the crank for the antenna tuner.
It will fit just behind the crystal/cat whisker setup. I only need to
compute the area for the plates, then I can make and experimental
version to test it out on the set.
The Trimm Featherweight headphone I am using is the loudest of all the
ones I have around. But the thing is, it only measures about 300 ohms.
I've read that 2000 ohms is what you are looking for in crystal
earphones... and so I was trying other sets. I have a 1,200 ohm set,
two sets which read 2000 and 3800 ohms. But non of them are as loud as
the 300 ohm Trimm.
Still, the sound is very low, as built. To isolate the
reasons, I thought I would swap out the different
components. First I replaced the cap I made with a factory .002
Mfd one. This did not change the performance at all, so the
theory I made a "leaky" cap was out. I sent for some 1N34 germanium
diodes, and clipped one to the cat whisker and crystal cup, with the
whisker raised. The sound was suddenly loud and clear! This test
exonerated my antenna, coil, slide switch, ground, whisker post and
rod, wiring, and so on. I realize now that the natural galena crystal
is just way less "sensitive" than the factory diode. Since I have no
experience in these radios I cannot know how sensitive a different
galena crystal might be... but now that I know the crystal is the
performance culprit, I will get a chunk of galena and try other pieces.
The local station, WLNA, comes in... but weak, like I said (China comes
in louder). As an experiment I drove to a park (before I had the
germanium diodes) within a half-mile of
the antenna. I drove about 2' of brass rod in the ground, and stretched
a 30' antenna up to a tree. I tied it to the tree with string to
insulate it. For the life of me, I could not get a sound! At home, the
station came in weak. Within spitting distance of the antenna, not a
peep. Go figure.
I finally, four years later
(2011), got around to finishing the radio. I made a glass cover for it,
and added copper plated brass feet. I did not turn the feet, they are
drawer pulls.
The
radio almost done. It is all shellacked, and I
polished all the brass
with White Diamond on my Unimat. I still need to add feet, a log scale
to the oak bar above the antenna tuner, and make a plexi case for it. I
also will make up an antenna, ground wire with small stake, and
instruction booklet to keep in the drawer.