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Have you been wondering what could be a new use
for that old oscillation transformer you had left over from your spark days?
Well, look what I have cooked up in my kitchen. A shortwave crystal set using
some old spark radio components.
While innocently rummaging through eBay I stumbled upon this transformer
and capacitor. Since not too many people understood what this stuff was (including
the seller), I took the opportunity to grab these treasures. I knew that coil would
make a good tuned circuit for shortwave, but I didn't know how close it would be
to what I wanted. I ended up with a radio that would tune the 49, 41 and 31 meter
shortwave bands.
The standout component in this radio is that coil or as they called it
in the old days, an oscillation transformer. It was a device added to a ham
radio operators spark station, in an attempt to keep his signal in the 200
meter range. Have you ever keyed a spark rig? I did at the
AWA Museum. It is quite a thrill.
I sent my call sign on the key. (I hope the transmitter wasn't connected to
an antenna.) Anyway, I'm getting a little off the track here, but you should
expect that from me by now.
In keeping with the old time look, I went with a cat whisker detector
for this old radio. I'm casting my own pyrite detectors too.
As you see in the pictures, the coil has several clips hanging off
of it. They are for ground, antenna, detector tap and one wire to the stator
of the variable capacitor. All these taps can be moved to provide the best
signal sensitivity and selectivity. The further up the coil from ground, the louder the
stations are (to a point) but the selectivity gets worse. It is fun experimenting
with the coil taps.
The variable capacitor was in poor condition when I received it. One
of the insulators was gone, turned to powder. It was an easy job to take a little
piece of styrene and make another insulator. Then after a session with the
steel wool, the capacitor was presentable. That dial scale was another eBay
treasure.
The wood base is solid oak, stained and poly coated. Everything mounted
easily and those white ceramic stand off insulators give the rig a nice
finished look
I do cheat by placing a 1N34A diode in the circuit, in place of the
pyrite detector. After I get the
radio tuned, then I take out the diode and poke around the pyrite for the
hot spots. This was a fun radio to make. I hope you like it too.
Guten Empfang wünscht Dave - N2DS
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