Impoverished Radio Experimenter #3
You
can't build much of anything, mechanical or electrical, unless you have useful
measuring tools. The first part of this new volume shows you how to build
a simple impedance bridge that will allow you to measure inductors and capacitors
to two significant digits. That means that if you find an old choke in a radio
chassis in the alley, this will tell you whether it is 2.5 henries or 17 henries.
This bridge will tell you if that old paper capacitor you removed from the
underside is still .01 mfd or has changed.
Not
only can you test old commercial parts of unknown value, you can also build
your own capacitors and inductors (see Voice
of the Crystal) and measure the results. Then you can intelligently use
the parts to build all kinds of radio gear.
The circuit is simple, in use for 70 years, and is remarkably accurate. It's like a crystal ball that looks inside mysterious inductors and capacitors and gives you a good idea of what they are.
The bridge needs an audio tone. If you have a simple audio oscillator, you're set. But if not, you'll find plans for a simple one IC oscillator that will put out a 150-1500 cycle tone. Use it on the bridge or for other tests.
And
you need a pair of earphones to listen to the tone inside the bridge circuit
as you make bridge adjustments. A pair of 2000 ohm headphones is best, but
you'll get plans for another very simple one IC amplifier that will make a
pair of low-impedance of "Walkman" type headphones, available almost anywhere,
"look like" a pair of 2000 ohm phones. That means you can use Walkman earphones
on that crystal or regenerative receiver you're building. It also gives a
volume gain of 20 (200 optional) that will make measurements easier and bring
in more stations.
You'll learn how to make pointers for knobs, and shielded cases from low-cost lumber that will make your home-built gear look like expensive antique radios.
You'll
see a large homemade variable capacitor built from surplus printed circuit
boards and shaft collars, and the simple one-tube shortwave regenerative receiver
that was built around it.
Your impedance bridge will allow you to experiment with audio filters that will dramatically improve the selectivity of a regenerative receiver so that you can copy code in crowded amateur radio bands.
And you'll get some ideas for that larger power supply you'll need for more advance receivers and transmitters.
Same nuts-and-bolts how-to. Heavily illustrated. More ideas than you'll be able to pursue in a month of Sundays. Get a copy! 5-1/2 x 8-1/2 booklet 48 pages
No. 22733 ... $6.95
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