T EVIS i - AmericanRadioHistory.Com

Code Practice Oscillator
(Continued from page 41)
streetcar. Probably the only difficulty
encountered would be the stares
of fellow passengers.
Group or Class Practice: While the
unit was originally designed for
private use, it is capable of giving sufficient
loudspeaker volume for a class
of ten to twenty students, where the
background noise is kept reasonably
low. Use the circuit arrangement
shown in Fig. 4.
A 3" to 6" PM loudspeaker is used
(better results are obtained with the
larger speaker) together with a standard
audio output transformer, such
as the Stancor Type A -3856. The
primary of the audio transformer is
connected in place of the headphone
connections (Fig. 3).
Where desired, the loudspeaker and
transformer could easily be mounted
in a small metal utility box or wooden
baille and plug -in leads identical to
the headphone leads provided. The
basic oscillator would then serve
equally well for classroom (group)
or private use, as required.
For practice in group sending, a
hand -key may be provided for each
student, with all the keys connected
either in series or in parallel. If connected
in series, all students except
the one actually sending close the
switches of their keys Practice can
he carried on in "round robin" fashion,
with first one, then another, and another
student sending a group of test
words.
Audio Signal Source: The completed
oscillator will serve as a satisfactory
portable audio signal source for signal
injection testing of phonograph amplifiers,
public address systems, or the
audio section of radio or television
receivers.
In this application, either the "Key"
or "Phones" terminals should be
shorted together and a potentiometer
connected between the other pair of
terminals (from 500 to 47,000 ohms
will be satisfactory ... higher output
voltages will be obtained with the
higher resistance values). Output is
obtained from the center arm of the
pot and one of the outer terminals. A
blocking condenser should be used if
the audio signal is fed into circuits
where d.c. cannot be tolerated.
In general, the audio signal obtained
will not be a sine wave, but it
is still useful for general purpose
signal injection tests.
The transistor code practice oscillator
described, useful in its own
right, also makes an excellent construction
project for the experimenter
desiring to work and to gain familiarity
with transistor circuits and applications.
Work with comparatively
simple projects helps to develop the
experimenter's knowledge of transis-
-
tor behavior and to instill the confidence
necessary to undertake increasingly
complex and more difficult
projects.
April, 1953
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