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even a small wooden box may be used
for housing the components. Shielding
is unimportant.
The author used a Bud CU -3001
"Minibox" (3'4" x 21/s" x 11% "), and
finished it in black glossy enamel.
However, the reader may prefer to
use an already finished unit (Bud also
supplies this same size "Minibox" in
a gray hammerloid finish at about the
same price). Decals were used for
lettering.
In order to fit the audio interstage
transformer into the small metal box,
it was necessary to remove the mounting
"ears" on the transformer frame.
This was accomplished by simply
bending the "ears" back and forth
with a pair of heavy pliers until they
broke off.
Since the battery life is quite long,
it was found practical to make the
battery connections by soldering leads
directly to the battery terminals
rather than by providing special clips.
After the connections were made, the
batteries were taped together and the
terminals insulated by wrapping with
Scotch electrical tape.
The interior view of the oscillator,
given in Fig. 2 clearly shows the
general parts arrangement used by
the author in constructing the model.
It was only found necessary to mount
the phone tip jacks and the slide
switch. The other parts are held immobile
by the compact wiring and by
the pressure of the case when the
cover is in place.
Parts Substitutions: The circuit
given is not critical and a number of
substitutions is possible to suit
the requirements of the individual
builder.
Almost any standard 1:3 interstage
audio transformer should give satisfactory
results for use as TI, although
the builder may find it necessary to
experiment somewhat with the lead
connections. The author originally
tried the entire secondary winding
(the two green leads, Fig. 3), but
found that better results were obtained
when only half the winding was
used.
Resistor R may best be chosen experimentally*
to give the tone desired
by the builder (as mentioned previously,
the size of this resistor has some
effect on the frequency of operation).
If too small a value resistor is used,
the current drain may be large, while
if too large a resistor is used, the
oscillator may not work. Satisfactory
values will generally fall between 2500
and 15,000 ohms.
A rotary switch, toggle switch, or
lever switch may be substituted for
the slide s.vitch (S,) if preferred, but
the slide switch is the least expensive
and most compact.
If the t uilder prefers, a different
size penlig it cell or an ordinary flashlight
battery may be substituted for
B,. A Mal'ory mercury cell may also
be used here. and will give much
longer life than a zinc -carbon cell
(the Mallcry RM1000 is an excellent
choice).
The headphone and band is likely
to prove a little bulky in the pocket.
Where maximum convenience is desired,
it is suggested that the builder
use a Telex midget earset (Type
A9680, 2000 ohms) in place of the
standard wadphone unit shown in
Fig. 5. Equally satisfactory results
will be obtained. Crystal headphones
should not be used, however.
Substitutions may be made for other
components where necessary or desirable,
for example, miniature "hearing
aid" type plugs and jacks may be
used in place of the phone tips and
tip jacks, .?tc.
Lead lengths can be varied to suit
the indivic ual user. It is even practicable
to mount the hand -key directly
on the case of the oscillator and to
wire it directly into the circuit, where
desired by the constructor. An alternative
is to mount the hand -key and
oscillator an a small flat wooden or
plastic base.
Applications
In addition to the rather obvious
applicatior of the transistor code practice
oscillator for use as a personal
unit for pi ivate use, it is well adapted
to other uses. A few of these are given
below:
Mobile Practice: Military radio
operators, especially, make wide use
of hand -keys mounted on straps and
fixed to one of their legs, for communication
work while in a plane,
"jeep," or command car. Practice in
this technique may be obtained by
A 15.0(10 rotentiometer may be used here to
drtrrmin. t h.. mit. ial value for It
Fig. 2. Chassis view with cover removed.
it
R:J
9 i
GREEN
(NOT USEO1
3_4
IIIIiI éP
PHONES
KEY
R -4700 ohm. !/z w.., .. used in model (select
size for desired tone -see text)
S,- S.p.s.t. slide switch
volt battery (Burgess #7 penlight
cell)
B_-15 sole hearing aid battery (Burgess U10)
T, -3:1 interstage trans. (SNC #IP331)
CK722- Raytheon transistor
1 -Bud "Minibox" CU -3001
I- Hand -key
1 -1000 ohs headphone unit
4 -Phone tip jacks
Fig. 3. Complete schematic of oscillator.
TO PHONE
TERMINALS
PR(
SEC.
STANDARD OUTPUT TRANSFORMER
SMALL PM
SPEAKER
Fig. 4. Circuit variation using a speaker.
providing a strap (made from webbing
or from an old belt) on the hand -
key so that it can be easily strapped
to the leg.
By slipping the oscillator in a
pocket, practice can be obtained while
riding in an automobile, bus, train, or
(Continued on page 99)
Fig. 5. Complete code practice setup including oscillator, key, and headphone. Unit is compact enough to be carried in a suit pocket.
April, 1953 41