July 1960 Popular Electronics
[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular
Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from
this article was written today, the title would more likely be,
"One IC Pocket Radio," and rather than a couple dozen resistors,
capacitors, and inductors (and a transformer), the might be one
or two decoupling capacitors. Everything else would be contained
within the integrated circuit. There are plenty of single-chip radio
circuits available from distributors like Digi-Key, Newark Electronics,
etc. Oh, and how many of you even know what a phenolic board looks
like? Better yet, how many of you you identify the unique smell
of one heating up or burning due to component overheating? If you
can't, then consider yourself lucky, because the probably means
you're 30-40 years younger than I am, and you have that much longer
to live then me ;-)
See all articles from
One Transistor Pocket Radio
Reflex and regenerative circuits are combined in this sensitive
and stable radio.
By Alvin Mason
and constructing a one-transistor pocket receiver is a challenge
to any experimenter. A good many "pocket" receivers are either too
large or too bulky for true "pocket" operation. Or they simply don't
possess enough sensitivity and gain to pull in stations without
an external antenna.
The little receiver described here
gets around both of these weaknesses. It uses a combination of reflex
and regenerative action to cut size and components to a minimum
and increase sensitivity to striking proportions. The complete unit
measures only 4" x 2 1/2" X 3/4". And it's powerful enough to pull
in every local station on the dial with no external antenna at all!
. Because of the "reflex"
action of the circuit, a single transistor is made to amplify the
signal twice - once at radio frequencies and again, after detection,
at audio frequencies (see "How It Works"). To simplify the circuit,
a diode is used as a detector, leaving the transistor free to do
nothing but amplify.
Also acting to increase the circuit's
simplicity and stability is the regeneration hookup. The circuit
is designed so that the amount of positive feedback or regeneration
doesn't control the overall sensitivity as is usually the case with
regenerative detectors. What's more, there is no regeneration control
or annoying oscillation to contend with.
Since the remarkable
efficiency of this little set doesn't depend on regeneration alone,
only a limited amount of regeneration is used. Its stability is
evidenced by the fact that, once adjusted, the set is as stable
as most non-regenerative detectors.
B1-15·volt battery (two Eveready
404E's or . equivalent in parallel)
capacitor (Lafayette MS-445 or equivalent)
miniature electrolytic capacitor C3-30-µf., 25-volt miniature
electrolytic capacitor C4-.0005-µf., ceramic capacitor
C6-Gimmick capacitor (see text)
L1-Antenna coil for C1 (Superex 2004 or equivalent)
L2-Six turns of #26 insulated wire wound on L1 (see text)
All resistors 1/4 watt
control with s.p.s.t. switch S1 (Lafayette VC-28 or equivalent)
S1-S.p.s.t. switch (on R4)
T1-Coupling transformer (Philco
32-4763-2 or equivalent-see text) 1-2000-ohm impedance earphone
(Lafayette MS-368 or equivalent)
1-4" x 2 1/2" x 3/4" plastic box
1-4" x 2 1/2" x 1/16" phenolic
Misc.-Tuning dial, knob for volume control, wire,
One transistor does the work of two in this highly efficient
circuit. The signal is amplified twice-once at radio frequencies
,and, after detection, at audio frequencies.
HOW IT WORKS
One transistor and
one diode are employed in a circuit that combines the advantages
of both reflex and regenerative action. Because the signal passes
through transistor Q1 twice - once as r. f. and once as a.f.
- the transistor is properly described as operating in a "reflex"
circuit. Adding to the already high efficiency of this circuit
is the regeneration furnished by gimmick capacitor C6.
In operation, the r.f. signal picked up by antenna coil
L1 is tuned by coil-capacitor combination L1-C1 and induced
into secondary coil L2. Fed directly into the base of transistor
Q1, the r.f. signal is amplified and passed to transformer T1.
A portion of the signal from Q1's collector is returned to Q1's
base by capacitor C6 to provide additional gain through regeneration
, The signal induced in T1's secondary is detected by diode
D1, smoothed by capacitor C5, and returned to the base of Q1
through volume-control R4 and coupling capacitor C2.
Transistor Q1 again amplifies the signal, this time at audio
frequencies. The audio signal from Q1's collector is fed through
the primary of T1 to the earphone.
Although a Philco r.f. transformer was used as T1 in the
model, this particular transformer is available only from authorized
Philco distributors and may prove hard to get. However, T1 is in
no way critical - a number of transformers were substituted for
the Philco unit, and most of them worked satisfactorily.
The Argonne AR-162 (available from Lafayette Radio, 165-08 Liberty
Ave., Jamaica 33, N. Y., for $2.95) seems to be a good substitution.
A miniature output transformer measuring only 1" x 3/4" x 3/4",
the AR-162 has identical center-tapped primary and secondary windings
of 500 ohms with a d.c. resistance of 18 ohms. You'll have to remove
the transformer's strap and laminations to fit the unit in the small
plastic box specified in the parts list. But you'll find that this
bit of disassembling proves no problem (see illustration on next
page). The windings are light enough to be held in place with a
strip of transparent tape. The center-taps are not used.
. The chassis is a piece of Formica
or phenolic board about 4" x 2 1/2" x 1/16". Depending on the size
of the components, the chassis should fit into a small plastic box
measuring about 4" x 2 1/2" x 3/4". Homemade printed circuitry was
used on the model, but standard wiring will do just as well. Most
of the component leads are long enough to permit point-to-point
wiring, but a transistor socket was used to prevent possible damage
to the transistor when soldering.
Coil L2 consists of six
to nine turns of No. 26 insulated hookup wire wound on the "ground"
end of L1 and spaced 1/16" from it. "Gimmick" capacitor C6 is made
up of two 1/2" lengths of insulated hookup wire twisted together
several times to form a small capacitor.
It's a good idea
to layout all parts and drill most of the holes in the chassis before
starting assembly. Since the wiring is relatively simple, you should
be able to take your time and do a good job. As with any construction
project, time spent in careful wiring will payoff in the long run.
Parts are mounted on a phenolic board;
R2 and C6 are on the reverse side.
After ail parts have been mounted and soldered in place, double-check
all connections. Now, with the switch off and battery B1 in place,
plug in the transistor. Turn on the set and rotate the volume control
to full on. Select a station, preferably the strongest one on the
dial. Listen for distortion. If necessary, either loosen the coupling
in capacitor C6 by untwisting the leads slightly or by snipping
off the leads bit by bit until the distortion disappears.
Once adjusted, the set should be nearly as stable as the superhet
in your living room. And it's a safe bet that in sensitivity and
portability this little unit will have few equals.
Coupling transformer T1
must be a miniature unit.
If Arqonne Type AR-162 is used, it
can be reduced
in size by removing the strap and laminations.