[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 (if any) are hereby acknowledged.
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
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
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! Reflex Circuit
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)
C1-365·µµf. variable capacitor
(Lafayette MS-445 or equivalent)
C2-10-µf., 25-volt 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
R4-10,000-ohm volume control
with s.p.s.t. switch S1 (Lafayette VC-28 or equivalent)
switch (on R4)
T1-Coupling transformer (Philco 32-4763-2 or
equivalent-see text) 1-2000-ohm impedance earphone (Lafayette MS-368
1-4" x 2 1/2" x 3/4"
1-4" x 2 1/2" x 1/16" phenolic board
dial, knob for volume control, wire, solder, etc.
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
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.
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.Operation
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.