July 1946 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Applying the adjective
"portable" to electronics equipment in the vacuum tube era was often a stretch
of the term. Usually, portable radios were backpack ordeals with a separate
battery carried by second person. They can be seen in a lot of the old War
War II movies. It was really not until the advent of the "Handie-Talkie" two-way
radio sets that a truly hand-held, self-contained system was available. This
"Inside the Handi−Talke" article from a 19467 issue of Radio−Craft magazine
provides a very detailed look at the internal components and of the schematics.
"Walkie−Talkie" was another name often given to the Handi−Talkie, although some
might argue that there is a distinct difference.
Handi−Talkies used to be widely available on eBay, but are very scarce at
this time - and the sale prices reflect that reality. A lot of the listings use
the term Handi−Talkie as a ruse to be included in a search of the real item.
The Walkie-Talkie - March 1955 Popular Electronics,
Self-Contained Handie-Talkie - June 1944 QST, and
The New Handy-Talkie - December 1942 Radio-Craft,
Walkie-Talkies: Something for Everyone - April 1974 Popular
Handie-Talkie - June 1944 QST,
Inside the Handie-Talkie
- July 1946 Radio-Craft.
Inside the Handie-Talkie - War's Smallest Two-Way Radiophone
This Handi-Talkie chassis view shows the main components.
Handi-Talkie antenna switch detail.
By Robert F. Scott, W4FSI
The Handie-Talkie is perhaps the most widely known and well liked radio set developed
during the war. Its efficiency at short ranges and its compactness created a demand
for this set in practically all branches of the service. Called by the Signal Corps
"Radio Set SCR-536". the Handie-Talkie is built into a metal case 3 5/8 x 5 3/8
x 15 3/4 inches long. This space contains a receiver-transmitter complete with batteries,
microphone and ear-phone. Compactly built and weighing but 5 1/2 pounds, it permits
the operator to use it like a French type telephone.
The SCR-536 is designed for two-way voice communication over distances up to
one mile. The effective range is often decreased by terrain features like hills,
steel buildings, heavily wooded areas and power lines located between the two stations.
On the other hand, the range may be increased by locating the transmitter on the
top of a tall building or a hill where the signal will have an unobstructed path
to the receiver. Fool-proof net operation is made possible by using crystal control
in the receiver and transmitter circuits. This feature permits instant operation
by untrained operators who would not be able to make frequency adjustments on other
types of sets. These sets may be tuned to practically any predetermined frequency
between 3500 and 6000, kc by the selection of proper coil and crystal combinations.
Frequency changing in these sets is done by men who have been trained to do this
Operation is further simplified by the omission of several conventional controls.
The only ones on the set are the "on-off" switch and a fingertip operated "push-to-talk"
button. The "on-off" switch is mounted inside the set and is operated by the telescoping
antenna. When the antenna is fully extended, the switch is thrown on and when retracted,
is automatically turned off.
Coils and crystals for changing Handie-Talkie frequencies.
Bottom view of Handie-Talkie. Important features are identified.
By careful selection of operating frequencies, these sets may be used in nets
with such other sets as the SCR-245 and SCR-193 which employ variable frequency
control and may be tuned to the frequency of the SCR-536.
How it Operates
The receiver portion of the set uses a conventional superheterodyne circuit with
a 3S4 r.f. amplifier, a 1R5 oscillator-mixer, 1T4 i.f. amplifier" 1S5 second detector,
first a.f. amplifier and a.v.c. A 3S4 audio amplifier supplies approximately 0.18
watt- to a small dynamic earphone.
The grid circuit of the r.f. amplifier is tuned by a series combination of the
antenna, antenna loading coil and a 0.006-μf fixed mica condenser, as shown in
Fig. 1. The plate circuit of this stage is loaded by a permeability-tuned coil and
is capacity-coupled to the signal grid of the 1R5.
Crystal control of the receiver oscillator frequency is maintained by connecting
a quartz crystal between the No.1 grid and screen grids of the 1R5. Excessive oscillator
voltage on the signal grid is reduced by a 7-μμf ceramic condenser connected
between the signal and oscillator grids. This condenser neutralizes a portion of
the oscillator frequency voltage by returning it to the oscillator grid out of phase
with the internally coupled voltage.
The 1T4 and 1S5 are connected in the standard i.f. and second detector circuits
common to most battery receivers. Midget i.f. transformers are used throughout.
The secondary winding of the input transformer is loaded with a 1-megohm resistor
to broaden the i.f. tuning.
Impedance coupling is used in the output circuit to prevent plate current from
flowing through the earphone.
When the set is in operation, it is a receiver unless the "press-to-talk" button
is depressed. A diagram of the complete set is shown in Fig. 2. All switches are
in the "receive" position.
The Transmitter Circuit
When the set is used as a transmitter, switching converts the r.f. amplifier
and oscillator-mixer portions of the receiver to a master oscillator-power amplifier
transmitting circuit. The first a.f. and power amplifiers of the receiver are converted
to microphone amplifier and modulator for the transmitter.
While transmitting, the "push-to-talk" button must be depressed. The circuit
of the set is then changed to that of the diagram in Fig. 3, and a crystal of a
frequency 455 kc. lower than the receiving crystal is switched into the circuit.
The output of the oscillator anode is capacity coupled to the grid of the 3S4 r.f.
power amplifier. The r.f. tank coil is switched into the circuit and tuned by a
0.00014-μf variable tuning condenser. This combination of coil and condenser
is called a "tank" circuit. The antenna loading coil is connected in a pi network
tuned by the tank tuning condenser and the capacity existing between the antenna
Fig. 1 - Handie-Talkie schematic switched to the "receive" position.
Letters refer to switches shown in schematic form in the drawing below. Note that
only one side of the two 3S4 filaments are heated when the set is used as a receiver.
Tuning is controlled by the oscillator crystal.
Fig. 2 - Schematic of the war's most famous piece of communications
equipment - the Handi-Talkie. Made by Galvin Motorola for the Signal Corps, it was
used "in the air, on land and on the sea." Each set employs two crystals ground
to frequencies 455 kc apart. The 1R5 acts as a Pierce oscillator in both transmitting
and receiving circuits. The 14-section changeover switch is lettered to agree with
the other two figures. Early Handie-Talkies had crystal earphones, but later ones
used the inductor type illustrated in these diagrams.
Fig. 3 - As a transmitter, the Handie-Talkie is a four-tube set.
The 1R5 functions as master oscillator in a Pierce circuit, driving one of the 3S4's
as r.f. power output tube. The 1S5 and the other 3S4 are speech amplifier and modulator,
Heising system being used.
The dynamic microphone is transformer-coupled to the grid circuit of the 1S5.
This tube functions as a speech amplifier and is resistance-coupled to the 3S4 modulator.
The grid return of the 3S4 modulator is tapped into the grid leak circuit of
the oscillator tube, to provide proper operating bias of the modulator. The a.f.
choke in the plate lead serves as the coupling choke for modulation of the power
amplifier. A portion of the a.f. output of the modulator is applied to the earphone
to provide a sidetone for monitoring transmission.
The "push-to-talk" switch is made up of 14 sections, which perform the intricate
switching functions which convert the receiver to a transmitter. Each section of
the switch has been given a letter to simplify a study of the diagrams. In Fig.
1, all sections of the switch are shown in the receive position. All of the switching
functions are evident from the diagrams.
Many of the resistors and condensers of this set are mounted in small circular
"cups" that fit around the underside of the tube sockets. Terminal lugs are mounted
on the rim of the "cups" so that the parts may be connected to the external circuits.
Later models abandoned the cup system and used straight terminal strips, as shown
in the photo on page 684.
The transmitter is easy to adjust. A crystal and plug-in coil for the desired
frequency are plugged into the unit and the antenna fully extended. The "push-to-talk"
switch is held down and the r.f, amplifier tank condenser adjusted to give minimum
amplifier plate current. The plate current is metered by removing a jumper located
at the lower end of the chassis.
The receiver is tuned by inserting coil and crystal combinations for the desired
frequency. The receiver oscillator crystal is ground to a frequency 455 kc higher
than the transmitter operating frequency. The receiver is then aligned by conventional
Direction Finder, Too
The Handie-Talkie used as a homing device.
In the latter days of the war, an accessory was added to the SCR-536 which permitted
it to be used for direction finding for locating a command post or unit whose location
is not known, or for "homing" in fog or darkness.
This is a directional loop antenna used in place of the non-directional telescoping
antenna. A modification kit was also provided that permitted the operator to use
The loop consists of two windings, a single turn secondary winding and a four
turn primary winding. The primary of the loop is tuned by a 100-μμf variable
condenser. The loop assembly is connected by cable to a matching transformer that
matches the impedance of the loop to the input impedance of the receiver. The matching
transformer screws to the antenna cap stud and consists of a tuned auto-transformer.
Its output lead is equipped with a clip that fastens to the vertical antenna.
When the loop and transformer have been tuned to give maximum response from the
desired station, the loop is rotated through 360 degrees. There will be two points
where the signal strength drops to a sharp null. These points will occur when the
loop is broadside .to the direction of the signal source. This will indicate that
the signal is coming from one of two directions 180 degrees apart. To determine
which is the true direction, a "Sense Button" permits the body to act as a vertical
antenna. Rotating the loop 90 degrees in each direction from the null will produce
a stronger signal in one direction than in the other. When the loop is turned to
the loudest signal, an arrow on its base indicates the true direction. The thumb
is then removed from the "Sense Button" and the loop turned to the original null.
The operator may then direct himself toward the transmitting station by traveling
in a direction perpendicular to the plane of the loop in null position.
Posted October 29, 2021