Burst Encoder (USA)
The AN/GRA-71 was a burst encoder
that was built for the US Army by
Stenographic Machines in Skokie (Illinois, USA).
It allowed pre-recorded
coded messages to be broadcast in
at very high speed, in order
to minimise airtime and avoid the risk of
Direction Finding (DF).
It was initially supplied
with the American AN/GRC-109 Special Forces radio
— known by the CIA as the RS-1 spy radio set —
and was connected directly to its (modified) T-784 transmitter.
It was also used in combination with other radio sets,
such as the PRC-64A,
the PRC-74A and the RS-49.
The GRA-71 was a modular system,
consisting of separate recording and
play-back devices. Messages were recorded in
morse code onto
industrial-grade magnetic recording tape,
using a passive entry device
(i.e. without batteries).
The complete system is shown on the right.
The recording tape itself was housed in a small cartridge that was
suitable for a dead drop and could
be attached to either the recorder
or the play-back device (the keyer).
All components were small enough to be carried on the body and could
easily be concealed by a field agent.
A complete GRA-71 consists of a water tight container
in which the various components are stored. Each item has its own slot,
so that it can easily be checked for completeness.
All items can be removed from the container (see below), except for the
Keyer Adapter which is a permanent part of it.
It connects the Keyer to the radio via the
two cables at the back.
Each cable has a suitable connector at one end,
whilst the other end disappears into the container.
The cables are shown here in their storage position, with
protective caps at the end.
The complete kit consists of the following items:
Each item is described in more detail below.
Burst encoders like the GRA-71 were very popular with the
American Special Forces (SF) during the Vietnam War, as
they drastically shortened the time needed to occupy the
limited frequency space that was available at the time. The CIA used
burst encoders also to limit the risk of interception and
Direction Finding (DF)
by the enemy. The GRA-71 was used well into the 1970s
when it was replaced by digital systems.
The price of a single GRA-71 unit was US$ 759.14 .
Messages are recorded onto magnetic tapes (CA-3B), using one
of the two supplied coders, much like using a domestic tape recorder.
One coder (CO/B-8) has a disc with the 26 letters of the alphabet.
It allows letters to be recorded directly in
Alternatively, the dash-dot coder (CO-3B) could be used, allowing
any morse character to be composed manually (e.g. numbers and punctuation marks).
Once the recording was complete, the Keyer (KE-8B) was connected to the
transmitter in order to play back the message at high speed (approx. 300 WPM).
The diagram above clearly shows the flow of information when using the AN/GRA-71.
At the left are the two possible routes that the user can choose. Whether the user
selects the CO-3B dot-dash coder or
the CO/B-8 alphabet coder, the messages are
always stored as a sequence of (morse) dots and dashes onto a
CA-3B tape cartridge.
The tape has two separate tracks:
the upper track is used to record the dashes, whilst the lower track is used for
recording the dots.
In the above example, the text NOW IS THE TIME
(in morse: -· --- ·-- ·· ··· - ···· · - ·· -- ·)
is recorded onto a tape
cartridge. The coders are mechanically constructed in such a way that an
accurate timing is guaranteed for the dashes, dots and the spaces between
letters and words.
Once the message is complete, the cartridge is removed
from the coder and attached the the actual Keyer (KE-8B).
As the keyer can not be connected to old valve-based transmitters,
the KA-3 Keyer Adapter had
to be used as an interface for the higher voltages and currents.
Later transistor-based transmitters, like the PRC-64A,
could be connected directly to the KE-8B Keyer.
Pre-coded messages are recorded onto standard magnetic (ferro) tape,
similar to the tape used with domestic open reel audio recorders.
Each cartridge contains 12.5 feet of 428 industrial-grade recording tape
manufactured by 3M, on which the dots and dashes of the
are stored as a series of pulses in two individual tracks
(see the illustration above).
When recording the morse signals, the tape is advanced mechanically
in such a way that accurate timing for the dots and dashes is guaranteed.
When the tape is played back by the KE-8B Keyer, it is advanced at
a speed of 4.5" per second by the wind-up motor. This way, the dots
are 3 ms long, and the dashes take 10 ms.
When in storage, a hinged lid protects the tape against dirt and damage.
Two identical tape cartridges are supplied with the set.
When used for espionage, a tape cartridge is small enough to fit inside
a concealment like a dead drop.
Once a full message is recorded onto the tape, the cartridge is removed
from the coder and will automatically rewind itself by releasing an
internal spring mechanism that has been wound-up during recording.
As a result, the tape will return to the start of the message;
ready for the Keyer.
This coder enables an operator to record messages in
onto a CA-3B Tape Cartridge as a sequence of letters.
Each messages is stored as a series of
dots and dashes that are stored onto
two individual tracks of the tape (see the illustration above).
In order to use the CO/B-8, the operator does not have to be
familiar with the morse alphabet.
He simply selects the required letter and
pushes down the handle.
The handle activates a small built-in power generator,
which produces just enough energy to electrically store
the dots and dashes onto the ferro-magnetic recording tape.
Numbers and punctuation marks can not be created with this coder,
but numbers can be replaced by letters.
Spaces between letters are inserted automatically.
Spaces between words can be inserted with the
The CO-3B coder was supplied as an alternative to the heavier
CO/B-8 alphabet coder (see above). It had only three buttons:
one for dots, one for dashes and one for spaces, and allowed
each possible morse character to be composed.
This coder required the operator to be familiar with the
and was often preferred as it was the lighter
of the two. Like the CO/B-8, the CO-3B is a passive device
that does not require an external electrical power source.
Pushing down a button produces just enough electrical energy
to store the data onto the tape.
Alternative index disk
The CO/B-8 coder was supplied with two alphabet discs:
one disc with the letters A-Z in reverse order and an inner
ring (in red) that contained the letter A-Z in the correct
order. The image on the right shows this disc mounted on
the CO/B-8 coder. At the reverse side, this disc contains
the alphabet in reverse order (Z-A).
A spare disc was supplied with the letter (A-Z) in the
correct order, corresponding directly with the morse
characters A-Z. The numbers (0-9) are always sent as
letters (A-J or Q-Z).
The KE-8B (also known as the KY-468) is the actual Keyer and can be
regarded as the heart of the system. It contains
electronic transistor-based circuits and a spring-loaded
wind-up mechanism for playing back pre-recorded tape cartridges.
In addition it needs a 12V DC power supply.
The keyer can be connected directly to a solid-state (i.e.
transistorized) transmitter, using the 7-pin Amphenol socket at
the rear. It is then powered directly by the 12V of the transmitter.
When using the Keyer in combination with a valve-based transmitter,
such as the T-784,
the KA-3 Keyer Adapter is needed as an interface.
Before sending a message, the spring-loaded mechanism inside the
Keyer should be fully wound-up, by turning the hand-operated
crank clockwise several times.
This produces enough energy to keep it running for approx. 1 minute.
A small hinged door at the front, protects the reading heads
from dust and damage. After opening the door, the
Tape Cartridge (CA-3B) containing the message, is
attached to the Keyer.
Next the ON/OFF switch at the rear
is set to the ON position, after which the wind-up motor starts
running and the message is sent. At the end of the message,
the transmitter will be turned OFF automatically, approx.
one second after the last character has been sent. The keyer
should then be stopped manually, by sliding the ON/OFF switch
back to the OFF position.
The Keyer can also be used to erase a Tape Cartridge by
winding-up the mechanism, attaching a pre-recorded tape,
sliding the ERASE switch to the upper position (and keeping
it in that position) whilst sliding the ON/OFF switch to
the ON position. The ERASE switch will now be locked-in.
Also at the rear of the Keyer is a switch
marked IDY (identification). When engaged, it uses the dot-channel
of the Keyer to send a continuous sequence of dots at a
rate of 300 wpm.
When using the Keyer (KE-8B) in combination with a valve-based
transmitter, such as the T-784 of the
RS-1 spy set (AN/GRC-109),
the Keyer Adapter (KA-3) should be used as an interface. It converts
the signals from the Keyer to the high-voltage high-current signals
needed for the valve-based circuits of the transmitter.
It is not needed for transistorized transmitters.
The transmitter is modulated by switching the anode of the Power
Amplifier (PA) valve in the rithm of the
The Keyer Adapter also contains an R/C timing circuit that enables the
oscillator of the T-784 transmitter for the duration of the
transmission. Once the last character has been sent, the oscillator
is disabled after approximately one second.
Additionally, the Keyer Adapter contains a power supply circuit
that converts the 6.3V AC filament voltage of a transmitter
into 12V DC needed for the transistor-based circuits in the Keyer.
A soft camel hair cleaning bush is supplied to allow the contact
and the mechanical parts to be cleaned regularly.
It is usually stored in the container, in a small hole between
the alphabet coder and the keyer.
The brush itself works like a lipstick; take off the cover and rotate the shaft to
reveal the brush. Rotate backwards to retract the brush again.
Operator usage and maintenance of the AN/GRA-71 is explained in the small
(A5 size) instruction manual TM 11-5835-224-12 [A].
The booklet contains 76 pages with full instructions, maintenance tips and
examples of messages. It also contains a series of black and white photographs
of the various parts, and explain the operating principle of the devices.
Although original copies of the User Manual are rare, good
quality reproductions are relatively easy to obtain and are often found
on auction sites like Ebay.
The manual shown in the image on the right shows a reprint of the 1969 issue,
that is based on the initial version of 1964. This release includes the
changes made in 1966.
In addition, the Depot Maintenance Manual TM 11-5835-224-35 (also shown
on the right) contains
full circuit diagrams, parts lists and circuit descriptions of
the various modules of the GRA-71. It is available for download below [B].
The Keyer (KE-8B) has a 7-pin
Winchester M-series socket  at the rear,
allowing the unit to be connected directly to a transistorized (solid-state)
transmitter. The transmitter needs to supply 12V DC for the electronic circuits
inside the Keyer. The pin-out of the socket is as follows:
Note that the SIGNAL lines are galvanically isolated from the 12V circuitry.
In most cases, a purpose-built cable will be needed between the
Keyer and the transmitter.
When used in combination with a valve-based transmitter, such as the
T-784 of the RS-1 spy set (AN/GRC-109),
The Keyer Adapter (KA-3)
(part of the GRA-71 container) is used as an interface.
A fixed cable on the KA-3 (marked TRANS) has an M10P connector at the end,
with this pinout:
J2 - Keyer socket (M7S)
The 6.3V filament power from the transmitter (AC or DC) is supplied by
the transmitter on pins H (6.3V) and J (GND) of J1. This voltage is converted
by the KA-3 into 12V DC that is available on pins H (+12V) and C (-12V)
of J2. Pins C and F of J1 are both connected to the screen grid of the PA
valve of the transmitter, which is keyed via the cathode of the PA valve
using the signal at pin B. Pin A provides a timed signal to the cathode of
the oscillator valve, keeping the oscillator ON whilst sending a burst
message. The HT voltage (B+) is received from the transmitter on pin E.
J1 - Transmitter socket (M10S)
Most solid state radios can be used in combination with the
GRA-71, although in some cases a special cable is needed for
this. Apart from the radios listed below, the GRA-71 was used
with a variety of Cold War spy sets, both in the USA and in
Europe. In The Netherlands, the GRA-71 was used by Korps
Commando Troepen (KCT), the Dutch Special Forces, in combination
with the TRC-77 radio. This radio was also very popular with the
Special Forces in Vietnam at the time.
Supersedes TM 11-5835-224-45, 15 February 1967,
and TM 11-5835-224-45P, 24 June 1966.
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© Crypto Museum. Created: Monday 03 August 2009. Last changed: Friday, 30 June 2017 - 07:12 CET.