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AN/GRA-71
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 morse code 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.
  
All items packed inside the container (lid removed)

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.   
All items packed inside the container (lid removed)

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 [4].
 
GRA-71 closed container GRA-71 with its lid taken off GRA-71 with all items stored inside the container Protective caps removed from the connectors Complete GRA-71 with all modules outside the container Clearly showing the two cables of the Keyer Adapter Keyer Adapter GRA-71 with Keyer attached to the T-784 transmitter

 
Operation
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 morse code. 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).

AN/GRA-71 Block Diagram

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.

AN/GRA-71 Tape Layout

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.
 
Components
CA-3B Tape Cartridge Cartridge CO/B-8 Alphabete Coder Coder 1 CO-3B Dot-Dash Coder Coder 2 KE-8B Keyer Keyer KA-3 Keyer Adapter Adapter Alternative alphabet disk Index Camel hair cleaning brush Brush Operating and Maintenance Manuals Manuals

 
Tape cartridge CA-3B (MA-9)
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 morse code 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.
  
CA-3B Tape Cartridge

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.
 
Closed Tape Cartridge Two CA-3B Tape Cartridges CA-3B Tape Cartridge Close-up of the opened Tape Cartridge Side-view of the opened Tape Cartridge Close-up of the tape guides Tape transport mechanism CO-3B dot-dash coder with tape cartridge attached

 
Alphabet coder (CO/B-8, MX4496)
This coder enables an operator to record messages in morse code onto a CA-3B Tape Cartridge as a sequence of letters. Each messages is stored as a series of accurately timed (morse code) 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 word-space button.
  
CO/B-8 Alphabet Coder

 
CO/B-8 Alphabet Coder Attaching the tape cartridge to the Alphabet Coder Alphabet Coder with Tape Cartridge attached Handle raised Operating the handle Pushing the handle down Close-up of the recording head Adding a word-spacing

 
Dot-dash coder (CO-3B, MX-4495)
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 morse alphabet 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.
  
CO-3B Dot-dash Coder

 
CO-3B Dot-dash Coder Dot-dash coder in operation Compact dot-dash coder Opening the dot-dash coder Placing the tape cartridge Attaching the tape cartridge CO-3B dot-dash coder with tape cartridge attached Close-up of the tape cartridge mounted on the CO-3B coder

 
Alternative index disk (for CO/B-8)
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).
  
Alternative alphabet disk

 
Keyer (KE-8B, KY-468)
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.
  
KE-8B Keyer

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.
 
Closed Keyer with crank in storage position KE-8B Keyer Keyer rear panel Operating the crank to wind-up the mechanism Placing the Tape Cartridge on the Keyer Attaching the tape cartridge Keyer with Tape Cartridge attached Putting the crank in its storage position

 
Keyer Adapter (KA-3, MX-4498)
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 morse code. 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.
  
Keyer Adapter

 
Keyer Adapter Keyer Adapter: top view of the empty container Close-up of the Keyer Adapter Keyer (with tape cartridge) connected to the Keyer Adapter GRA-71 with Keyer attached to the T-784 transmitter

 
Soft cleaning brush
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.
  
Camel hair cleaning brush

 
Closed cleaning brush Camel hair cleaning brush Using the cleaning brush

 
Manuals
Operator usage and maintenance of the AN/GRA-71 is explained in the small (A5 size) instruction manual TM 11-5835-224-12 [1]. 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 [2].
  
GRA-71 Manuals

 
Connections
The Keyer (KE-8B) has a 7-pin Winchester M-series socket [3] 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:

KE-8B pinout, looking into the Winchester M7S socket.

J2 - Keyer socket (M7S)

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:

KA-3 pinout of J1, looking into the Winchester M10S socket on the transmitter.

J1 - Transmitter socket (M10S)

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.
 
Designators
Description CIA Army designator #  
Alphabet Coder CO/B-8 MX-4496/GRA-71 1  
Dot-dash Coder CO-3B MX-4495/GRA-71 1  
Tape Cartridge CA-3B MA-9/GRA-71 2  
Keyer KE-8B KY-468/GRA-71 1  
Keyer Adapter KA-3 MX-4498/GRA-71 1  

 
Compatible radios
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.
 
References
  1. US Army, AN/GRA-71, Operator and Organizational Maintenance Manual
    TM 11-5835-224-12. Reprint, June 1964 (digital copy).

  2. US Army, AN/GRA-71, Depot Maintenance Manual / Repair Manual
    TM 11-5835-224-35. 25 July 1969. 1

  3. Winchester, M-Series connectors - datasheet
    Date unknown. Retrieved December 2012.

  4. Peter McCollum, GRA-71 Burst-Coder
    Website 1999-2012. Retrieved December 2012.

  1. Supersedes TM 11-5835-224-45, 15 February 1967, and TM 11-5835-224-45P, 24 June 1966.
Further information

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