GRA-71 Coder-Burst Transmission Group
NSN 5820-00-056-6856

Brooke Clarke, N6GCE, 2000 - 2020


I have one of these sets.  It works with the T-784/GRC-109 (see Fair Radio Winter Spring 1997 catalog page 11 for the T-784/GRC-109 Xmitter.  The xmitter has modifications to increase it's bandwidth, to allow Morse code to be sent at a very high data rate (300 WPM).  This implies that some receiver would also need to have an especially wide bandwidth to receive the signal from the transmitter.  Note that these xmitters were used by special forces units operating behind enemy lines.  By speeding up the transmission  it was much more difficult for an intercept operator to pick up for two reasons, first it "spread" the bandwidth making it harder to hear on a conventional receiver and it reduced the time that the transmitter was on the air giving "lower probability of intercept".  These were used in Vietnam by forward observers.  The GRA-71 uses magnetic tape recorder technology to store the messages.  Each of the component parts has a heavy duty military label with the manufacturer: Arvin Industries, Inc. and the contract number: FR-36-039-E-5-15509(E).  I think that Arvin later changed their name to Motorola.
Overall Photo  Another Photo from Toronto Surplus
There was an earlier version(scroll to # 190) called the M-108 that had a similar look, only it used paper tape in which holes were punched.  It also sent high speed Morse code by contact closure.  If you have any info on the M-108 or know about the radios it was used with, please let me know.

This keyer was introduced for the GRC-109 transmitter, but will also work on a number of other transmitters like the PRC-64 (TM 11-5820-552-15), PRC-74A or B  and the PRC-104.

There is a Russian version that has a telephone type dial with 12 positions (1, 2, 3, 4, 5, 6, 7, 8, 9,< ...>, 0, <space>).  It uses metal tape in a removable cartridge.

KY-468/GRA-71 Keyer (KE-8B Keyer) Serial 104

Controls End Photo    Tape Head End Photo
This is the unit that reads the prerecorded magnetic tapes and modulates the T-784/GRC-109 transmitter.  It also has the capability of erasing a tape.

The lid is similar in concept to a Zippo lighter and can be removed.  Once the lid is removed the tape magazine can be attached to the top.  There is a two track tape head, the gear to drive the tape magazine and two pins with groves to allow the magazine to attach to the keyer.  On the large face there is a manual crank to wind up the clockwork mechanism that will drive the tape over the head.  There is a governor mechanism to regulate the tape speed.

On the bottom face are:

An Amphenol 6-pin socket with 5 pins on the outer circle and a single pin in the center.  This connects to the cable in the main box.  It is a slightly different connector than the one that goes to the transmitter so that they can not be connected to the wrong unit.
A black sliding switch with the label "IDY" that actuates a switch that causes a series of dots at 300 Words Per Minute..
The ON OFF switch in mechanically interlocked with the ERASE switch.  When turned ON the tape moves and the data is sent.  If the ERASE switch is lifted and held while the power switch is turned to ON, then the tape will be erased.

Inside there are two circuits that look to have almost the same components indicating that one is for "dots" and the other is for "dashes".  When using magnetic tape, this type of coil  tape head and the "mechanical pulse generator" the only type of data that can be recorded is a pulse, not a level.  Therefore you can not record a dash, only a pulse on the dash head. I think that when the dash electronics see a pulse they wait for a time abut the length of a dash, close the switch that causes the transmitter to start sending and keep sending until 2 or three gear teeth have been turned.  When the dot electronics see a pulse they wait a dot time then turn on the transmitter for one gear tooth.  This is all guessing since I have not yet connected the GRA-71 to a transmitter for testing.  The tape plays at 4.5 inches/second.

It is 3 1/8" high, 3 1/8" wide and 1 3/16" thick.  The newest date code found on the transistors in this unit was 6613 indicating the 13th week of 1966, or mid March 1966.

Connector Pins

Chassis ground
12 V return
Signal open NPN collector
10 Ohms to D
Signal return, emitter
+12 V <= 40 ma

The Connector was made by Amphenol as their hexagonal Section 6 but is now made by WPI in Salem as their 126 series.
It is also made by Continental connector.  Available from William Perry.

MA-9/GRA-71 Magazine, Recording Tape (CA-3B Cartridge) Serial 205 & 207

There are two tape magazines included with the GRA-71 set.  They have a spring that causes them to automatically rewind if they are removed from any of the other units.  The lid is like that on a Zippo lighter and is designed to slip off it's hinge.  When the lid is removed about 1/4" of both the take up and supply reels are visible. The supply reel has ten radial white lines engraved on it so that the operator can see that the tape is advancing or playing.  There are two holes that have a thin sheet metal plate also with two holes that slides and is spring loaded.  The end of the sheet metal slider is also the spring loaded latch that holds the lid closed.  The sheet metal slider latches the tape magazine onto the other units and the latch part can be pressed to release the magazine, allowing the spring to rewind the tape. The tape is 12 feet long and can be replaced.  The magazine is 3 3/16" wide, 1 7/8" high and 1" thick.

MX-4495/GRA-71 Dot Dash Coder, Tape (CO-3B Coder) Serial 104

This is one of the units that writes data onto magnetic recording tape in the tape magazines.

It has a hinged lid that is the same size as it is.  The two track  tape head is at the end with the same provisions as the MX-4496 to mount the tape magazine and a gear wheel to advance the tape.
There is 12 feet of tape in a cartridge that will play for 32 seconds.  This is good for 125 groups of 5 letters.

Along the bottom edge from left to right are three push buttons marked "." "S" and "_". That is dot, space and dash.  The space button just advances the tape without writing any data.  The dot sends a pulse to the "dot" recording head and advances the tape one gear tooth.  The dash button writes a dash to the dash head and advanced the tape 2 or 3 (it is not clear to me) gear teeth.  It uses the same "mechanical pulse generator" as the MX-4496 described below.

Note that this tape recorder and the MX-4496 write to the tape using no batteries or outside electrical power.  This means that someone could use this kit to write tapes in a location that did not have any electrical power to be sent at a transmitter site that may be operated by someone else besides the person that posses the GRA-71.  Since no batteries or other electrical power is needed to write the tapes this system is extremely reliable. A little over 3" high, 2 5/8" wide and  1 3/16" thick.

MX-4496/GRA-71 Wheel Coder, Tape (CO/B-8 Coder) Serial 104

MX-4496This is one of the units that writes data onto magnetic recording tape in the tape magazines.

It can be fitted with one of two wheels that can be turned to 26 different positions with a nice feeling detent action.  One wheel had the letters of the alphabet in a clockwise direction in white engraved letters on a black background.  It also has the numbers from 1 to 9 then 0 clockwise starting with the letter A.  The other wheel also has 26 positions.  The letters of the alphabet nearest the outside are in a counterclockwise direction and are white.  Inside these letters there is another alphabet in red letters going clockwise and with the letter A (red) next to the letter Z (white).  Inside the red letters A through Q and the numbers 1 to 9 and 0.  In the center along the edge of the MX-4496 closest to where the tape magazine fits (the top) is a red index mark abut the same width as one letter.  To the upper left of the wheel is a black push button that advances the magnetic tape one unit but does not write any data on it.  There is a "U" shaped handle that has it's hinge points about half way along the right and left sides and the handle part at the bottom.  In the bottom part of the handle there is a fold out extension that when folded out extends the length of the handle to make it easier to operate it quickly and surely.

At the top there is a hinged cover that can be removed from it's hinge to allow attachment of a tape magazine.  The tape head has two tracks. There is a gear that mates to the tape magazine to advance the tape with each stroke of the lever.

Inside the unit there is another wheel made of light brown colored insulating material like printed circuit board material but with no metalization.  Mine has the letter T marked on it.  This wheel is positioned by the outer code wheel.  There are 12 pins arranged in two columns of 6 each. One column is for "Morse code "dots" and the other column is for "dashes".  In a CCW direction the standard Morse alphabet is encoded in the wheel.  For example "A" is the right column in row one and the left column in row two (dot dash).  As the lever ii raised it just ratchets up and has no action.  When the lever is lowered it causes the pins to lift first in row 1 then in row 2 and so on through row 6.  In Morse code the 26 letter are represented by no more than four elements (dots or dashes).     For the 4 element code there are: 2 (single character letters, E T) + 4 (two element characters A I M N) + 8 (three element characters D G K O R S U W) + 16 (four element characters B C F H J L P Q V X Y Z and 4 that are not used) for a total of 30 combinations.

As the lever is being pulled down a wheel with a ratchet mechanism causes a lever to move rapidly inside a coil of wire.  The coil looks like what you would see on a relay.  But in this case there are permanent magnets and an iron frame around this "mechanical pulse generator".  Each time the toothed ratchet wheel snaps the lever a pulse of electricity is generated.  There are two switches that can route the pulse to either the left or right recording head depending on whether a pin in the left column or the right column is allowed to raise up into the internal code wheel.  There are also 6 cams that act like a car odometer to cause the sequential activation of rows 1 through 6.  This unit is about 4.5" top to bottom, 3.25" wide and 1.25" thick.

It looks like a dot causes the tape to advance one gear tooth but a dash causes it to advance 2 or 3 teeth and after the active elements are through the tape does not advance.  This seems to be a good thing since it would not waste the tape.

28 Aug 2003 - Upon correcting some typos on this page it occurs to me that the two tracks on the tape are not dot and dash, but rather key down and key up commands.

MX-4498/GRA-71 Keyer Adapter (KA-3 Keyer Adapter) Serial 104

The box has a built in power supply for the Keyer and a cable to go to the T-784/GRC-109 Transmitter and another to go to the Keyer.  Note that the keyer and power supply are the only components that have active electronic circuits.  The box has pockets to hold all the components as well as another external code wheel for the MX4496 tape coder and a camel hair cleaning brush.  My box does not appear to have ever had an external label.  A couple of stick on labels have been removed.  A white paint stencil inside the lid is "IMP" and using hand number stamps what may be the serial number: 556047.  My box came missing the front panel that holds the power supply.   I have replaced the panel with an aluminum panel with holes that match the rubber liner for the pockets to hold all the components.  It is about 9' long, 6" wide and 5.5" high with the cover on.  It weighs less than 10 pounds.
The newest date code found on the transistors in the power supply was 6613 indicating the 13th week of 1966, or mid March 1966.

Transmitter Connector Pins

Osc Cathode
PA Cathode
C & F
Screen Grid
B+ Supply
6.3 V AC/DC

GRC-109 Jumper Plug

A + B + L
E + L
C, D, F, H, J

The PRC-74

CX-10239 GRA-71 to PRC-74 CableCX-10239/PRC-74

PRC-74(?B & C) HF Transceiver can connect directly to the GRA-71 Keyer (without using the keyer adapter) by means of the CX-10239/PRC-74.   See TM 11-5820-590-12 available on line at ETM.

The CX-11468/U is mentioned in the PRC-74 manual as suitable for use with the GRA-71 Keyer, but this cable is for using a hand key to send CW.  It has a standard U-229 connector on the radio end and a couple of spade terminals on the other to connect to a key.  Pin "A" for ground and pin "E" for the key.

Note that a SSB HF radio can NOT use the PTT line for CW since there's no carrier when there's no audio.  So these radios typically have an audio generator that sends a tone when a different line (typical mil HF radio AUDIO connector pin "E") is grounded.

WPI 126 Connnector

A, C, F
Chassis Gnd
12 V return
A, C, F
Chassis Gnd
12 V return

CW Key
Open NPN collector
signal return

* The PRC-74(?B & C) has a power supply connection just for the GRA-71.

There is also a CX-11468/U cable that probably looks the same.  Notice it has the /U Universal suffix rather than the PRC-74 specific suffix.

AN/GRA71-PRC316 Adaptor Unit (British A16 High Speed Morse)

uses what appears to be the same tape cartridges and works with their A16 field HF radio.
Photo1 and Photo 2..

GSH-17 Receiving System for GRA-71 Transmissions

RD-265/GR Recorder Reproducer NSN 5835-00-901-1086

This is part of the GSH-17 system for receiving the GRA-71 transmissions.  Other parts of the receiving system include the CV-1716/GR that converts the 455 kHz IF receiver output into an audio frequency on-off keyed tone of about 10 kHz.  The RD-138/GR is another audio recorder.  Following is a quote form TM 11-5835-228-34.

2. System Application
a. The recorder-reproducer is a three-channel, audio frequency record and playback unit that uses three-track endless loop magnetic tape cartridges. Two of the channels are used to record and reproduce audio information. The third channel is used to record and sense a control signal (cue tone). The recorder assembly is primarily intended to record, on magnetic tape, high-speed Morse code data transmitted at speeds up to 300 words per minute. The outputs of the two audio information channels in the reproducer assembly, are separately applied to individual earpieces of a binaural headset. The cue tone signal is recorded at the start of each message on the third track of the tape. A third channel in the recorder-reproducer is used to sense the presence of the cue tone. Whenever the cue tone is sensed (record or playback operation), the tape drive is disabled. In the record operation, it prevents the recording of additional information over the initial recording. In the playback operation, it insures that the tape is in the proper position to play back the information from the beginning of the recording.
b. The recorder-reproducer is used with two communications receivers, operated in diversity mode. The intermediate-frequency (IF) output of each receiver is converted to 10 kilocycle (kc) and applied to the audio channels of the recorder- reproducer. The continuous-wave (cw) information is recorded on magnetic tape at a speed of approximately 15 inches per second. The recorded information is later played back on a reproducer with a variable speed of 0.5 to 2 inches per second (ips) (10 to 40 words per minute (wpm)), thereby enabling the operator to select a speed which is compatible with his copying ability. The playback speed selected (0.5 to 2 ips) reduces the recorded 10-kc frequency to a lower frequency (approximately 333 cycles per second (cps) to 1,333 cps).

RD-60 Code Recorder

I think this is a pen on paper tape receiver that can receive 300 WPM Morse code transmissions.
23 Oct 2004 - The Parts list manuals TM 5805-253-20P and -34P are on Logsa, but not anything more useful.
9 Feb 2010 - the manual may be dated 3-Sep-54 according to the Historical Office at Fort Monmouth

Lightweight Tacfire AN/PYC-1 (BCT) and AN/PSC-2 (DCT)

This may be the replacement for the GRA-71. TB 11-5820-890-12 has information on connecting it to the RT-1523(C)U SINCGARS radio.


Military Commo Equipment List

Listing of the GRC-109 and component parts, some photos.

This file lists some observed info on the GRC-109, RS-1, RS-6, PRC-64, and GRA-71 equipment.

Clandestine Radio Equipment of the United States' Cold War Era -  2nd edition (preliminary), 1999 Pete's GRA-71 page -


FM 24-24 Signal Data References: Signal Equipment

This manual is on line maintained by the U.S. Army.  Chapter 4 Section 3: Auxiliary Radio Equipment has a  GRA-71 listing

TM 11-5835-224-12 = 019010.pdf on ETM

Operator and Organizational Maintenance Manual
Coder-Burst Transmission Group AN/GRA-71
May 1964

TM 11-5835-224-34P

Direct Support and General Support Maintenance Repair Parts and Special Tools Lists (Including Depot maintenance Repair Parts and Special Tools) For Coder-Burst Transmission Group AN/GRA-71
(NSN 5820-00-056-6856)
July 1978

TM 11-5820-474-14

Operator, Organizational, and Field Maintenance Manual:
Radio Set AN/GRC-109
18 May 1962

TM 11-5835-224-12

Operator's and Organizational Maintenance Manual
Coder-Burst Transmission Group AN/GRA-71
(NSN 5820-00-056-6860)
With changes C 2 and C 5

TM 11-5835-224-35

DS, GS and Depot Maintenance Manual
Including Repair Parts and Special Tool Lists
Coder-Burst Transmission Group AN/GRA-71
July 1969

MWO 11-5820-474-35/1

Modification of Radio Set AN/GRC-109 to make it Compatible with Coder-Burst Transmission Group AN/GRA-71

TB 11-5820-890-10-12

Operation of Lightweight Tacfire AN/PYC-1 (BCT) and AN/PSC-2 (DCT) with SINCGARS ground radio sets

TM 11-5835-228-34

DS and GS Maintenance Manual Recorder-Reproducer, Sound RD-265/GR

TM 11-5835-228-20P

Organizational Maintenance Repair Parts and Special Tools for Recorder-Reproducer sound RD-265/GR
(NSN 5835-00-901-1086)


The GRA-71 Burst-Coder by Peter McCollum
KL-43 Burst Communications on a Budget -
ҹqx Spy Radio  -  a display of the PRC-316 & GRA-71 attachment 

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