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AN/GRA-71 Burst Encoder (USA)
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This page is under construction
The AN/GRA-71 was an American burst encoder that allowed pre-recorded
coded messages to be broadcast in morse code at very high speed, in order
to minimise the risk of interception and detection. It was often supplied
as part of the American RS-1 spy set
(a.k.a. AN/GRC-109), but it was also used with
other types of (spy) radio, such as the SP-20.
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A complete GRA-71 unit consists of a water tight container in which the
various components are stored. Each item has its own unique bay in the
container, 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 the container.
It connects the actual Keyer to the radio set by means of the two wires
at the back. Each cable has a suitable connector at one end, shown here
with protective caps, whilst the other end disappears into the container.
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Messages are recorded on magnetic tapes using one of the two supplied coders,
much like using a domestic tape recorder. One coder 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 could be used, to allow any morse
character to be recorded (e.g. numbers and punctuation marks).
Once the recording was complete, the keyer was connected to the transmitter
and was used to play back the message at high speed (approx. 300 WPM).
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The complete kit consists of the following items, which are further described
below:
- MA-9, Recording tape magazine [2x]
- MX-4495, Tape coder (dot-dash)
- MX-4496, Tape coder (wheel)
- Alternative index wheel for MX-4496
- MX-4498, Keyer Adapter (fixed)
- KY-468, Keyer
- Soft cleaning brush
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The pre-coded message is recorded onto standard magnetic (ferro) tape,
similar to the tape used with domestic open reel audio recorders.
Tape of a fixed length is stored inside a small cartridge, which was
called Recording tape magazine.
When stored, a hinged lid protects the tape against dirt and damage.
Two identical tape cartridges are supplied with the set.
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MX-4495, Tape coder (dot-dash)
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MX-4496, Tape coder (alphabet)
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Alternative index wheel for MX-4496
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A soft 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.
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Operator usage and maintenance of the AN/GRA-71 is explained in the small
(A5 size) instruction manual TM 11-5835-224-12
[1]
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The booklet contains 76 pages with full instructions, maintenance tips and
examples of messages. It also contains a series of black and white photographs, showing the various parts.
The manual shown in the image on the right is the 1969 issue,
that is based on the initial version of 1964. This release includes the
changes made in 1966.
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The AN/GRA-71 keyer was designed for the SP-1 spy set
(GRC-109).
The keyer is also known to have been used with other types of (spy) radio,
such as the PRC-64,
PRC-74 and
PRC-104.
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- US Army, Coder-Burst Transmission Group AN/GRA-71
TM 11-5835-224-12 (Operator and Organizational Maintenance Manual)
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