Spy radio
Burst encoders
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AN/PRC-64   Delco 5300
USA Spy Radio Set

The AN/PRC-64 was a true spy radio set, developed by Delco in the USA in the early 1960s as a possible successor to the AN/GRC-109 (RS-1). It was intended for use by Special Forces (SF) and for espionage activities. The design was based on the Delco 5300 that was used by the CIA. It was extremely popular with the Special Operations Group of the Australian Army in Vietnam.
During their operational life, most of the PRC-64 units were upgraded to PRC-64A, which allowed faster burst transmissions in combination with the AN/GRA-71 burst encoder (see below) that was connected to the 7-pin morse KEY socket.

The image on the right shows a typical PRC-64A radio set with the main accessories connected. Apart from the sweet little bakelite morse key, an even smaller key is present on the radio itself. Contary to other spy sets of the same era, the PRC-64 was also suitable for voice transmission, hence the presence of a dynamic microphone.
AN/PRC-64 ready for use

The PRC-64 was an extremely compact unit for its time and measures only 25 x 13 x 12 cm. Even when packed together with the accessories in the canvas carrying bag, the set measures less than 35 x 14 x 14 cm and weights less than 3.5 kg including the battery. The unit is powered by an internal battery and is switched on by opening the top lid. The small size however comes at a price, as the HF output power is no more than 5 Watt (CW morse) or 1.5 Watt (AM voice).

Frequency selection of the PRC-64 is crystal controlled, limiting its operation to 4 preset channels. Receiver and transmitter each have their own set of 4 crystals to allow split-frequency operation. The frequency range is from 2.2 to 6.0 MHz. RX and TX frequencies are usually written in a table inside the top lid. The crystals were stored in a compartment behind the table.
AN/PRC-64 packed in its canvas bag The closed PRC-64 radio box (watertight) The opened PRC-64 box showing the controls AN/PRC-64 ready for use Operating the internal morse key Close-up of the controls of the PRC-64 Receiver and transmitter controls The serial number plate, indicating that this unit was used by Australian troops

The entire PRC-64 radio is no larger than an average lunchbox. Opening the top lid, reveals the control panel where all switches and adjustments are nicely arranged. The front panel can be roughly divided into four sections: the receiver (top left), the transmitter (bottom left), the battery (top right) and the input/output section (modulator, bottom right).

PRC-64 controls

The battery compartment accepts a BA-1509/PRC-64 battery that supplies three voltages: +4V, +12V and +28V. These batteries were purpose-built for this radio and are no longer available. Earphone(s) and microphone are connected at the bottom right, where also the blue socket for an external key or burst keyer is located. The RX and TX channels can be selected independently, but is limited to just 4 channels each. For this, suitable crystals have to be inserted inside the radio.

Compared to earlier spy radio set, the built-in Antenna Tuner has a limited range, increasing the importance of the correct antenna (wire) length. Note that the channel selectors also act as a Frequency Range (BAND) selector, limiting the range for each channel as follows:
Delco 5300
  1. 3.0 - 3.9 MHz
  2. 3.8 - 4.9 MHz
  3. 4.8 - 6.3 MHz
  4. 6.2 - 8.0 MHz
  1. 4.6 - 6 MHz
  2. 2.3 - 2.85 MHz
  3. 2.8 - 3.65 MHz
  4. 3.6 - 4.7 MHz

The PRC-64 was derived from the Delco 5300 radio that was built especially for use by the CIA. The Delco 5300 is physically identical, but has a black case, rather than green, and has a different frequency range (3.0 to 8.0 MHz). Furthermore, some of the controls on the front panel are red rather than black. At the moment we don't have a picture of the Delco 5300 available.
  • Delco 5300
    Frequency range: 3 - 8 MHz. No 0.5/6kHz bandwidth switch. This is the original version of the radio, that was used by the CIA for clandestine operations. The internal morse key is a true (adjustable) telegraph key rather than a microswitch (see below).

  • Delco 5300A
    Same as the above, but with 0.5/6kHz bandwidth switch for the receiver.

  • PRC-64
    Frequency range 2.2 - 6 MHz. This is the Army variant of the Delco 5300. The 0.5/6kHz bandwidth switch is available on the receiver and the radio has a simpler battery-test circuit (24V only). Furthermore, the transmitter uses a larger crystal (CR-89) and the morse key has been simplified (microswitch). Probably developed around 1964.

  • PRC-64A
    Same as the PRC-64 above, but modified for use in combination with a high-speed burst keyer. The modification consists of a daughter board on the transmitter module and was probably issued around 1968.

Delco 5300
From Ray Robinson in Australia [4], we received two photographs of Delco 5300 sets. One is a plain Delco 5300 as used by the CIA for clandestine operations, and the other one is a Delco 5300C. The latter is shown in the image on the right.   
Delco 5300C. Copyright Ray Robinson [4].

Standard Delco 5300. Unknown author [5]. Delco 5300C. Copyright Ray Robinson [4].

Burst encoder
In order to avoid evesdropping and direction finding by the enemy, it was necessary to be on the air as short as possible. The PRC-64 was therefore modified to the PRC-64A, to allow an AN/GRA-71 burst encoder to be connected. The burst encoder connects to the same Winchester M7S socket as the external morse key.

 More information
GRA-71 with all items stored inside the container

Morse key
The PRC-64 is usually operated with an external key when sending messages in morse code. A small bakelite morse key was therefore supplied with the set. It has a small piece of wire (50 cm) with a 7-pin connector at the end. and connects to the blue KEY socket at the front right of the radio. The same socket is used to connect an external burst encoder.

For emergency purposes, the PRC-64 also has a tiny little internal morse key on its front panel. It is located to the left of the KEY socket. It has a flattened side to allow the case lid to pass by.
The small bakelite morse key

The small bakelite morse key External morse key with cable and connector Mounting rig at the bottom of the morse key External morse key in operation The 7-pin connector on the external morse key, shown here with a protective cap. The blue KEY input socket used for connecting an external key or burst encoder. AN/PRC-64 ready for use Operating the internal morse key

Earphones and microphone
The PRC-64 was usually supplied with two ear pieces; one for each ear. It also allowed a second person to listen to the incoming message. At the center of the front panel are two PHONE sockets.

Unlike most other spy sets of the same era, the PRC-64 can also be used for the transmission of voice messages. A small dynamic microphone is supplied, which connects to a socket at the far right of the front panel. When used for speech, the mode-selector has to be set to AM.

Earphone Earphone Earphone with cable and plug Earphone sockets Close-up of the dynamic microphone Dynamic microphone Accessories AN/PRC-64 ready for use

Storage bag
When unused, the PRC-64 is normally stored inside a small rugged canvas bag, together with the main accessories, such as microphone, ear pieces, morse key and antennas.

The image on the right shows the canvas bag with the PRC-64 radio at the center. Two small pockets - one at either side of the bag - are used to store the accessories. Microphone, ear pieces and morse key are stored in one pocket, whilst the antenna reels are stored in the other one.
Removing the PRC-64 from the canvas bag

AN/PRC-64 packed in its canvas bag The PCR-64 stored in the canvas bag Removing the PRC-64 from the canvas bag Empty canvas bag The closed PRC-64 radio box (watertight) Leftmost pocket containing microphone, ear pieces and morse key The antennas in the rightmost pocket Closing the straps of the canvas bag

Due to the modular construction of the PRC-64, the radio is easily serviceable. After removing 10 bolts around the edges of the front panel, the entire radio can be taken out of the case.
As the entire radio is bolted to the front panel, and all controls and connections are on the front panel, the radio can be operated outside the case. After removing the knobs and some bolts from the front panel, each module can be removed easily.

Some modules, such as the receiver, can even be operated when removed, by using an extension cable, making it easier to adjust it.
Interior removed from the watertight case

Interior removed from the watertight case PRC-64 out of the case PRC-64 out of the case Transmitter module Interior detail Receiver module Crystal sockets Close-up of the receiver showing the mechanical filter

External KEY
An external telegraph key can be connected to the 7-pin Winchester M7S socket that is present to the right of the internal morse key. The same socket can be used to connect an external Burst Encoder such as the AN/GRA-71. The pinout of this socket is compatible with the the KE-8B Keyer (KY-468) that is part of the GRA-71 kit. The internal wiring of this socket is as follows:

Looking into the KEY socket of the PRC-64

An external key should be connected between the KEY input and GND (pins D and F). A +12V power source is provided by the PRC-64A for the external (burst) keyer (pin H), using GND for the negative terminal (pins A, C and F). When using a 5-way 1-to-1 cable, the KE-8B Keyer of the GRA-71 can be used directly with the PRC-64A. Pins B and E are not used at either end.
Memories of a Patrol Sig
During the early 1970's, John Trist from Deniliquin (Australia) was a so-called Patrol Sig. He was trained on the AN/PRC-64 for 6½ weeks before being deployed to the Jungle, using the 64 Set as a lifeline from places like Borneo, Malaya and Vietnam.

In 2014, John Trist looked back on his days in active service and wrote an article for the Australian Branch of the Royal Naval Amateur Radio Society, in which he gives a account of the operation of the AN/PRC-64 in the jungle [6].

 Read the full article

  1. Operator and Depot Maintenance Manual for Radio Set AN/PRC-64A
    Department of the Army, US Army Publications Center. Including full circuit diagrams.
    TM 11-5820-552-15. 16 November 1970.

  2. Radio Set AN/PRC-64, Final Report
    Defense Documentation Center for Scientific Information.
    Army Concept Team in Vietnam, 15 May 1965. Unclassified.

  3. Technical Report No. 67-01, Radio Set, AN/PRC-64
    U.S. Army Limited War Laboratory, Unlimited Distribution.
    Stanley D. Peirce, April 1967

  4. Ray Robinson, Photographs of Delco 5300 and 5300C
    Received December 2013.

  5. Unknown source, Photograph of Delco 5300
    Image taken from Ebay by [4].

  6. John Trist, Memories of a Patrol Sig
    Royal Naval Amateur Radio Society, Australian Branch, Newsletter, Issue June 2014.
    Reproduced here by kind permission of the RNARS-A. December 2014.

Further information

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Crypto Museum. Last changed: Sunday, 16 August 2015 - 16:00 CET.
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