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AN/PRC-77
Portable military VHF/FM transceiver

AN/PRC-77 was a military portable VHF transceiver, also known as a backpack radio, introduced by the US Army in 1968 as the successor to the AN/PRC-25. The radio was used heavily during the Vietnam War, and differs from its predecessor in that its RF power amplifier stage (PA) is fully solid state, whereas the PA of the PRC-25 was made with thermionic valves (vacuum tubes) [1].

The PRC-77 consists of an RT-841 transceiver and some additional components, made by various companies. It is suitable for secure voice operation, by selecting the so-called X-MODE and adding an external voice encryption unit, like the KY-38 (NESTOR) or the KY-57 (VINSON).

For the European market, it was made by Telemit Electronic GmbH in München (Germany). Telemit improved the radio in several ways, including changing the channel spacing from 50 kHz to 25 kHz. As a result, the radio is still interoperable 1 with most modern military VHF/FM radios today.
  
PRC-77 military radio

Both receiver and transmitter cover the 30-75.95 MHz frequency range, divided over two bands, in steps of 50 kHz (or 25 kHz for the European version). 2 The receiver has a sensitivity of 0.5 µV, whilst the transmitter delivers between 1.3 and 4 Watts to the 50Ω antenna. The device measures 273 x 273 x 100 mm and weights 6.2 kg. It should powered by a 12.5 to 15V DC voltage that is usually supplied by military batteries, such as the (BA-4386/U, BA-398/U or BA-55984) [1].

  1. Not compatible with the SINCGARS frequency hopping mode (FH).
  2. Commonly designated AN/PRC-1177.

PRC-77 military radio
PRC-77 seen from the front
Handset with U-229 connector
Foldable antenna with gooseneck
Battery pack - top
PRC-77 radio and TST-7698 voice encryptor
Installing the battery pack
Power socket (for connection to voice encryptor)
A
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A
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PRC-77 military radio
A
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PRC-77 seen from the front
A
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Handset with U-229 connector
A
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Foldable antenna with gooseneck
A
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Battery pack - top
A
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PRC-77 radio and TST-7698 voice encryptor
A
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Installing the battery pack
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Power socket (for connection to voice encryptor)

Features
The diagram below provides a quick overview of the controls and connections of the PRC-77, all of which are located at the front panel (which is at the top when carried on the back of a soldier). The device is powered by internal batteries, or by an external power source – such as the battery of a vehicle – connected via the POWER socket. It has provision for a local antenna (fitted to the antenna base mount) or an external one, that can be connected to the BNC socket at the top left.

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The radio has two bands: (1) 30 to 52.950 MHz and (2) 53 to 75.950 MHz, selectable with the BAND selector at the bottom left. The desired frequency should be set with the two rotary dials at the centre. The leftmost one is used for setting the MHz, whilst the rightmost one is for the kHz. The standard version of the radio has a channel spacing of 50 kHz, but some units – made by Telemit in Germany – have a 25 kHz spacing. On such device an extra +25kHz switch is present.

For regular use, a standard handset with an U-229 connector should be connected to one of the audio sockets at the top right. When using voice encryption, the encryptor should be connected to the power socket, and the handset should be connected directly to the external encryptor.


Voice encryption devices used with the PRC-77
Tactical voice encryptor (NESTOR)
Wide-band Voice and Data Encryption Unit (VINSON)
Hagelin CVX-396 (SVZ-B) voice encryptor
Siemens MSC-2001
Timmann TST-7698 voice encryptor

KY-38
KY-38 was one of the first secure voice units that provided real high-end encryption. It is part of the NESTOR family of devices – developed by the US National Security Agency (NSA) – and was introduced during the Vietnam War in 1964.

The KY-38 was a large and heavy unit, that has nearly the same size as the radio itself. The image on the right shows an American soldier during the Vietname War, carrying a PRC-77 in manpack configuration with a KY-38 mounted at the bottom. Note that the handset is connected to the KY-38 rather than to the radio itself.

 More information

  
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KY-57
KY-57 was a small tactical wideband voice encryptor that was developed in the mid-1970 by the US National Security Agency (NSA) as a replacement for the NESTOR devices (KY-38).

It features the highly secret SAVILLE encryption algorithm and was for many years the crypto 'workhorse' of the US Armed Forces.

 More information

  
KY-57 voice encryption unit

CVX-396
In the mid-1970s, Telemit added the CVX-396 voice encryption device to the European version, which was in fact a rebranded device made by Crypto AG (Hagelin) in Switzerland.

It has the same width as the PRC-77 and can be mounted on top of it. Note that it is less secure than the Swiss national version SVZ-B, which looks nearly identical.

 More information

  
CVX-396 with open door

MSC-2001
In 1978, Siemens in München (Germany) introduced the MSC-2001, a compact tactical voice encryption unit that could be mounted to the side of the PRC-77. It was initially developed especially for the PRC-77, but was later adapted for use with other radios as well.

It can hold up to 8 different 50-bit cryptographic keys, selectable with a rotary switch at the front.

 More information

  
MSC-2001 voice encryptor

TST-7698
in the mid-1980s, Telescurity Timmann (TST) developed the TST-7698 voice encryptor that used an LPC-10 or a RELP vocoder, allowing it to be used on HF, VHF and UHF radio channels, as well as via analogue telephone lines.

The device is constructed in such as way that it can be fitted to one of the short sides of the PRC-77, using a special mounting bracket.

 More information

  
 TST-7698 voice encryptor

Parts
AN/PRC-77 manpack radio
Battery pack
Handset with U-229 connector
Foldable gooseneck antenna
Mounting bracked for encryption unit
Radio   RT-841
The RT-841 radio is the core part of the set. It measures 273 × 273 × 100 mm and weights 6.2 kg. It can be used in a vehicle by using a suitable mounting kit or, more commonly, as a man­pack unit mounted in a metal carrying frame.

For portable use, it is powered by an internal battery pack and uses a local antenna, mounted directly to the control panel. For mobile use, power can be supplied directly to the POWER socket, whilst an external antenna can be connected to the BNC socket marked 'ANT'.

  
PRC-77 military radio

Battery pack
The radio was usually powered by an internal battery pack, of which various types were available: chargeable and non-chargeable. The image on the right shows the BA-386-A/PRC non-chargeable type, that accepts 10 standard 1.5V D-size cells.

The battery pack is installed at the bottom of the radio and has a socket that should mate with a connector inside the radio. The battery pack is protected by a metal cover.
  
Battery pack open

Handset
The radio should be used with a standard dynamic handset with an U-229 connector at the end of its coiled cable, such as the one shown in the image on the right made by TMC.

Note that, when using an external voice encryptor, another type of handset might be required.
  
Handset with U-229 connector

Antenna   AT-892/PRC-25
By default, the PRC-77 is used with its own antenna that should be mounted on the screw terminal marked ANT at the bottom left of the control panel. It consists of a gooseneck with a foldable steel-tape part mounted on top and is also known as bush-whip. When folded, this antenna requires very little space.

When using the PRC-77 with an external antenna, it should be connected to the BNC terminal marked ANT. In that case, the foldable antenna should be removed and a cover plate should be installed on the screw terminal.

  
Foldable antenna with gooseneck

Mounting bracket
Several mounting brackets were available for mounting an encryption device to the body of the PRC-77, such as the one shown in the image on the right, which was made especially for the TST-7698 by Timmann Telesecurity (TST).

Other types of brackets were available for the other encryption devices listed above.

  
TST mounting bracket seen from the rear

Battery pack - top
Battery pack - bottom
Battery pack open
Socket on battery pack
Battery connector
Handset with U-229 connector
Foldable antenna with gooseneck
Folded antenna
B
×
B
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Battery pack - top
B
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Battery pack - bottom
B
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Battery pack open
B
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Socket on battery pack
B
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Battery connector
B
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Handset with U-229 connector
B
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Foldable antenna with gooseneck
B
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Folded antenna

Interior
The AN/PRC-77 is a service-friendly device that was designed in such a way that it could be repaired in the field and that it could be built under licence by different companies in different parts of the world, such as Assiciated Industries (USA), Telefunken (Brasil) and Telemit (Germany).

The device is housed in a watertight enclosure that consists of a die cast aluminium control planel and an extruded cases shell. The interior can be accessed by releasing just four bolts, located at the rear side of the front panel grips.

After releasing the bolts, the case shell can be removed and the interior — which is completely mounted to the rear of the control panel — will be exposed. The image on the right shows the various modules that are visible at the bottom of the radio. This side also exposes the extremely complex MHz-stepping mechanism of the tuner.
  
Interior seen from the bottom

Additional (smaller) modules are located at the upper side of the radio, mounted onto a hinged panel that can be swung to the side in order to get access to the tuning assembly, the PA stage and the audio circuitry mounted below it. Further details can be observed in the images below.

Opening the PRC-77
Interior seen from the bottom
Interior - top view
Crystal filter
Relay
Interior seen from the top, with hinged upper panel removed
Interior - bottom view
MHz stepping mechanism
C
×
C
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Opening the PRC-77
C
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Interior seen from the bottom
C
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Interior - top view
C
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Crystal filter
C
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Relay
C
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Interior seen from the top, with hinged upper panel removed
C
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Interior - bottom view
C
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MHz stepping mechanism

Specifications
  • Frequency
    30 - 86 MHz
  • Bands
    2 (30-52.95 MHz and 53-75.950 MHz)
  • Channels
    920
  • Spacing
    50 kHz (25 kHz on some units) 1
  • Output
    1.3 to 4 Watts
  • Range
    8 km
  • Power
    12.5V and 2.5V supplied by battery pack
  • Antenna 1
    AT-271A/PRC — 3 m multi-section whip
  • Antenna 2
    AT-892/PRC-25 — steel tape bush-whip
  • Weight
    6.2 kg
  1. Some units were modified for a 25 kHz channel spacing and a reduced voice bandwidth of 6 kHz. These units were generally designated AN/PRC-1177 and can be recognised by an extra switch (25 kHz) at the front panel.

Documentation
  1. AN/PRC-77 Operator's and Organizational Maintenance Manual
    TM 11-5820-667-12. US Army, 1 January 1987.

  2. AN/PRC-77 Hand Receipt Manual
    TM 11-5820-667-12-HR. US Army, December 1978.

  3. AN/PRC-77 Repair Parts and Special Tools
    TM 11-5820-667-20P. US Army, 15 August 1989.

  4. AN/PRC-77 Depot Maintenance Repair Parts
    TM 11-5820-667-34P. US Army, 15 August 1989.

  5. AN/PRC-77 Technical Manual
    TM 11-5820-667-34LD. US Army, 30 September 1976.
     Interconnection diagram
     Mechanism Exploded view

  6. AN/PRC-77 Direct Support, General Support and Depot Maintenance
    Circuit diagrams of the individual modules.
    TM 11-5820-667-35. US Army, February 1968.

  7. EMP Hardening Investigation of the PRC-77 Radio Set
    Ottawa (Canada), February 1993.
References
  1. RigReference, Military AN/PRC-77 Portable VHF Transceiver
    Retrieved March 2018.

  2. Wikipedia, AN/PRC-77 Portable Transceiver
    Retrieved September 2019.

  3. Radionerds.com, AN/PRC-77
    Retrieved September 2019.

  4. Brooke Clarke, PRC-77 Back Pack Squad Radio
    Retrieved September 2019.
Further information
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Crypto Museum. Created: Sunday 11 March 2018. Last changed: Thursday, 19 September 2019 - 10:09 CET.
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