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Tesla PS-31   PR-31 · Troika
Portable surveillance radio

The PS-31 was a rather large semi-portable radio station with voice scrambling, developed and manufactured by Tesla in Czechoslovakia. It consists of various components, including the actual PR-31 transceiver. The radio was introduced in the early 1980s and was intended for police surveillance, covert operations, observation tasks and secure communication. As it was also used by the secret service of the former DDR, the Stasi, it is also known as the Secure Stasi Radio.
 
The PR-31 is a simplex radio that operates in phone (voice) in the 80MHz band. It has provisions for 70 pre-determined channels. Each user is issued a special code-key that is used for identification. At the start of each transmission, the ID is transmitted to the master station.

The radio is normally used inside the protective leather case that can be fitted to a belt, or carried on the shoulder, using the provided strap. When not in use, the handset can be slotted onto the shoulder strap.
  
The PR-31 radio station with helical antenna and handset

A separate handset contains a combined microphone/speaker and connects to the radio by means of a special 7-way screw-on plug. On top of the handset is a rather 'heavy' push-to-talk switch (PTT). At the front of the handset is a red EMERGENCY button and a red LED.
 
The original packaging of the PS-31 The PS-31 stored in the original packaging Complete view of the PS-31 radio The PR-31 radio station with helical antenna and handset PR-31 ready for use PR-31 ready for use The channel selector Close-up of the 2-digit LED display

 
Code key
The PR-31 allows a secret identification number to be inserted at the start of each transmission. This way, the master station can varify the identity of the person at the other end. This is particularly useful in situations where agents were working in shifts and had to hand the radio over to the next person on watch.
 
The operator has to insert his key into a socket on the front panel of the radio. It has a special rig to ensure that it is always inserted the correct way around. They key itself has two rows of metal notches which together make up the unique ID.

After inserting the key, the user has to activate it by turning it 1/4 clockwise, so that it lines up with the '1' mark. The key has to be left in place for the duration of the session.
  
Inserting the ID key

 
Key in pocket A typical ID key with ID label and storage wallet Identification key Close-up of the ID key Extreme close-up of the teeth of the ID key Inserting the ID key Turning the unit ON Close-up of the key and the antenna

 
Handset
The PR-31 has a 7-pin connector at is front panel. The connector is used for connection of a handset, such as the one shown here. This handset is supplied with the radio as a standard accessory. It is used both as a speaker and a microphone and the PTT switch (push-to-talk) is mounted on the top.

At the front panel is a large red button and a red LED. The red button can be pressed in case of an emergency. The radio then transmit the distress code. When not in use, the handset can be clipped to the shoulder strap.
  
Using the handset

 
Using the handset The complete handset with cable and 7-pin plug Close-up of the PO-31 handset with microphone/speaker, Emergency button and LED Connecting the handset How to clip the microphone to the shoulder strap, using the special slot. Clipping the microphone to the shoulder strap

 
Antennas
The PR-31 is supplied with two different antennas: a long rather stiff antenna and a small flexible helical antenna. The long one gives the best operating range, whereas the helical is used for convenience.

The image on the right shows the helical antenna being inserted in its socket. A screw-on terminal is used, similar to TNC.
  
Fitting the helical

 
Helical antenna Fitting the helical The long antenna, stored inside the top half of the packaging. The long antenna, stored inside the top half of the packaging. Close-up of the bottom of the long antenna Close-up of the key and the antenna

 
Battery
The PR-31 is powered by 12V battery pack that is inserted into a slot at the bottom of the radio. Normally, two battery packs were supplied with the unit, so that one could be charged externally, whilst the other one is used. It is also possible to recharge the battery whilst it is seated in the radio, by using two contacts at the bottom.

The battery contains 10 1.2V NiCd cells with a capacity of 500mAh. It is quite likely that the batteries of the surviving radios have died and can no longer be recharged.
  
Close-up of the battery pack

If you want to bring an old radio back to live, it might be a good idea to replace the batteries with NiMH cells.
 
Close-up of the battery pack The battery pack to be inserted at the bottom of the radio Close-up of the battery pack and the battery slot Inserting the battery Locking the battery in place

 
Charger   PZ-31
The PZ-31 was a fast battery charger for the PR-31 radio set, in which the radio could be placed stright up.
 
Radio station
The complete PS-31 radio station, consists of the following components:
 
  • Radio PR-31
  • Handset PO-31
  • Helical antenna PA-31
  • Fixed antenna PA-32
  • Battery 12V (10 rechargeable NiCd cells)
  • Leather carrying case
  • Code-key
  • Documentation
Documentation
  1. Souprava Prenosne Radiostanice Nabijec PS-31, Navod Kobsluze A Udrzbe
    User instructions, 1988 (Czechoslovakian)

  2. Funksprechgerät PS-31, Unterhaltungs- und Bedieningsvorschrift QD 008 95
    User instructions, 1988 (German)

  3. Tesla Pardubice k.p., Gerätekarte PSK 49904
    Warranty and inspection card, issued 10 November 1988 (German)

  4. Tesla Pardubice k.p., Liste des Funksprechgerät QD 02958
    Checklist

  5. PR-31 Circuit Diagrams (off-site)
    Retrieved June 2016 from [1].

References
  1. Remesla Jirikov, Schemata ruznych pristroju
    Website (Czech). Visited June 2016.

Other Stasi radios

Further information

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Crypto Museum. Created: Thursday 29 March 2012. Last changed: Friday, 22 July 2016 - 13:56 CET.
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