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Short-range agent communications

The UHER SCRAC 1 was a device for Short-Range Agent Communications (SRAC), developed in the early 1960s in Czechoslovakia, probably by Správa 6 2 for use by the secret state police (StB) and the secret intelligence service Správa 1 3 (espionage). The device was used by agents and spies for delivering a recorded (verbal) message to a nearby Electronic Dead Letter Box (EDLB).
The device was used in combination with an UHER Report 4000 tape recorder from the early 1960s. The agent recorded his message verbally on the tape at the lowest speed (2.4 cm/s). He then placed his UHER recorder in his car and installed the SRAC onto the sockets at the right. He also connected the car radio antenna to it.

Next, he would drive his car to the agreed drop zone and, whilst driving in that area, play back the message at the highest speed (19 cm/s). The SRAC shown on the right is in fact a transmitter that operates in the 88-108 MHz FM radio band.
UHER 206/2 electronic dead letter box

A receiver, hidden in the drop zone, would pick-up the broadcast and record it at the highest speed. It could be placed in another car, in which case the handler might also be driving around in that area, or it could be placed in a fixed (unmanned) position, in which case it was a so-called Automatic Dead Letter Box. The handler would then collect the recorded message(s) later.
The two DIN connectors at one of the long sides of the device are placed in such a way that they mate with the outer two DIN sockets at the right side of the UHER Report 4000 recorder. The device gets audio from one of these connectors, whilst the other one connects the transmitter to 7.5V DC from the recorder's internal batteries.

The image on the right shows the SRAC installed to the right side of an UHER Report 4000 and connected to the car antenna, ready for use. The clandestine transmitter will be activated as soon as the play button of the recorder is pressed.
UHER 4000 Report-S with EDLB

Selecting the UHER Report 4000 for this purpose was a clever move. It was available off-the-shelf from every good HiFi store, and could be bought without attracting any attention. Furthermore, walking around with an UHER recorder in a European city would be considered normal, whereas a similar device from an East European manufacturer would certainly have raised eyebrows.
Briefly transmitting a secret message via the FM broadcast band would not attract much attention in the early 1960s as the 88-108 MHz band was rather empty in those days (especially when compared to the situation in 2015). As the segment above 100 MHz was unused in most West Europen countries, it wouldn't cause any interference with legal broadcast stations either.

As the transmisson was relatively short and the target was moving, interception would have been unlikely and triangulation by means of Radio Direction Finding (RDF) would not be effective.
Close-up of the EDLB with the cover taken off

In practice, the selected frequencies were often very close to existing radio stations, so that they would not be noticed by the radio monitoring services of the guest country. In theory, the transmission could be picked up accidently by someone tuning in to a regular broadcast station, but given the low power and the limited range of the device, this was not considered a problem.

The device shown in the image above is labelled 206/2 and came from a batch of similar units that were numered consecutively from 206/1 to 206/9. Apparently these units were each tuned to a different frequency in the FM-band. It is unknown how many units were built, but it must have been substantial, as they were used in all countries where Czechosovakia had a presence.
  1. UHER SRAC is most likely not the name of this device, but since the device was designed for short-range agent communications (SRAC) and because it is used in combination with the UHER Report tape recorder, we have given it this nickname name.
  2. Správa 6 refers to Government Department 6: Communication Technics.
  3. Správa 1 refers to Government Department 1: Intelligence (espionage).

UHER 206/2 electronic dead letter box Rear side (antenna socket) UHER 4000 Report with Czechoslovakian EDLB EDLB system Installing the EDLB UHER 4000 Report-S with EDLB Close-up of the EDLB with the cover taken off Front view with it lid open


Interior of the UHER SRAC

The UHER SRAC is in fact a low-power miniature FM transmitter, housed in a metal enclosure that measures just 10 x 4 x 2 cm (without the connectors). It has one socket for the car radio antenna and two fixed connectors that mate with the outer two DIN sockets at the right side of the UHER Report 4000 tape recorder. Via these two connectors, the transmitter receives power and audio.
The design of the transmitter is simple, yet very effective. Inside the metal enclosure are three compartments: the oscillator (left), a driver stage (center) and a Power Amplifier, or PA, (right).

The oscillator is free-running, and also acts as the modulator by applying line-level audio from the UHER directly to it. Using a free-running oscillator instead of a crystal-based one should not be considered a problem as the transmitter is properly shielded and most domestic receivers in the FM broadcast band generally have an Automatic Frequency Correction (AFC) anyway.
Inside the EDLB

The transmitter does not need a separate power source as it is powered directly from the charge socket on the UHER recorder to which it is connected. Note the gap in the 6-pin DIN connector on the SRAC. This prevents the batteries from being disconnected when the connector is inserted into the socket, as is normally the case when an external battery charger is connected.

Considering its age, the transmitter is well-built, using only first class components. Frequency alignment is possible by adjusting the coils and the output trimmer through the three holes in the lid of the case. Note that the transistors used in the device are made by Western manufacturers such as Telefunken and Philco. The trimmer is from Tesla.
Inside the EDLB UHER 206 EDLB transmitter - interior Interior seen from the rear Top view Oscillator/modulator section Pre-amplifier section PA section Another view of the PA

The UHER SRAC has three connections. The antenna socket accepts the coaxial connector from a standard car radio antenna. The other two are male DIN connectors that are soldered to the transmitter's frame. They mate with the outer two DIN sockets at the right side of the recorder. The pinout of these sockets is as follows:
This is the (mono) phone/radio socket of the UHER recorder. It carries both input and output signals at line-level, but for this application we only need the recorder's audio output on pin 3.
  1. -
  2. Ground (chassis)
  3. Audio
Only two wires are needed on this socket. Note that, like the UHER recorder, the SRAC has the (+) terminal of the battery connected to the chassis. This means that the circuit is powered by -7.5V.
  1. -
  2. -
  3. Ground (7.5V battery +)
  4. -
  5. Power (7.5V battery -)
  6. -
  1. Anonymous, UHER SRAC electronic dead letter box transmitter - THANKS!
    UHER 206 SRAC kindly donated by anonymous former user. July 2015.

  2. UHER Report 4000 IC, Technical description with circuit diagram
    March 1963. Website HiFi Engine. Retrieved April 2015.

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Crypto Museum. Created: Friday 07 August 2015. Last changed: Friday, 22 July 2016 - 13:48 CET.
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