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Burst encoders
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DÁVAČ   TI-485
Tape-based morse burst encoder - wanted item

DÁVAČ 1, or TI-485, or SIRIUS D, was a tape-based burst encoder that was used for sending messages in morse code at very high speed. It was developed in 1959 in Czechoslovakia by Správa 6 2 for use by the secret intelligence agency (StB) and by Správa 1 (espionage). The fully transistorised device was supplied with the SIRIUS spy radio set and was succeeded by MĚSIC.

The image on the right shows the burst encoder as it is shown in the technical manual [A]. The device measures 190 x 72 x 50 mm and weights 1.40 kg. It has the same length as the receiver and the accessory box, but is 3 cm less wide.

The device is connected to the transmitter by means of a short cable with a rectangular 6-pin plug at either side. The recorder is powered via this cable and delivers its modulated signal to the transmitter when in playback mode. The device is only suitable for (pre-coded) numerical messages consisting of the digits 0-9. The numbers are recorded onto a magnetic tape.

Once the message is recorded and the transmitter is tuned to the desired frequency, the message can be played back at high speed by pressing the REPLAYING button at the top left. It takes just 15 seconds to send a message of 200 groups of 5-digits each, which greatly reduces the risk of interception and radio direction finding. When playing back, the signal from the pickup head is amplified and sent to the transmitter, where it toggles the main oscillator in order to produce CW.

A later variant of DÁVAČ is MEŠIC (moon) which was supplied with the SIRIUS III radio station. It is nearly identical to the DÁVAČ, but has its endless tape located at the top. Furthermore, it features a different keypad and has a different socket for connection to the SIRIUS III transmitter.

  1. DÁVAČ is the Czech word for 'keyer'.
  2. Správa 6 refers to Government Department 6: Communication Technology.

DAVAC burst encoder. Photograph via [2].
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DAVAC burst encoder. Photograph via [2].

Operation of the device is pretty straightforward. After connecting it to the transmitter (via the 6-pin socket at the right side), it can be switched ON by sliding the ON/OFF switch to the right. First, the tape should be fully ereased by pressing the ERASE-button. The tape will then run for approx. 15 seconds during which time a Direct Current (DC) is supplied to the recording head.

DAVAC burst encoder. Photograph via [2].

Once the tape is ereased, a new message can be recorded. Press the START button to initiate a recording session. Each time one of the number-buttons is pressed, the tape is advanced and a rotating drum with interrupt switches causes the selected number to be recorded on tape as a series of dashes and dots (morse code). When the message is complete, the STOP button should be pressed. Now set the transmitter to the desired frequency and press the REPLAYING-button.

The interior of the DÁVAČ can be accessed by removing the 6 screws around the edges of the control panel. The entire assembly may then be removed from the case. The device is built around a metal chassis with the motor and the coded drum taking up one half, and the electronics taking up the other half. The tape mechanism is located at one of the short sides.

The image on the right is taken from the manual and shows the device as seen from the bottom. At the upper right is the DC electromotor that drives the mechanics. Immediately below that is the coded drum that consists of 10 discs of which the notches represent the dashes and dots that make up the morse code characters 0-9.

At the far left is the actual recording mechanism that can be accessed from the short side of the device. It consists of an endless magnetic tape that is held in a small compartment, a pressure roller, a capstan, a record/playback head and several tape guides, much like an old recorder.
Interior of the Davac recorder. Click for a close-up.

As far as we know, there are no surviving examples of this device, which is why we can only show the black-and-white photographs from the manual here. The device is very similar to the later MĚSIC burst encoder however, so for a closer look at the mechanism, please follow this link.

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DÁVAČ should be connected to the SIRIUS transmitter (TI-466 AB) via the rectangular 6-pin socket at the right side. This socket is identical to the one at the front of the transmitter. As the burst encoder should be placed in front of the transmitter, only a short 1:1 cable is needed.

  1. Modulation (+)
  2. LT (6.3V AC)
  3. LT (6.3V AC)
  4. HT in
  5. HT out
  6. Modulation (GND)

  • TI-485
  1. TI-485 'DÁVAČ' technical manual (with circuit diagram)
    20 December 1959. Scanned August 2015 from faded original.
  1. Anonymous, DAVAC technical description - THANKS!
    Documentation kindly donated by anonymous former user. July 2015.

  2. Louis Meulstee, Wireless for the Warrior, Part 4 Supplement
    Forthcoming. Accessed August 2015.
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Crypto Museum. Created: Tuesday 04 August 2015. Last changed: Thursday, 29 June 2017 - 19:20 CET.
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