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Burst encoders
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R-394K   Strizh - Стриж
Russian spy radio set (Swift) Mark I and Mark II

The R-394K was a Russian spy radio set that was developed in the early 1980s in the USSR as the successor to the R-354. The device is fully self-contained and uses an internal battery as its power supply. It features an analogue transmitter and receiver, and an analogue tape-based burst encoder. A later version had a digital burst encoder (shown here). The R-394K was codenamed Strizh (Swift) and was short lived. It was replaced in 1987 by the fully digital R-394KM.
At least two different versions of the R-394K exist and they are both extremely rare. The first one has a built-in tape-based burst encoder. Very few of these are in the hands of collectors and we are still looking for one (see below).

The image on the right shows the later, even rarer, version of the R-394K. It can be viewed as a half-way version between the earlier analogue R-394K, which uses a magnetic tape, and the later R-394KM, which was fully digital. The transmitter is located at the top right, whilst the leftmost unit is the digital burst-encoder.
Controls on the R-394K spy radio

The radio is housed in the same metal case as the later R-394KM. All electronics are located in the bottom part, including the battery, whilst some of the accessories are stored in the top lid. When not in use, the headphones are stored behind a special bracket that is part of the top lid of the metal case. Also in the top lid is a 2-size screwdriver and a small metal box with spare fuses.
Closed case of the R-394K R-394K with open lid, ready for use. Controls on the R-394K spy radio Using external power External power supply Transceiver of the R-394K Operating the burst encoder Morse key and connector for external key or external burst encoder

  • R-394K Mark I
    This is the most common version of the R-394K. It consists of a transmitter and receiver with an analogue PLL, and a mechanical tape-based burst encoder that is fitted at the left.

  • R-394K Mark II
    This is an extremely rare version of the R-394K that has a digital burst encoder fitted at the left, instead of the mechanical tape-based one. It is believed that this variant was short-lived and that it was replaced soon after its introduction, by the R-394KM.

Modular design
Two different versions of this radio are known: one with an analogue (tape-based) burst encoder, and one with a digital burst encoder. Apart from the burst encoder, the two radios are identical. The radio has a fully modular service-friendly design. The main wiring between the units is fixed inside the chassis and each unit plugs straight into the chassis by means of sub-D connectors.
Apart from the battery, there are 4 functional units, or Blocks, as the Russians call them:
  1. Transmitter (TX)
  2. Burst Encoder
  3. Receiver (RX)
  4. Power Supply Unit (PSU)
Four female-type sub-D sockets are mounted on a stub inside the chassis, in such a way that they mate with the male-type connectors on each of the units. Additional guide pins at the bottom are provided to firmly lock the units in place.
Top view of the chassis after removing all units

Each unit is fully self-contained. In case of a defect, a module can easily be removed and replaced by a new one, without losing the alignment of the other units. This way it was possible to replace the analogue tape-based burst encoder with a digital one when new techniques became available. The digital burst encoder is a functional replacement for the old one that had become obsolete.
Top view of the chassis after removing all units PSU connector RX unit connector Burst Encoder connector Perspective view of the empty chassis Another perspective view of the empty chassis DB15 plug for connection to the chassis

R-394K Mark I
Below is an overview of the control panel of the earlier R-394K with the analogue tape-based burst encoder at the far left. The Russian text has been replaced by suitable English translations. At the center of the leftmost unit is a large 'blob' which is actually a cover that protects the plastic tape cassette. At the bottom left is the start button of the tape player, with a warning: rewind the tape before starting. The external power connector at the top left is not present on all models.

Earlier version of the R-394K, with tape-based burst encoder.

R-394K Mark II
The drawing below shows the control panel of the later R-394K. In the image, the Russian text has been replaced by suitable English translations. At the left is the digital burst encoder that is described further down this page. It is connected to the rest of the radio set via an internal 15-way sub-D connector (DB15). Immediately to the right of the burst encoder is space for a 15V battery. At present, we do not known if there was a connector for an external power source.

Later version of the R-394K, with digital burst encoder.

Apart from the burst encoder, the two versions of the R-394K are idential. The antenna and a suitable counterpoise are connected to the terminals at the top right of the transmitter. The bottom two units are the receiver and the power supply unit. An external 10-button morse keypad, can be connected to the PSU. All modules (blocks) are fully described below. The digital burst encoder of the R-394K Mark II is identical in operation to the encoder of the later R-394KM.
Power Supply Unit   PSU
The unit at the bottom right (block 2) is the power supply unit (PSU). The battery voltage (12V) is distributed throughout the radio by this unit. Furthermore, a stabilized voltage of 8V is produced for the external morse generator and several other parts. Three black rectangular shadow-type push-buttons at the bottom left are used to selected the main mode of operation [3].
The leftmost button selects the receiver, whilst the rightmost button (marked with red text) enables the transceiver. The middle button is used to enable the calibrator (marker). The radio is turned off by pressing one of the 3 buttons half-way down, so that they are all released.

The switch at the top left is used to turn the overhead lamp on or off. The lamp is mounted to the body of the transmitter and is intended for reading the scales of the frequency dials. The switchis shown here in the off-position. Spare lamps are stored inside the top lid of the case.
Block 2 - PSU

To the right of the switch is a 5A fuse that is used for protection of the internal battery. The small switch at the far right can be used to check the battery voltage. When pressed, the small meter at the top right of the transmitter will show the actual battery voltage. At the bottom right is the morse key that can be used to manually send messages in morse code, without using the burst encoder. To the left of the morse key is a 10-pin expansion connector with the following pin-out:

pinout of the 10-pin expansion socket, when looking into the socket.

Please note that the original manual is wrong about the pinning of this connector. Furthermore, the pinning of the same socket on the later R-394KM is completely different. A device that is designed for one model will get damaged when connected to the other one! So, be careful.

An external morse key can be connected to this socket between KEY and GND. The socket provides a stabilized +8V for the external morse generator (early model) and the raw +12V battery voltage for any other device. The PSU also produces a stabilized 25V for the transmitter power amplifier (PA). When in TX-mode, this voltage is also available on the expansion socket.
PSU front panel (block 2) Block 2 - PSU PSU interior Another view of the interior of block 2 Part of the 25V power supply Interior of the built-in morse key Russian precision potentiometer Modified DB23 connector

Transmitter   TX
The transmitter (block 1) is located in the top right. It is a very complicated but very clever combination of 3 more or less identical frequency units (VFOs) that are mixed in such a way that the full range between 1500 kHz and 13.499 MHz can be used with an accuracy of ± 500 Hz. All the necessary internal frequencies are derived from a single 12 MHz crystal oscillator [3].
The image on the right shows the transmitter once it is removed from the chassis. Each of the functional blocks is housed in its own shielded case, most of which are purpose-built die-cast aluminium blocks. The blocks are mounted on an axle in such a way that they can easily be 'turned' out of the way when servicing the unit.

The images at the bottom of this section give a good impression of the build quality of the transmitter. Behind each of the three frequency dials is an adjustable capacitor that is 'trimmed' mechanically in order to make the scale linear.
Transmitter unit (block 1)

The transmitter circuit is extremely complex. In order to make it more comprehensible, we've divided it into a number of simpler block diagrams. Let's first have a look at the main quartz-controlled 12 MHz oscillator. After buffering the 12 MHz signal, it is divided several times. All intermediate frequencies are distributed throughout the transmitter and are used at various places. At the far right is the calibrator that produces a 500 kHz signal for the receiver (block 3) and an combined 20/100 kHz signal for the transmitter (press the calibrator switch on block 2).

R-394K transmitter main oscillator

The actual transmitter consists of three independent VFOs that each produce a carefully worked out frequency range. VFO-1 controls the output frequency x1000. It is fed by a 500 kHz signal and produces frequencies in the range 17.5 - 29 MHz. The two other VFOs (2 and 3) are driven by a 20/100 kHz signal and produce lower frequencies in the range 2 - 3 MHz.

R-394K transmitter block diagram

The block diagram above shows the frequency range of each VFO (in red). At the bottom of the diagram are four mixers that combine the outputs from the three VFOs with the 1 MHz and 12 MHz signals from the main oscillator. Finally, a low-pass filter (LPF) is used to ensure that only frequencies in the range 1.5 - 13.499 MHz are used. Behind the filter is the Power Amplifier (PA) and the Antenna Matcher. The connections at the right are the antenna (A) and counterpoise (C).

Note that the frequency from VFO-3 is first divided by 20 before it is applied to the first mixer. Immediately behind the divider is a phase shifter that is used to modulate the morse signal from the burst encoder when Phase-Modulation is selected (F1). Now lets look at the VFO in more detail. The block diagram below shows a single VFO, which is in fact an analogue PLL:

R-394K single VFO

The reference signal is entered on the left, whilst the manually adjusted free-running oscillator is at the right. Both signals are compared in a phase detector that drives a DC Amplifier, followed by a Proportional Integrating Filter and finally a Voltage Amplifier that applies a correction signal to the free-running oscillator. Once the frequency scale is near a frequency that is a multiple of the reference signal, the PLL locks-in and the indicator light is driven by the DC amplifier.
The image on the right shows scale calibration in progress. The calibrator is turned on (middle button on the PSU) and the dials are each set to a marker position. The indicator lights above the scales should then be lit. In case a lamp is not lit, adjust the dial until it does. Then adjust the hairlines with the two big screws below the dials.

Once the radio has been sufficiently warmed up (i.e. after approx. 10 minutes) and the scales are calibrated, any frequency can be adjusted with an accuracy of ± 500 Hz. The transmitter produces an HF output power of approx. 10 W.
The R-394K in use, showing the calibrator (marker) in operation.

This is sufficient for a range between 150 and 1200 km, depending on the antenna, the time of the day and the frequency in use. The radio can therefore be classed as medium-range. It can be operated in ambient temperatures between -20°C and +40°C and a humidity of 98% (at 35C).

Burst messages are usually sent in groups of 5 digits. When used with Amplitude Modulation (AM) the radio can send approx. 12 such groups per minute. However, when using Phase Modulation (PM), data can be sent at 167 groups per minute (select F1 on the burst encoder). PM is achieved by injecting the morse signal directly into the phase shifter of VFO-3 (see above).
Transmitter unit (block 1) Front view of the transmitter whilst adjusting the overhead light Interior of the transmitter 23-pin sub-D plug for connection to the chassis Removing the locking pin from the modules Folding-out the frequency units Close-up of a folded-out frequency module Outmost module in the transmitter
A frontal look at the frequency adjustments once the frequency module are all folded away. Perspective view inside the transmitter Perspective view inside the transmitter Close-up of a power transistor Close-up of a tuning capacitor Close-up of the adjustments of the outmost module Small module at the side of the transmitter Peeking inside the power amplifier (PA)
The R-394K in use, showing the calibrator (marker) in operation. Close-up of the dials when calibrating the scales. Note the black markers on the dials. The lamps indicate a lock-situation. Scale hairline adjustment

Receiver   RX
The Receiver (block 3) is a complete self-contained unit. It is connected to the fixed wiring of the chassis by means of a DB15 connector through which all power, control and HF-signals are supplied. The individual circuits are each built into their own metal cases, packed together as one compact modular unit that can easily be replaced. The receiver is a three-stage superheterodyne design with 190 fixed crystal-controlled channels, as shown in the block diagram below [3].
On the front panel are two channel selectors (marked I and II), plus a two-position switch (III) that allows the first channel selector (I) to choose between two banks of crystals (1 and 2). The same selector (I) also controls the pre-selection filter of the antenna input amplifier.

The 2nd IF consists of another bank of crystals, controlled by the other selector (II). This results in a 2nd IF frequency of 501.7 kHz. Finally, the 3rd stage, adjustable with a capacitor (TONE) converts the signal into audible LF frequencies that are amplified to headphones level.
Receiver block (3)

Below, the simplified block diagram of the receiver is shown. The first functional block is shown here as a band-pass filter, but it is in fact a two stage MOSFET-based pre-amplifier with a series of calibrated pre-selection filters, in order to obtain the best possible selectivity. The first channel selector (I) is a quite complex construction consisting of a 10-position 5-deck rotary switch amidst the three circuits it is connected to. The second selector is much simpler.

A 500 kHz calibration signal from the transmitter can be injected at the 3rd stage, so that the TONE adjustment can be zeroized. Furthermore, an audible tone from the burst encoder is injected directly in the LF output stage. It can be used to monitor any outgoing morse signal.
Receiver front panel Receiver block (3) Interior of the receiver block. On the left are the pre-selector and the two crystal oscillators of the 1st stage. Another view of the interior of the receiver block Close-up of Channel Selector (I). Check all the wires going to the pre-selector and the two crystal oscillators. Another view at the Channel Selector (I) Close-up of one of the crystal banks DB15 plug for connection to the chassis

Analogue Burst Encoder   wanted item
The initial version of the R-394K had an analogue tape-based burst encoder in the leftmost position. It allowed pre-recorded (encrypted) numerical messages to be played back at high speed in order to minimize the risk of interception and detection. A spare cassette was supplied with the radio and was stored inside a metal box that was integrated with the top lid of the case.
The leftmost unit consists of a blank panel, with a rectangular 'blob' in the middle. The blob is actually a lid that can be opened. Below the lid is the metal tape cartridge, which is quite similar to the one used with the R-353 spy radio.

The image on the right shows the transparent plastic cassette with the metal tape, that is reveiled when the lid is opened. The tape itself is perforated with square sprocket holes that are driven by a cog-wheel inside the tape player. It is unclear at present, how coded messages are recorded on the tape (internal or external).

The photographs of the earlier R-394K are kindly supplied by Italian collector Antiono Fucci [2]. More information can be found on his site. At present, we have no full documentation of the R-394K (only a partial manual). If you can supply additional details, please contact us. Furthermore, we are still looking for this model so that it can be added to our collection. Wanted item.
Burst encoder in the earlier R-394K. Photo copyright Antonio Fucci. Metal tape of the burst encoder in the earlier R-394K. Photo copyright Antonio Fucci. Play back unit of the burst encoder of the earlier R-394K. Photo copyright Antonio Fucci.

Digital Burst Encoder
Sometime in the mid-1980s a digital burst encoder was introduced. It fits the leftmost slot and offers a plug-in replacement for the ageing analogue tape-based burst encoder (see above). The new encoder is functionally identical to the integrated burst encoder of the later R-394KM.
The burst encoder consists of a red 5-digit 7-segment LED readout, 10 push-buttons and two red LEDs. It is also similar in operation to the external burst encoder that was supplied with the post Cold War Severok-K spy radio.

At present, the digital burst encoder in our R-394K is not working as parts of it have been removed as part of the demilitarization process. We hope that it will be possible to restore the original functionality in due course. When this happens, we will provide a full operational description of this unit.
Operating the burst encoder

For the time being, please refer to the built-in digital burst encoder of the R-394KM.
Operating the burst encoder The burst encoder to the left of the battery compartment Connector on the burst encoder Connector on the burst encoder Morse key and connector for external key or external burst encoder

The R-394K was usually supplied in a large wooden box, complete with a number of accessories, spare parts and full documentation. At present it is not exactly known which parts were supplied and no complete kit has been found to date. For the time being, we have to rely on the items listed in the manual.

According to the manual [3], a separate hand-operated power generator was supplied in order to charge the batteries in the field whenever no mains power was available. Charging the batteries is not an easy task and takes several hours, for just a few minutes of operation.

The generator is identical to the generators supplied with other Russian spy radios.

Almost any type of headset can be used with the R-394KM. In most cases, a common USSR military headset was supplied, with rubber ear pads and elastic head bands. Such headsets are commonly used with military radio sets in tanks etc. Headsets are connected to the two-pin socket on the left of the front panel of the radio.   

The antenna is usually stored inside a cavas packet to the right of the radio. Depending on the way the radio is used, a second antenna might be needed as a counterpoise. The second antenna (see the images below) can be stored in the same pocket.   

Antenna mast
In order to setup the antenna as required, a light-weight telescopic glass fibre mast is supplied. It allows the antenna wire to be mounted free from obstacles and the earth. A ground pin is supplied to prevent the mast from sliding away.

The mast and the ground pin are stored inside a canvas bag that can be strapped to the radio or the canvas raincoat (see below).

Canvas raincoat
The radio is already painted in the usual Russian 'sand colour' camouflage tint and has suitable padding at the bottom to allow the radio to be carried on the back of the radio operator.

The lid of the unit is firmly closed with clamps and a gasket, to protect the radio against dirt and water. Further camouflage is possible by fitting the canvas raincoat shown on the right.

A set of spare fuses and lamps is stored inside a small metal box that is fitted inside the top right corver of the top lid. The fuses are all 5A types of typical russian size. They are used by the PSU (block 4). The lamps are spares for the indicator lights above the three frequency scales of the transmitter.

The image on the right shows the spares box with its lid open. At the top left (inside the box) is a piece of insulating tape that can be used for small repairs.
The opened spares box

The generator with the crank folded out Headset Antenna Extra antenna The packed mast Telescopic antenna mast Canvas raincoat Spares box

A complete R-394K radio station comes with an an extensive set of documents that are usually stored inside the large wooden storage box. The documents include the operator's manual and full circuit diagrams of the analog and digital parts. Some of the books are marked 'Secret'.
Unfortunately, most of the original documention has been lost or has deliberately been destroyed by the authorities. For the R-394K featured on this page, we only have a partly complete manual available. The manual features the earlier (tape-based) model. No description of the digital burst encoder has been found to date.

The image on the right shows our current manual. The text СЕКРЕТНО (Secret) has been removed from the front cover and approx. 20 pages and circuit diagrams have been ripped out as part of the demilitarization process.
Close-up of the front cover of the R-394K manual

You can help us by supplying a complete manual, or a copy, and any other additional documentation, information or items. Any contribution is most welcome.
Front cover of the R-394K manual Close-up of the front cover of the R-394K manual First page (index) Removed pages... Impulse diagram Transmitter circuit diagram

  1. Louis Meulstee, R-394KM
    Wireless for the Warrier. Volume 4. September 2004. ISBN 0952063-36-0.

  2. Antonio Fucci, R-394K Transceiver
    Website with photographs of the earlier R-394K. Retrieved February 2012.
    Photographs of earlier R-394K on this page used with kind permission.

  3. Radio Station R-394K, Technical Manual (Russian)
    Original manual of the R-394K. Incomplete (pages removed). Date unknown.

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Crypto Museum. Created: Wednesday 18 January 2012. Last changed: Sunday, 02 October 2016 - 08:59 CET.
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