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Filin
Body wearable direction finder

FILIN (Russian: ФИЛИН) 1 is a body-wearable direction finder and intercept receiver, developed around 1970 in the former Soviet Union (USSR). It exists in three different versions, and was used throughout the Cold War for locating clandestine (spy) transmitters and for intercepting adversary agent communication. It was used by Eastern Block secret services like the KGB and the Stasi/MfS.

The receiver was designed for concealed (covert) operation, and has a curved shape to allow it to be carried on the chest easily, hidden under the operator's clothing. It is powered by an external 12V source, usually provided by a rechargeable battery that could be attached to the waist belt.

Depending on the application, the user picked one of the three supplied antenna types. The image on the right shows a typical FILIN model A, that was used by the feared Ministerium für Staats­sicherheit (MfS, or Stasi) of the former DDR (East-Germany). It is modified in several ways.
  
FILIN-A body-wearable receiver

The most obvious modification is the addition of an RF pre-amplifier which is bolted-on at the bottom of the case. It makes the receiver more suitable for intercepting secret agent-to-agent communication, and can be bypassed with a slide-switch at the side. The East-Germans also replaced the Russian antenna socket with a regular BNC one, that was more commonly available in the DDR. FILIN was supplied with a range of accessories, packed in an unotrusive suitcase.

FILIN is clearly modelled after early body-wearable direction finders like Soyka and R11-PA, which in turn are modelled after the German WWII Gürtelpeiler made by Kapsch in Austria. It is currently unknown how many devices were made, but they were in production for several years, and were used in most Warsaw Pact countries. FILIN was later complemented by the aperiodic SINITSA.

  1. Filin is the Russian word for owl. Like most Soviet covert equipment of the era, it was named after a bird.

Suitcase Filin-A with accessories in suitcase FILIN-A body-wearable receiver Indicator used as field strength meter Indicator FILIN battery pack Loudspeaker with safety-pin at the rear
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Suitcase
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Filin-A with accessories in suitcase
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FILIN-A body-wearable receiver
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Indicator used as field strength meter
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Indicator
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FILIN battery pack
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Loudspeaker with safety-pin at the rear

Features
The diagram below gives an overview of the controls and connections on the body of the receiver. FILIN is usually carried on the chest, with the frequency scale at the top, visible to its operator. A 12V DC power source (ПИТАНИЕ), a miniature loudspeaker (ТЛФ) and a field strength indicator (ЛИН) can be connected at the right side. A suitable wire or loop antenna is connected at the top.


The receiver is switched ON with the upper slide switch on the rightmost sloped panel. A rotary switch – at the far right of the top panel – is used to selected the desired type of modulation – AM or FM (ЧМ) – and signal – PHONE (ТЛФ, telephopony) or TONE (ТЛГ, telegraphy). The desired frequency is adjusted with the recessed tuning knob to the left of the frequency scale, and can be fine-tuned with a dial on the sloped front panel, close to the Sensitivity and Volume controls.

Power Supply
Filin could be powered in various ways. It is typically be powered by a 12V DC source connected to the socket marked Питание (Power) on the side of the receiver, but accepts any DC voltage between 9 and 15V. It comes with two types of batteries: A rechargeable battery block and an assembly, packed in green cloth, to carry three flat-pack 4.5V batteries.

Models
As far as we know, three different versions of Filin were made, each with its own frequency range. They are designated by the first three letters of the Cyrillic (Russian) alphabet: A, B and V (or in Russian: А, Б, В). The model is engraved on the left of the top panel of the receiver.

The image on the right shows a typical top panel of a Filin model-A. It is marked as БЛ.П-А, which is the abbreviation of Блок Приемник-А (Unit Receiver-A). The serial number is engraved directly below the model designator (110088).

  • Filin A (А)* /24-63 MHz
  • Filin B (Б)* / 60-150 MHz
  • Filin V (В)* / 148-308 MHz
*) Russian designator
  
Model and serial number

Filin (model A) body-wearable intercept receiver Front panel with additional controls Controls on the front panel Top panel Side panel with connections Antenna socket and MODE-switch Model and serial number Adjusting the frequency
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Filin (model A) body-wearable intercept receiver
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Front panel with additional controls
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Controls on the front panel
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Top panel
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Side panel with connections
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Antenna socket and MODE-switch
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Model and serial number
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Adjusting the frequency





Parts and accessories
Suitcase Body-wearable intercept receiver
Receiver
Rechargeable battery pack Alternative battery holder Power converter 12/24V Headphones (miniature speaker) Various wire antennas Loop antennas
Power voltage and signal strength meter Cable for connecting the indicator to the receiver Leather carrying belt
Suitcase
The Filin receiver was usually packed inside a common unobtrusive suitcase, together with all accessories. Various designs of this suitcase are known, with the one shown in the image on the right being one of the smaller variants.   
Suitcase

Reciever
The image on the right shows the actual receiver. It has a curved shape to allow it to be concealed on the human body more easily. It was available in three versions, each with its own frequency range.
  • А (A) / 24-63 MHz
  • Б (B) / 60-150 MHz
  • В (V) / 148-308 MHz
  
FILIN-A body-wearable receiver

Accumulator
By default, Filin was powered by a rechargeable battery, consisting of a bakelite container with 7 cells of 1.5V each. This gives a total of 10.5V. It is shown in the image on the right.

The container is curved – just like the receiver itself – so that it can be worn more easily on the body, attached to the belt. It has a fixed power cable, with a 4-pin connector at the end that mates with the power socket on the body of the receiver.
  
FILIN battery pack

Battery
As an alternative, an assembly was supplied that allowed three standard flat-pack 4.5V batteries to be connected in series, producing 13.5V.

The flat-pack battery assembly is shown in the image on the right. It has a fixed power cable, with a 4-pin connector at the end that mates with the power socket on the body of the unit.
  
Alternative battery assembly, allowing two 6V cells to be used.

Converter
The receiver could also be powered by an external source, such as the battery of a car or truck. A special power connection box (2) was supplied for this purpose. The box has a switch that allows selection between 12V and 24V.

The box is shown in the image on the right. One side of the box is connected to the external 12V or 24V source. The other side has a typical 4-pin rectangular plug that mates with the power input socket of Filin (ПИТАНИЕ). The switch on the front panel of the box should be set to the correct voltage 12V (12В) or 24V (24В).

  
Power converter

Headphones
FILIN was supplied with two miniature speakers, known as NOVA (НОВА), one of which is a spare one. The speaker has a safety pin at the rear, which allows it to be attached to the clothing.

The speaker has a 70 cm long cable with a coaxial screw plug at the end, indentical to the ones used on the indicator cable. These plugs are extremely hard to find, so we have supplied a drawing with the exact dimensions.
  
Loudspeaker with safety-pin at the rear
Wire antennas
FILIN was supplied with three antennas, each of which was made for a specific application. For simple interception of agent communication, it was sufficient to use the one-element antenna, which consists of a single with with a crocodile clip for attachment to the operator's clothing.

For simple radio direction finding tasks, and for improved interception of agent communication, it was better to use the two-element, or V-type, antenna. Depending on the version, it consists of two wires with crocodile clips at the end, or with straps for attachment to the wrists or ankles.

  
V-antenna with anke-straps

Loop antenna
In addition to the simple wire antennas shown above, the receiver was sometimes used with a loop antenna, that could be worn on the body, similar to a bra. The loop antenna is best used for radio direction finding tasks, and requires the operator to rotate his body in order to find the direction with the minimum signal strength.

Neither of the antennas is resonant, and are intended only for use in close proximity of the (clandestine) transmitter.
  
Loop antenna ('bra')

Indicator
Some FILIN kits were supplied with the indicator (ИНДИКАТОР) shown in the image on the right. It has a meter, a two-position switch and two sockets, and serves two purposes: (1) a voltage checker and (2) a field strength, or FS, indicator.

With the switch set to ПИТ and the battery pack connected to the ПИТАНИЕ socket, the voltage of the battery pack (or a connected car battery) can be checked. With the switch set to ВЫХ and the ЛИН output of the receiver connected to the ВЫХ socket, it acts as a field strength indicator.

  
Indicator

Cable
This cable was used to connected the indicator to the line output (ЛИН) of the receiver, in order to use it as a field strength indicator. The cable is approx. 80 cm long and a coaxial screw plug at either end. Note that the Н/ПР switch on the body of the receiver has to be placed in the Н-position when using the field strength indicator.

This cable was only supplied if the indicator was also present. Nevertheless, it is often missing from the surving FILIN sets. Note that the plugs for this cable are very hard to find.

  
Cable for connecting the indicator as field-strength meter

Leather belt
Filin was supplied with a leather belt that allowed the receiver to be worn on the chest. The belt has 4 hooks (2 at either side) that are attached to the metal brackets at the corners of the receiver, as shown in the image on the right.

In the middle, the belt has a buckle that can be used to adjust it to fit tightly around the waist. Over time, different designs of belts were supplied. The images below show a brown belt with safety catches and a simple black hook-on belt.
  
Leather belt attached to Filin

Suitcase Suitcase Filin-A with accessories in suitcase Inside the case lid Inside the case FILIN battery pack Opened battery pack 7 rechargeable cells inside the battery pack
Indicator used as field strength meter Indicator connected to the line output of the receiver Indicator used as battery checker Indicator Leather belt Another view of the leather belt Leather belt attached to Filin Loudspeaker with safety-pin at the rear
Single-wire antenna V-antenna V-antenna with ankle straps V-antenna with anke-straps Loop antenna ('bra')
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Suitcase
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Suitcase
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Filin-A with accessories in suitcase
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Inside the case lid
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Inside the case
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FILIN battery pack
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Opened battery pack
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7 rechargeable cells inside the battery pack
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Indicator used as field strength meter
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Indicator connected to the line output of the receiver
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Indicator used as battery checker
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Indicator
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Leather belt
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Another view of the leather belt
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Leather belt attached to Filin
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Loudspeaker with safety-pin at the rear
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Single-wire antenna
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V-antenna
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V-antenna with ankle straps
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V-antenna with anke-straps
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Loop antenna ('bra')

Checklist
Inside the lid of the suitcase was a checklist with all items and their location. Various layouts of the case exist and a different number of accessories was supplied with each version of the receiver (depending on the frequency range). Furthermore, the contents were changed and items were swapped for alternatives regularly without altering the checklist.

Location of the parts inside the suitcase

The drawing above shows one of the case layouts that was used. The list below has been compiled from various checklists found inside the top lid of the cases. It shows the location number of each item in brackets at the end of each line. This number corresponds with the drawing above. The number is also imprinted at the bottom of the case.

  1. Приемник
    Receiver (1)
  2. Блок питан. аккум.
    Rechargeable battery (6)
  3. Ремень
    Belt (7)
  4. Антенна V
    2 x V-Antenna (8)
  5. Антенна Штюрь
    Rod antenna (8)
  6. Блоцк Питан. Бат.
    Battery block 3 x 4.5V (9)
  7. Телефон
    2 x Speaker (10)
  8. Кабель (l: 800)
    Cable (9)
  9. Отвертка
    Screwdriver (ED 76-09) (11)
  10. Отвертка
    Screwdriver V 150 x 0.4 (12)
  11. Фильтр
    HF Filter (5)
  12. Power converter (2)
  13. Комплект запасн. частей
    Set of spare parts (8)
Please note that different layouts were also used and that certain parts were sometimes stored in a different place. During the receiver's lifetime, some parts were removed and other were added. The items in the bottom part of the case are held in place by means of leather straps. Location (2) is hidden under item (1). A leather belt (7) is supplied to allow the receiver to be carried on the chest. Position (4) is believed to be reserved for an extra filter or for the indicator. The purpose of position (3) is unclear. It is possible that it was used to store an (optional) battery charger.

WARNING — Never carry the suitcase by its handle. The grip of the suitcase is made of poor-quality plastic which has become brittle after all these year. Carrying the suitcase by the grip, might cause it to break. As a general rule: never carry a museum piece by its grip.
Interior
Filin is built inside a die-cast aluminium frame, with aluminium covers at the front and rear. The interior is exposed when the front and rear panels are removed. This is done by removing the small bolts at the edges of the panels. The image below shows the font side of the interior.

The frequency dial, which is normally at the top, is shown here at the bottom. At the bottom right are the controls. The various pre-amplifiers, filters and IF-sections are housed inside a series of shielded boxes. At the top right is the power supply that generates the necessary voltages.

In order to improve the reception of weak signals, the Stasi (secret police) of the former DDR (East-Germany) added an extra 2-stage pre-amplifier which is bolted to the bottom of the receiver. It allows Filin to be used at greater distances from the intercepted object.
  
Filin interior

The pre-amplifier is curved in the same way as the receiver itself and can be accessed from the bottom, by removing an aluminium panel. The antenna signal from the BNC connector on top of the unit, is fed directly to the pre-amplifier. The output of the amplifier is then used as input to the receiver. Operation of the pre-amplifier can be controlled by a small slide-switch mounted at the side. It has two positions: FERN (FAR, pre-amplifier enabled) and NAH (NEAR, pre-amplifier bypassed). It is the only switch that is marked with German text rather than Russian.

Filin interior Filin interior Font view of the interior Rear view of the interior Close-up of the interior in the area of the controls Close-up of the bottom left corner Close-up of the MODE-selector (interior) Close-up of the interior in the area of the controls
Close-up of some small circuits The PCB inside the HF pre-amplifier that was added by the DDR Close-up of the pre-amplifier Close-up of the Near/Far switch of the pre-amplifier FAR/NEAR switch on the pre-amplifier at the bottom Operating the FAR/NEAR switch
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Filin interior
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Filin interior
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Font view of the interior
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Rear view of the interior
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Close-up of the interior in the area of the controls
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Close-up of the bottom left corner
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Close-up of the MODE-selector (interior)
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Close-up of the interior in the area of the controls
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Close-up of some small circuits
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The PCB inside the HF pre-amplifier that was added by the DDR
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Close-up of the pre-amplifier
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Close-up of the Near/Far switch of the pre-amplifier
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FAR/NEAR switch on the pre-amplifier at the bottom
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Operating the FAR/NEAR switch




Indicator circuit diagram

The image above shows the circuit diagram of the indicator. It is clearly visible that the box contains two seperate circuits. The upper circuit is the battery condition meter. Resistor R1 is used to create an artificial load, whilst R2 is dimensioned to give a proper reading on the voltage scale. The lower section is the field-strength meter. D3 is a zener diode that protects the meter (M1) against excess voltages. The circuit is shown here with the switch (S1) set to ПИТ (voltage). As we swapped the meter (M1) for a different type, we had to change R2 and R3 accordingly.

It is not clear where the indicator was stored inside the suitcase. One possibility is that is was stored in position (6), instead of the rechargeable battery pack. Another possible location is (4) which has the proper footprint for the indicator, but is a bit too close to the receiver (1).


Restoration
When we obtained our FILIN set, the meter of the indicator was broken. The kit had been stored in a moist place for many years. As a result, some accessories were oxidized and the meter was completely blocked. The scale of the meter had deteriorated so much that it was beyond repair.

In order to bring the indicator back to life, we replaced the meter by a similar (Russian) type of the same era. As this meter has different characteristics, some of the resistor value inside the indicator had to be changed accordingly.

A few years later we found a nearly identical meter on a fleamarket in Germany, which has allowed us to restore the indicator to original condition.
  
Defective meter

Indicator Charger carried in a hand Connections of the charger Defective meter Indicator in use as field-strength meter Indicator in use as field-strength meter Indicator Indicator used as field strength meter
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Indicator
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Charger carried in a hand
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Connections of the charger
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Defective meter
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Indicator in use as field-strength meter
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Indicator in use as field-strength meter
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Indicator
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Indicator used as field strength meter

Connections
Power socket
Below is the pinout of the power socket on the body of the receiver, when looking into the socket. Also shown is the circuit diagram of the 12V/24V power selector that is available as an accessory.

  1. (-) 12V
  2. (-) 12V
  3. (+) 12V
  4. (+) 12V
    Filin power converter circuit diagram
Speaker socket
Below is a detailed drawing of the rather rare speaker plug. The same plug is used on the UFT-421 body-wearable covert radio, and can be difficult to find. For this reason the dimensions have been given as well. With the proper tools it might be possible to create a usable alternative.

The rather strange loudspeaker plug

The speaker is relatively high-impedant (approx. 2000 ohm) and produces a loud signal. If the speaker appears to be dead, it can easily be checked with a multi-meter. The (DC) resistance should be approx. 200 ohm. In many cases, the wires inside the connector are broken.


Help required
We are still looking for the user manual and the technical manual (including circuit diagrams) of this receiver. If you have any of these available, or if you have additional information, please contact us. Furthermore, any stories about the use of this receiver are most welcome.


References
  1. Paul Reuvers and Marc Simons, Filin-A portable intercept receiver S/N 110088
    Crypto Museum, Investigation March 2012.

  2. Louis Meulstee, USSR Portable Intercept Receivers
    Wireless for the Warrier. Volume 4. September 2004. ISBN 0952063-36-0.
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© Crypto Museum. Created: Monday 03 August 2009. Last changed: Sunday, 02 December 2018 - 08:44 CET.
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