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Bicho
Spanish valve-based radio bug · 1953

Bicho 1 is a wireless radio frequency (RF) covert listening device (bug), build with three thermionic valves (vacuum tubes), developed in the early 1950s in Francoist Spain, probably for industrial or political espionage, early in the Cold War, before Spain was a member of the United Nations (UN).

The device produces approx. 150 mW of RF power at a frequency on or near 6.865 MHz in the 40 metre band, using amplitude modulation (AM). It was commonly used in combination with an externally connected carbon microphone.

The bare device measures 115 x 70 x 26 mm and weights just 210 grams. It is powered by 1.4V (LT) and 90V (HT), that were supplied by an external LT/HT battery pack or – more likely – an external 220V AC mains power supply unit (PSU). With a properly installed (wire) antenna, the device has a range of several hundred metres.
  
Spanish valve-base bug 'EL BICHO'

The device is built with parts from various European countries but has a spanish signature, most notably the wiring layout that is taped to the enclosure. The device shown here was discovered in 1990 by construction workers in the French customs office (douane) of Le Perthus, just south of Perpignan (France), when the border with Spain was being converted into an EU internal one [1].

It is likely that it had been planted there in the early or mid-1950s, when the French customs office was built or refurbished. Perpignan is the southmost major French city before the Spanish border. In the days before the A9 autoroute, the main road from south-east France to Barcelona crossed the Spanish border in the small town of Le Perthus into El Pertús on the Spanish side [5].

The device had just enough RF power to reach the Spanish customs office about 300 metres to the south, at the other side of the border, where the listening post (LP) was most likely located.
  
The French-Spanish border at Le Perthus in 1953, as seen from the French side [4]. Click to enlarge.

When it was discovered, the wires were cut-off by the construction workers, who probably had no idea what they had found. As far as we know, the accompanying power supply unit (PSU) and the original microphone have not survived. Judging from the construction of the enclosure and its interior, it was not a one-off, but was probably part of a small – professionally produced – series.

  1. As the actual name or designator of this device is currently unknown, we have nicknamed it BICHO (pronounced: bi-choh), which is Spanish for bug, software error or hidden microphone.

Spanish short-wave radio bug 'EL BICHO'
Spanish valve-base bug 'EL BICHO'
Spanish 'Bicho' bug
Wiring table
Antenna label
The French customs office at Le Perthus around 1953 (the lower white building on the right) [4]
The French customs office at Le Perthus seen from the Spanish side [source unknown]
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Spanish short-wave radio bug 'EL BICHO'
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Spanish valve-base bug 'EL BICHO'
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Spanish 'Bicho' bug
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Wiring table
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Antenna label
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The French customs office at Le Perthus around 1953 (the lower white building on the right) [4]
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The French customs office at Le Perthus seen from the Spanish side [source unknown]

Features
The image below shows the features of the (partly reconstructed) listening device. The actual bug – nicknamed EL BICHO – is at the centre. At the right is a piece of wire that acts as the antenna. It can be hidden inside a wall or behind a ceiling. At the bottom left are the power wires, which should be connected to a 1.4V/90V power source, such as a battery or a mains PSU (not shown).

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At the top left is the microphone, which is connected to the device by means of a shielded cable, in order to avoid picking up the 50 Hz hum from the power lines in the building. Judging from the circuit diagram, the device was used with a carbon microphone, such as the one shown here.


Circuit diagram
Below is the circuit diagram taken down from the actual device in our collection. It is built around three thermionic valves (vacuum tubes) and comprises a microphone pre-amplifier (V1), an AM modulator (V2) and an RF oscillator (V3). The latter is driven by a 6865 kHz quarz crystal (X1).


At the left is the audio pre-amplifier (V1). Note the 300Ω resistor (R1) between the microphone and the LT rail (+1.4V DC). This suggests that the device was used with a carbon microphone. The output of the amplifier (V1) is taken from the anode and supplied via C2 to the input of the modulator valve (V2). At the far right is a crystal oscillator built around a 3S4 (V3). It delivers an RF output of approx. 150 mW to the antenna, via the tuned circuits at the top right (L1/C5/C6).

As the power to the modulator (V2) and the oscillator (V3) is supplied via L2 – which has a large inductance of several H – the modulator will load the HT' rail in the rythm of the sound from the pre-amplifier, causing the voltage on the HT' rail to vary accordingly. As a result, the power to the oscillator valve (V3) will vary as well, resulting in Amplitude Modulation (AM) of the RF output.

Two capacitors, C7 and C8, were missing from the device when we received it, but had clearly been present, as part of their legs and the solder joints were still present. It is likely that they were destroyed when the device was discovered and pulled away from the antenna wire, as they were both mounted close to the antenna port. C8 decouples the HT' rail for RF signals and improves the operation of the oscillator. C7 is part of a series-resonance filter, together with L1c. When C7 was destroyed, the top-end terminal of L1c was broken off and had to be reconstructed.


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Interior
The bug is housed in a folded aluminium enclosure that consists of two parts: a base to which all parts are mounted and a case shell at the top. The case shell is bolted to the base by means of 6 screws divided over the four sides. After removing these screws, the case shell can be taken off.

This reveals the interior of the bug, as shown in the image on the right. The three sub-circuits are built around three thermionic valves (vacuum tubes) that are located side-by-side in the left half of the device. They are installed in square black sockets that we have not seen before.

The passive components (resistors, capacitors) are mounted directly to the contacts of the valve sockets. The resistors are not directly visible, as they are covered by the capacitors. The wiring from the outside (LT/HT power and microphone) enters the device in the bottom left corner.
  
Interior

The right half of the device accomodates – from front to rear – a large quartz crystal for the oscillator, the tuned circuit of the oscillator combined with the antenna coil, and a relatively large choke coil. The latter forms part of the AM modulator circuit. At the far right are two trimmers that can be adjusted from the outside: one for the oscillator circuit and one for the antenna. The antenna – typically a 3 metre long piece of wire – leaves the device in the top right corner.

Interior
Interior seen from the RF section
Interior
Interior seen from the valve side
Interior - top view
Reconstructed RF section
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Interior
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Interior seen from the RF section
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Interior
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Interior seen from the valve side
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Interior - top view
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Reconstructed RF section

Restoration
When the device featured on this page was discovered by construction workers in 1990, it was already ~ 35 years old. It had probably been installed covertly behind a wall or a ceiling in the mid-1950s and had done its work for several years before it stopped producing intelligence.

It is unlikely that it was ever detected by French security, as in that case the device would haven been removed. The fact that it was still there in 1990, indicates that it had probably died a few years after its installation. Either the valves were worn out, or the power source had been cut.

It is clear that before its discovery (and possibly also in the 30 years after that), it was kept in a moisturous and dusty place. The enclosure and the interior show severe signs of corrosion, and several solder joints and mounting posts were broken, as can be seen in the image on the right.
  
Interior before restoration

It was therefore unlikely that, when we received the device in 2021, it would still work. After the initial inspection, the valves were carefully removed, so that the pins and the valves sockets could be cleaned. Next, the broken mounting posts (C5, C6) and the broken solder joints were restored.

It was clear that some components were missing from the top right corner of the device (between C6 and L2) and that the antenna-end of L2c was broken off. This had probably happened when the device was discovered and subsequently removed by the construction workers in 1990.

By carefully studying the remains of the lost parts, were were able to make an educated guess of what had previously been located there. The broken (open) end of L1c was restored and capacitors C7 and C8 were added. In the circuit diagram above, these parts are identified in red.
  
Reconstructed RF section

The cut-off wiring was restored and the original valves were tested. Although all three heated up, they drew less current than expected and none of them showed any signs of emission. This was of course to be expected from a device that had probably been on the air 24/7 for several years.

It was decided to replace the valves 1 one at a time, so that each sub-circuit could be tested individually, starting with the oscillator (V3). As it did not produce an RF signal, C5 and C6 were turned until it did. The oscillator then produced a strong and stable signal at 6.865 MHz; the f1 or fundamental frequency of the quartz crystal. 2

Next, the modulator valve (V2) was inserted to see whether it could amplitude modulate (AM) the oscillator. In doing so, it became clear that the 10 nF coupling capacitor (C2) between V1 and V2 was broken. It has lost one of its legs.
  
Reconstructed capacitors

It was replaced by a smaller modern alternative that has been reworked to resemble the old one. To replace it, some of the other components had to be removed temporarily, which gave us a chance to take down the complete circuit diagram and check the values of the old resistors. 3

The modulator (V2) worked, as a 50 Hz hum could be heared in the receiver when the g1 of V2 was touched with a metal object. Next, the microphone amplifier valve (V1) was replaced, but the room audio from the microphone could not be heared through the receiver. As none of the voltages on the pins of V1 were within the expected range, it became clear that the other two 10 nF capacitors also had to be replaced.

Once this was done, the wiring at the bottom of the valve sockets was restored and the unit was tested again. This time it worked as expected.
  
Replaced capactors

As the amplifier circuit (V1) is dimensioned for a carbon microphone — the 300Ω resistor (R1) provides a bias voltage — we decided to use the carbon microphone from a telephone handset of the era, as this might indeed have been used with the device. The choosen microphone picks up any sound in the room and produces a crisp and clear audio signal in our surveillance receiver.

  1. The two 3S4 valves were replaced by the equivalent DL92. As the holes in the valve sockets are too large, the legs of the valves were bended outwards, just like it had been done with the original valves.
  2. The f2 was down by 60dB and the f3 was down by 40dB, so we assume that the device was used at the fundamental crystal frequency of 6865 kHz.
  3. These old composite resistors have an accuracy of 10% and are known to deviate over time. It is therefore important to check whether they are still within reasonable limits. This was indeed the case.

Spanish 'Bicho' bug
Spanish 'Bicho' bug
Interior (before restoration)
Interior before restoration
Interior - top view (before restoration)
Broken adjustable capacitor
Wiring
Loose contact
Corroded valve pins and socket
Reconstructed RF section
Reconstructed antenne terminal on the tank coil
Antenna wire with strain relief knot and series capacitor
Components partly disassembled from the valve sockets
Reconstructed capacitors
Replaced capactors
Replaced parts
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Spanish 'Bicho' bug
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Spanish 'Bicho' bug
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Interior (before restoration)
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Interior before restoration
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Interior - top view (before restoration)
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Broken adjustable capacitor
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Wiring
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Loose contact
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Corroded valve pins and socket
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Reconstructed RF section
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Reconstructed antenne terminal on the tank coil
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Antenna wire with strain relief knot and series capacitor
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Components partly disassembled from the valve sockets
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Reconstructed capacitors
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Replaced capactors
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Replaced parts

Problems
  • Wires cut-off
  • Several broken solder joints
  • Broken contact terminal on RF choke
  • Antenna trimmer broken
  • Corrosion in various places
  • Antenna tank coil broken
  • Two capacitors missing
Restored
  • Valve sockets (and valve contact pins) cleaned
  • Broken contact terminal of choke coil repaired
  • Broken antenna trimmer refitted
  • Anode trimmer mechanically re-aligned
  • External wiring restored
  • Antenna tank coil repaired
  • Antenna output circuit reconstructed
  • Both 3S4 valves replaced by DL92 (legs bended)
  • 800 pF mica capacitor replaced
  • All three 10 nF capacitors replaced
  • Carbon microphone added
  • 1 nF capacitor added between HT' and ground
Connections
Wiring
  • Blindado
    Shielded
    Microphone (hot)
  • Azul +AT
    Blue
    +HT voltage (75 to 90V DC)
  • Blanco +BT
    White
    +LT voltage (1.4V DC)
  • Verde común
    Green
    Ground (common)
Valve 1S5
The 1S5 is a miniature tube consisting of a diode and a sharp-cutoff pentode, developed for use as a combined detector and AF amplifier in battery-powered portable devices. In the bug featured here, it is used as a microphone pre-amplifier. Pinout as seen from the bottom of the valve.

 1S5 datasheet


Valve 3S4
The 3S4 is an audio pentode, also known as a beam tetrode, that was developed in the late 1940s for battery-powered operation in portable radios. It has two filaments which can be used in parallel or in series. In this case they are connected in parallel, so that they can be driven by 1.4V DC. The valve can be replaced by CV820 or DL92. Pinout as seen from the bottom of the valve.

 3S4 datasheet


Specifications
  • Year
    1953 (est.)
  • Origin
    Spain (see below)
  • User
    Spanish intelligence and/or police
  • Discovered
    1990, Le Perthus (France)
  • Frequency
    6.865 MHz
  • Output
    150 mW (est.)
  • Antenna
    Wire (3 metres est.)
  • Circuits
    Microphone amplifier (V1), AM modulator (V2) and Oscillator (V3)
  • Valves
    1S5 (V1), 3S4 (V2, V3)
  • Power
    LT: 1.4V DC, HT: 75 to 90V DC
  • Current
    LT: 230 mA, HT: 3 mA
  • Dimensions
    115 x 70 x 26 mm
  • Weight
    210 grams
Origin
Some of the components of the bug were manufactured in the UK, such as the crystal (STC) and one of the valves (Brimar = STC). The other valves are branded Sonitron (Spain). One of the capacitors is branded Jeepson, Barcelona (Spain). Furthermore, the text on the wiring layout label on top of the device is in Spanish. Considering this, plus the fact that it has a limited range and that it was found in a French customs office, it is very likely that the bug is of Spanish origin.

  • Valve 1S5
    Sonitron (Barcelona, Spain) 1
  • Valve 3S4
    Brimar = STC (Kent, UK)
  • Valve 3S4
    Sonitron (Barcelona, Spain) 1
  • 800 pF cap
    Jeepson, Barcelona (Spain)
  • 10 nF caps
    Unknown brand - 2/1952
  • Crystal
    STC, Kent (UK) - 7/1952
As several of the parts have date markings of 1952, it is assumed that the device was built not long thereafer, most likely between 1952 and 1954. This might have coincided with the building of the new customs office at Le Perthus, which was opened in 1953.

  1. Although this is not entirely certain, it is likely that SONITRON was a brand name of radio and television manufacturer Radiodina in Barcelona (Spain) at the time.

References
  1. Reinhard Glogowski, Spanish bug 'BICHO' — THANKS!
    Received April 2021.

  2. Wikipedia, Spain
    Retrieved April 2021.

  3. Wikipedia, Francoist Spain
    Retrieved April 2021.

  4. Geneanet, Vue générale et Douanes Françaises la fronière Espagnole
    Retrieved April 2021.

  5. Google Maps, Le Perthus / El Pertús
    Retrieved April 2021.
Further information
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Crypto Museum. Created: Sunday 25 April 2021. Last changed: Monday, 03 May 2021 - 20:12 CET.
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