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Capri   7042
Wire line carrier bug countermeasure

Receiver 7042 — codenamed Capri — was a Long-Wave (LW) receiver, developed around 1980 in the former DDR (East-Germany) by the Ministerium für Staatssicherheit (MfS), also known as the Stasi, for the detection of carrier frequency covert listening devices (bugs), connected via existing wiring, such as telephone lines, the mains power network and the antenna cables [1]. The device is part of the Stasi's so-called X-assignment: counter measures (German: Kontertechnik) [2]. It was manufactured at the Volkswirtschaft (VW) and was also known as Stasi project number 1079.

During the Cold War, carrier frequency bugs (German: TF 1 Wanzen) were a popular type of devices in the DDR for overhearing a room, as they used existing wiring (e.g. the mains power network, the telephone lines or the door-opener) for transport of power and the intercepted audio.

At the Stasi, this was part of their B-assignment (audio surveillance), which is why carrier bugs are known in Stasi terminology as TF-B devices. The Capri receiver – shown in the image on the right – was developed especially to detect TF-B bugs and make their intelligence audible again.
  
7042 receiver with earpiece connected

The device is powered by a small 9V battery and has terminals for connection of double-ended wiring, such as a telephone line or the mains power network. In addition it has a BNC socket for connection of single-ended wiring like the CATV 2 system. Inside the device is basically a Long-Wave (LW) radio receiver, which is capable of detecting and demodulating the carrier frequency of the bug, which is typically above the human audible range between 15 and 410 kHz. Common frequencies were 20 kHz, 30 kHz, 40 kHz, 60 kHz and 100 kHz, but others were used as well.

  1. TF = Trager Frequenz (carrier frequency). Wanze = bug.
  2. CATV = Closed Area Television, also known as Central Antenna Installation (CAI).

7042 receiver with earpiece connected Rear view Line terminals at the front of the device Stethoscope earpiece Earpiece Earpiece Adjusting the audio volume Illuminated frequency scale
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7042 receiver with earpiece connected
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Rear view
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Line terminals at the front of the device
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Stethoscope earpiece
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Earpiece
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Earpiece
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Adjusting the audio volume
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Illuminated frequency scale

Features
The diagram below gives an overview of the controls and connections on the body of the 7042. At the front are the sockets for connection of the wiring under test. At the rear is a DIN socket for connection of a speaker, or a pair of earphones which were usually of the stethoscope type. The unit is powered by an internal 9V battery and is switched on with the recessed switch at the left.


Immediately after switching the device on, noise will be heared through the earpiece. By turning the tuning wheel – which protrudes the case at the left – the entire 15 - 410 kHz range can be scanned. The two knobs at the right are for adjusting the RF input level and the audio volume.


Circuit diagram
Below is the circuit diagram of the receiver. The circuit is built around two integrated circuits (ICs) made by RFT, in the circuit diagram designated A244D and A211D. At the left are the two line terminals from which the signal is led through a transformer, a limiter, an RF gain potentiometer and a low-pass filter, before it is fed to the input of an LW receiver based on the A244D. At the output of the receiver is a detector (D3) and finally an audio amplifier, built around an A211D.

Capri (7042) circuit diagram

Please note that the circuits around the two ICs are largely as suggested in the application notes of their respective datasheets, which are available for download below. The green power indicator LED on the (+) rail — just behind the power switch — is also used as frequency scale illumination.


Interior
The 7042 is housed in a grey metal enclosure that measures 164 x 90 x 32 mm, including knobs and connectors. It has a removable bottom panel that is held in place by 2 recessed 2 mm screws. After removing these screws, the bottom panel can be taken off, and the internals are exposed.

The image on the right shows the interior, seen from the bottom. At the right are the terminals. Inside the unit is a single printed circuit board, or PCB, that fills the entire space, except for a small section at the rear end, which is reserved for the output socket and a common 9V battery.

Visible at the front are the two potentiometers for RF gain and audio volume, that are mounted on small metal sub-frames. At the other side of the PCB is the tuning capacitor which is used for the frequency adjustment. At the right, close to the terminals, is the input transformer and filter.
  
Interior

The circuit is built around two integrated circuits (ICs) that are made in the former DDR by RFT Kombinat Mikroelektronik: an A244D AM-receiver – which is the DDR equivalent of the western TCA440 – and an A211D audio amplifier – which is the DDR equivalent of the western TBA611. The two circuit are designed according to their respective applications notes, as suggested in the original datasheets, but with the tuned circuits dimensioned for LW rather than MW frequencies.

Bottom panel removed Interior Interior Battery fitted Input filter Audio amplifier and adjustable capacitor MW/LW receiver IC Audio amplifier IC
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Bottom panel removed
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Interior
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Interior
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Battery fitted
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Input filter
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Audio amplifier and adjustable capacitor
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MW/LW receiver IC
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Audio amplifier IC

Restoration
When we received our Capri receiver, it was in non-working condition. The knobs were missing, the cord of the earpiece was broken and the power amplifier IC (A211D) [B] was running hot as soon as a 9V battery was connected to the battery terminals, indicating that the IC might be bad.

A closer inspection revealed that there was a short circuit between the output of the IC and the case (ground), caused by a broken isolation ring between the PCB and a mounting stub. After replacing the isolation rings and re-assembling the receiver, everthing worked as expected.


Testing Capri
In order to check the functionality and performance of the Capri receiver, we tested it against a couple of orignal DDR bugs, connected to a simulated analogue telephone line. As the device is actually an AM receiver, it works best with Amplitude Modulated (AM) bugs, but is also suitable for the detection of Frequency Modulated (FM) carrier bugs. Here are the results of our testes:

33014
The first bug to be tested with the Capri receiver, was the Stasi's 33014 TF-B listening device. It is powered by the telephone line, and delivers its intelligence on a 24, 40 or 104 kHz carrier.

The receiver produced a strong and clear audio signal at a frequency of 40 kHz and on the harmonic frequencies: 80 kHz and 120 kHz.

 More information

  
Wired carrier frequency bug 33014-1

Bodil   33343
Capri was also tested with Bodil, which has a 60 kHz phase-modulated (PM) carrier. Although this signal is less suitable for Capri's AM receiver, the residual AM produced by an FM or PM modulated carrier is enough to make the bug audible again.

Due to the non-linear nature of the signal produced by Bodil, it also produces audible signals at 30, 90 and 120 kHz.

 More information

  
The BODIL telephone line carrier bug


Documentation
  1. A244D, AM Receiver up to 30 MHz, datasheet
    RFT, Kombinat VEB Halbleiterwerk Frankfurt (Oder). 1979.

  2. A211D, AF Amplifier, datasheet
    RFT, Kombinat VEB Halbleiterwerk Frankfurt (Oder). February 1976.
References
  1. Herbert Kunz, Capri 7042 receiver - THANKS !
    Personal correspondence, October 2018.

  2. Detlev Vreisleben, X-Auftrag: Kontertechnik
    Personal correspondence, October 2018.
Further information
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© Crypto Museum. Created: Sunday 28 October 2018. Last changed: Thursday, 01 November 2018 - 13:29 CET.
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