|
|
|
|
|
|
|
TSCM Stasi
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.
|
|
|
|
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.
|
-
TF = Trager Frequenz (carrier frequency).
Wanze = bug.
-
CATV = Closed Area Television, also known as Central Antenna Installation
(CAI).
|
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.
|
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.
|
|
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.
|
|
|
|
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.
|
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.
|
|
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:
|
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
|
|
|
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
|
|
|
|
|
|
Any links shown in red are currently unavailable.
If you like the information on this website, why not make a donation?
© Crypto Museum. Created: Sunday 28 October 2018. Last changed: Sunday, 23 August 2020 - 08:51 CET.
|
 |
|
|
|
