The device measures 260 × 165 × 63 mm and is shaped in such as way that it can be
placed on a table, with all controls on the horizontal and vertical front panel.
All connections, with the exception of the earphones, are at the rear.
At the top is a fairly large speaker, which produces a clear audio signal.
A typical listening post (LP) consisted of one or more of these receivers, placed
side-by-side. Each receiver is then responsible for monitoring a single VHF-H channel,
determined by a quartz crystal. Each receiver could be populated with 6 crystals.
The device could optionally be equipped with a descrambler board, which could be
used in combination with a body transmitter or bug,
that contained a compatible scrambler. It is not fitted in the device shown here.
The scrambler adds basic security to the setup, but as it uses simple
inversion of the audio spectrum,
it is no match for a professional interceptor.
The RX-905 was used by law-enforment and intelligence services like CIA, FBI
and the Police, mainly in the US.
It is currently unknown when it was first introduced, but based on the
design of the PCB and the electronic circuits, we estimate that it was designed
around 1980. Based on the date codes on the various components, it seems likely that
the device shown here was manufactured around 1990.
The device was succeeded around 1990 by the compatible digital RX-1000.
Below is the block diagram of the RX-905, based on observation of the PCB.
Note that the device has 6 separate crystal oscillators — one for each channel —
one of which is active at any given time, depending on the position of the
channel selector. The signal from the crystal oscillator is doubled and then
mixed with the intercepted signal (150-174 MHz), resulting in a 21.4 MHz IF1.
The 21.4 MHz signal is then mixed again in IF2, resulting in a 455 kHz signal
that is demodulated and amplified to speaker level. Separate outputs are available
for earphones and line. In addition, an optional descrambler board can be placed
between the FM demodulator and the AF amplifier.
The interior of the device can be accessed by removing four screws from the upper
case shell — two at the left and two at the right — after which the
case shell can be removed. To get full access to the PCB,
it will be necessary to remove the black front panel cover as well. This is done by
removing 4 screws, all knobs and the retaining nuts of the switches and potentiometers.
The circuit is well-designed and is mainly built with conventional parts from the 1970s
and 80s. At the rear end of the PCB is the mains power supply unit (PSU).
At the front left of the PCB is an unpopulated area.
Its function is currently unknown, but it is possible that it was there to accomotate
additional parts – such as a microcontroller and a display – for a different model.
As the highest observed date code on the internal components is 1989, we assume that
the device was manufactured in 1990, around the time that the successor – the
RX-1000 – was introduced. The RX-1000 is housed in an
enclosure with the same form factor.
DeviceAnalogue surveillance receiver
PurposeBug reception, frequency monitoring
ManufacturerAudio Intelligence Devices (AID)
ModulationFM narrow band
Mains110V or 230V AC (switch selectable)
Dimensions260 × 165 × 63 mm
The following channels were installed in the RX-905 with S/N 2318:
Any links shown in red are currently unavailable.
If you like the information on this website, why not make a donation?
© Crypto Museum. Created: Tuesday 16 January 2024. Last changed: Sunday, 21 January 2024 - 17:12 CET.