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CIA
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RX-905
VHF surveillance receiver

RX-905 is an analogue VHF surveillance receiver, introduced around 1980 by Audio Intelligence Devices (AID) in Fort Lauderdale (Florida, USA). It has six crystal-operated frequencies in the 150-174 MHz frequency band and was used for the reception of covert listening devices (bugs) and for monitoring of specific VHF radio channels. It was succeeded in 1990 by the digital RX-1000.

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.

RX-905 analogue VHF surveillance receiver
RX-905 analogue VHF surveillance receiver
Controls
Rear panel
Rear view
A
×
A
1 / 5
RX-905 analogue VHF surveillance receiver
A
2 / 5
RX-905 analogue VHF surveillance receiver
A
3 / 5
Controls
A
4 / 5
Rear panel
A
5 / 5
Rear view

Block diagram
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.


Interior
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.

Case lid removed - right angle view
Interior
Interior - right angle view
Front end
Six crystal oscillators (one for each channel)
1st and 2nd mixer
Dummy board to replace the (optional) descrambler
Unpopulated part of the PCB
B
×
B
1 / 8
Case lid removed - right angle view
B
2 / 8
Interior
B
3 / 8
Interior - right angle view
B
4 / 8
Front end
B
5 / 8
Six crystal oscillators (one for each channel)
B
6 / 8
1st and 2nd mixer
B
7 / 8
Dummy board to replace the (optional) descrambler
B
8 / 8
Unpopulated part of the PCB

Specifications
  • Device
    Analogue surveillance receiver
  • Purpose
    Bug reception, frequency monitoring
  • Manufacturer
    Audio Intelligence Devices (AID)
  • Model
    RX-905
  • Country
    USA
  • Year(s)
    1980-1990
  • Successor
    RX-1000
  • Band
    VHF-H
  • Frequency
    150-174 MHz
  • Channels
    6
  • Method
    Analogue, crystal-operated
  • Modulation
    FM narrow band
  • Antenna
    50Ω BNC
  • IF1
    21.4 MHz
  • IF2
    455 kHz
  • Mains
    110V or 230V AC (switch selectable)
  • Dimensions
    260 × 165 × 63 mm
  • Weight
    1490 g
Channels
The following channels were installed in the RX-905 with S/N 2318:

  1. 171.850
  2. 166.250
  3. -
  4. 171.475
  5. -
  6. -
Further information
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Crypto Museum. Created: Tuesday 16 January 2024. Last changed: Sunday, 21 January 2024 - 17:12 CET.
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