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Countersurveillance Probe Monitor

The CPM-700 is a broadband receiver for the detection and location of eavesdropping devices (bugs), developed and and built by Research Electronics Inc. (REI) in Cookeville (USA) around 1988. Although the device was introduced over 20 years ago, it is still for sale today (2013).
The image on the right shows a typical CPM-700 from the early 1990s, with the special RF probe attached to it. The probe has a built-in a pre-amplifier and a telescopic antenna, and can be waved around the room like a magic wand.

The device has two modes for detection: search and monitor. In search mode the receiver scans the entire frequency spectrum for RF bugs, using one of the RF/LF probes connected to the PROBE socket at the top right of the front panel. Please note that only original REI (active) probes can be used, as they contain some internal circuitry.
CPM-700 bug probe monitor

Once a room is sweeped, the unit can be set to monitor-mode and the treshold is adjusted so that the alarm is just silent. Whenever someone brings a new eavesdropping device into the area, or when a remote controlled or timed device becomes active, this will trip the acoustic alarm. Any eavesdropping device found, will be hear through the built-in speaker or, better, via the connected headphones. Apart from using one of the probes to pick up RF signals, the CPM-700 can also be used to check any type of cable for acoustic (audio) or carrier bugs using the AUX input on the right side. This can be used, for example, to check analogue telephone lines (PSTN).

The CPM-700 is powered by a removable battery pack that contains 8 penlight batteries of 1.5V each (12V) or 8 rechargeable NiCd batteries of 1.2V each (9.6V). It can also be power from the mains using the supplied mains adapter, or any other suitable external power supply unit. When using NiCd batteries, a slide switch in the battery compartment has to be set accordingly. This allows the batteries to be charged automatically when the external PSU is connected. The unit comes complete with a laptop-style carrying bag in which all accessories are nicely stored.
Laptop-style carrying bag Interior of the carrying bag CPM-700 bug probe monitor CPM-700 with many accessories Close-up of the CPM-700 front panel CPM-700 in operation Setup for detecting mains bugs Setup for detecting phone bugs

All controls and adjustments of the CPM-700 are at the front panel. The unit can be switch ON and OFF with the power switch at the bottom left. At the top left is the socket for the headphones and the volume adjustment. When headphones are used, the internal speaker will be disabled. The next section (MONITOR) controls the built-in alarm that has both an optical (red lamp) and an acoustic feedback (buzzer). The buzzer can be disabled with the SILENT/TONE switch.

Most of the right half of the front panel is taken by the clear LCD screen. The top half of the display contains the field strength indicator. The bottom half contains five indicators that reflect the current state of the device. At the far right is a BNC socket for connection of the probes. The unit comes with an RF probe (the magic wand) and an LF probe in the form of a power adapter. The latter is used to search for acoustic (audio) and for carrier bugs on the mains power lines.
CPM-700 with many accessories Auxiliary cable with two crocodile clips Telephone line patch cable Inductive probe LF (mains) probe HF probe (magic wand) Headphones CPM-700 interior

Despite the fact that the CPM-700 was developed in the late 1980s, it is extremely well built. The unit comes in a sturdy metal case with a metal front panel. A separate battery compartment is accessible via a metal panel at the rear by loosening two bolts. The CPM-700 has a surprisingly modern look and feel, which is probably why the design hasn't changed in all these years.
The interior can be accessed by removing four cross-head bolts from the sides. A large double sided PCB is mounted at the bottom. It contains most of the analogue and digital circuits of the CPM-700. A separate PCB hold the controls and adjustments of the front panel. It is mounted vertically at the front of the main board.

A rather large (and clear) loudspeaker is bolted to four mounting posts, in such a way that it lines up with the grid in the top cover. In order to prevent damage from leaking batteries, a separate compartment is present at the rear.
CPM-700 interior detail

The unit is very robust and most of the companents are easily accessible, making the CPM-700 a service friendly device. The only weak spot (if there has to be one) is the headphones socket at the top left of the front panel. If it is broken (as it was in our case), you will have a hard time replacing it, as the adjustments to its right are fixated with super glue, making it virtually impossible to removed the front panel. In our case, we fixed it by (carefully) cutting the existing socket to pieces first, then removing the pieces and finally replacing it with a proper alternative.
Technical specifications
  • Range
    15kHz-1MHz (LF probe), 50 kHz - >2GHz (RF probe)
  • Gain
    20 dB nominal (RF probe)
  • Sensitivity
    -38dBm (LF), -62dBm (RF)
  • Dimensions
    23 x 15.5 x 4.5 cm (main unit)
  • Weight
    1.1 kg
  1. Research Electronics Inc., Unit Operating Instructions, Model CMP-700
    Original User Guide. Date unknown, but probably 1988.

Further information

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Crypto Museum. Created: Monday 20 May 2013. Last changed: Wednesday, 02 December 2015 - 21:39 CET.
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