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Spectrum Monitor

The SM-2 Spectrum Monitor (SM) was a panoramic viewer that was developed by Glenn Whidden of Technical Services Agency (TSA) in the US in the mid-1980s, as the successor to the SM-1. It was an add-on to the Audiotel Scanlock Mark VB and the Scanlock 2000 bug finders, and allowed a small section of the frequency spectrum to be monitored in real time, whilst searching for hidden radio transmitters (bugs). In the UK, the SM-2 spectrum monitor was sold by Audiotel.

The unit can be connected to the 2MHz IF output that is available on a mini BNC socket at the front panel of the Scanlock. By making a 300 kHz section of the frequency spectrum visible, it becomes possible to look for hidden or masked low power bugs that transmit at a frequency very close to an existing strong broadcast station, or bug that use a subcarrier for its modulation.

The first model was the Spectrum Monitor SM-1. It was introduced in the late 1970s and sold for approx. US$ 2800. It was followed a few years later by the improved SM-2 that is shown here.
SM-2 Spectrum Monitor

The unit is about half the width of a Scanlock receiver and can easily be placed on top of it. It is powered by its internal 6V battery, or from an external 12V DC source. When powered externally, the internal battery is automatically charged. For connection to the Scanlock, a special coax cable is needed, with a normal BNC on one side and a mini-BNC at the other end. The SM-1 and SM-2 appeared to be a very useful add-ons when finding and identifying radio signals.

The monitor was available for many years and was even used with the later Scanlock 2000 and the Scanlock ECM, which features the same 2 MHz IF output. In the UK, the spectrum monitors were sold by Scanlock manufacturer Audiotel, who marketed them as rebranded products.

SM-2 Spectrum Monitor SM-2 Spectrum Monitor with RF cable RF cable for connection to the Scanlock Scanlock Mark VB with SM-2 Spectrum Monitor Scanlock Mark VB with SM-2 Spectrum Monitor SM-2 Spectrum Monitor on top of the Scanlock Mark VB Silent carrier from an RF bug Commercial radio station on the SM-2

All controls of the SM-2 are at the front panel. After applying a 2MHz IF signal from a Scanlock receiver and switching the unit ON with the power switch at the top right, the green phosphor CRT screen should immediately show a horizontal line with any radio signals displayed as a vertical deviation of that line. When properly adjusted, the currently received signal is shown at the center of the screen. The adjustments to the left of the CRT can be used to alter the scanning speed (RATE) and the part of the spectrum that is visible on the screen (WIDTH and CENTER).

Front panel of the SM-2 Spectrum Monitor

At the right are the adjustment for the RF (input) level and the AF (audio output) level for the headphones. The latter can be used to listen to the signal produced by the video output stage and can be used to identify certain signals acoustically. The small LCD screen can display the IF frequency at the center of the screen. In order to use it properly it has to be calibrated first, using the adjustments below it (MARKER). This way it is possible, for example, to measure the distance (in kHz) between a subcarrier and its audio or the distance between a radio station and a bug.

The SM-2 is housed in a heavy black aluminium case that consists of a bottom part with the front panel and the rear panel, plus a U-shaped top panel that is held in place with four bolts at the sides. After removing the four bolts, the top panel can be lifted upwards and is then removed.

The interior consists of four PCBs (three large ones and a smaller one), plus a small Cathode Ray Tube (CRT) that is used for displaying the spectrum. Each PCB is mounted to the bottom of the case by means of two supporting posts. The CRT is held in place with a mask at the front and an aluminium bracket half way down its neck.

The image on the right shows the interior of the SM-2, seen from the rear right. The image was taken whilst repairing the unit, which is why the RF board at the center is lifted up and the CRT is not visible here. The front panel is at the left.
Interior of the SM-2 with the RF board partly removed

Looking at the interior from the front, we see from left to right: the video board, the CRT, the RF board, the digital board and finally the smaller audio board. All board are interconnected with individial wires and a large number of connectors (18) some of which are hardly reachable. A metal frame that holds the battery is mounted between the RF board and the digital board. It is mounted in such a way that it can easily cause a short circuit on both of the adjacent boards.

Looking at the boards, it seems obvious that the digital board was added later and that it was designed by another person. It wasn't present in the earlier SM-1 Spectrum Monitor and contains a frequency counter that measure the frequency at the center of the screen (around 2 MHz).

The block diagram above explains how the SM-2 works. At the heart of the unit is the RF board that is located at the centre of the case. It consists of a pre-amplifier with an adjustable gain (RF), followed by the first mixer, where the signal is added to the signal of a varicap controlled free-running oscillator (VCO. This generator is driven by the horizontal deflection signal from the video board (saw-tooth). After a 2nd mixer stage, the LF signal is amplified and used to drive the vertical deflection of the CRT. The video signal is also fed into an LF (audio) amplifier, to allow some level of (acoustical) identification of the signal, using a pair of (external) headphones.

The signal from the sweep generator (VCO) is also used to drive a digital counter that is mounted at the front panel. When the image is properly centered on the screen, it displays the center frequency of the RF signal (2 MHz). In the example above a frequency of 2018 kHz is shown.

Interior of the SM-2 with the RF board partly removed Interior of the SM-2 with the RF board partly removed Interior of the SM-2 Interior of the SM-2 The opened SM-2 unit

The unit featured on this page, was added to our collection in 2009 after it had been in storage for over 10 years. When we first tried it out, it appeared to be broken. The only thing that came to live was the small LCD screen at the front panel, which was constantly showing the number '0'.

Although the interior of the unit seemed clean and original, a close inspection revealed that all of the small 10µF and 47µF capacitors on all of the boards had been leaking, causing damage to the PCBs and disrupting normal functionality.

In case you ever come across a similar problem, just replace all these low-quality electrolytical capacitors with proper ones, clean the boards and restore all suspicious solder joints. On the video board, the capacitors are close to the (thin) connections of the HT transformer, so it might be necessary to resolder the wires of that unit.
Repairing the SM-2 Spectrum Monitor

After the above was done, the video board came back to life and the screen showed a green horizontal line at the bottom. Further investigation showed that the sweep generator on the RF board wasn't working, so we had to replace the 2N4903 and two of its resistors. They had probably been damaged by a power surge via the horizontal deflection line from the video board.

Once the RF board was repaired, the RF signal was displayed nicely on the green CRT and the RF level could be properly adjusted. The only thing that was dead at this stage was the digital counter, which was still showing a '0' on its display. It turned out that the only analogue component on this board, the FET pre-amplifier at the input, was broken. After replacing the RCA40673 MOSFET at the top left, the counter worked fine again. The image above shows the SM-2 in our workshop, whilst reparing the RF board. The audio board is hanging out at the left.

Repairing the SM-2 Spectrum Monitor Leaking capacitors on the video board Interior of the SM-2 with the RF board partly removed Interior of the SM-2 with the RF board partly removed Part of the RF board. At the bottom left are the varicap and the VCO (2N4903) The digital counter. At the top left is the pre-amplifier (RCA40673 MOSFET) LF board Commercial radio station on the SM-2

  1. Technical Services Agency, Inc., Spectrum Monitor SM-1 Operating Manual
    The TSA Spectrum Monitor for use with the Mk Vb Scanlock. 1980.

  2. Technical Services Agency, Scanlock Mark VB Handbook
    Date unknown.
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Crypto Museum. Last changed: Wednesday, 29 January 2014 - 11:45 CET.
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