Auto Sweep Detector, EM10LM

 I spotted a couple of scanning receivers on an Internet auction site a few weeks ago (Dec 2011) and was intrigued enough to place a bid of £1. After all there might have been some useful bits and pieces in the pair, and being local (a charity shop in Boscombe near Bournemouth) wouldn't need to be posted at some great expense.

What exactly is an Auto Sweep Detector or a Scanning Receiver?

I think the best person to ask may be the chap at MI5 who was asked to use them at Number Ten.

Made by McLennan Engineering in late 1971 they were used up to at least 1997. How do I know this? Because the calibration charts on the inside of the lids gives me the date they were still in the factory, and one was fitted with a set of nine "D" size alkaline cells, all with NATO stock numbers. These were dated June 1997 and measured over 1.5-volts each.... as good as new! They probably hadn't been used much, if at all, because one of the tubular battery compartments had come adrift and effectively disabled the equipment.

After powering up the HF version with a bench power supply I discovered that it was a relatively insensitive receiver, which together with a simple probe, could automatically scan for signals. There are several modes of operation. In the automatic mode an electric motor tunes across the selected band and any detected signals are indicated by adding a tone to the demodulated signal before sending to the built-in loudspeaker or headphone jack socket. The tone presumably is added so that unmodulated carriers can be brought to the attention of the operator. Another setting on the mode switch allows one to listen to the signal without the added tone.

Manual tuning is possible so that a target signal can be accurately tuned for identification. As the equipment design dates to the late 60s only amplitude modulation is catered for. This is fine for listening to local aircraft as I was able to do, but hopeless for listening to wideband FM although narrowband FM would probably be resolved by slight off-tuning.

I wonder whether the MI5 chap ever detected a bug at Number Ten? I doubt it, although I'm sure some bugs were around during the cold war as MI5, MI6, the Foreign and Commonwealth Office, and GCHQ all had more than their fair share of Russian Agents.


So there it is... can anyone advise me whether this equipment was used by MI5, the Cabinet Office or CESG?

Now there's a question, of which I'm sure some readers will know the answer!

 As you can see there are six wavebands..

1.5 to 3.5MHz

3.5 to 7.5MHz

7.5 to 15MHz

15 to 35MHz

35 to 80 MHz

80 to 270MHz

It's a pity the designers didn't use a professional RF connector instead of the TV-style Belling Lee type.

Internal construction is typically that of a UK company, having that slightly amateurish look about it compared with offerings from the USA and Japan.

I remember the stuff we made at the Plessey factory in Liverpool where I worked for 20 years. Designs were mainly overseen by MoD and equipments were over-engineered to the extent that, if they hadn't already been obsolete by the time they were completed, would have lasted for hundreds of years.

 This is the brother of the EM10, the EM20.

As you can see the frequency bands extend from one to the other to give a continuous coverage

of 1.5MHz to 1000MHz

This unit uses a brass cavity to tune the UHF bands rather than the simple coils in the EM10.

This version has the date 1969 marked on its lid.

Since displaying these I was put in touch with the chap that built them at McLennan Engineering. Small world isn't it!

I recently bought a tiny little thing the size of a key fob which can detect the presence of transmissions at 2.4GHZ. Volumewise (including its lithium cell) it's about a quarter the size of one of the smaller knobs above. Progress...

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