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
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.