TELEPHONE No. 394
|Telephone No. 394
was first issued by the BPO in 1937, principally for use on CBS and CB
exchanges in rural areas [long lines], hence the provision of a
battery-assisted microphone (local battery) and external
connection of a magneto generator. This table model is Bakelite cased.
During World War II, these instruments with their provision for up to three buttons (keys), were also modified for use with 'Scrambler' Units, which were basically Frequency Inverters using thermionic valve. This phone was also used on long lines in CB and Automatic areas on extension plans.
Early units of the latter, referred to as 'Privacy Sets', were housed in heavy, bulky rectangular metal cases, but later versions were produced that became progressively lighter and more compact as smaller components, including more modern valves, became available. The most common 'Unit' was 'Frequency Changer No. 6', used extensively for wartime service (the post-war combination was the Telephone No. 710. and the very much lighter and compact transistorised version, 'Secrecy Unit No. 8').
Scrambler telephones always have green handsets, either moulded in green material or more commonly painted green. Similar-looking telephones can be found with red handsets but it is unclear what the other colour signified. Operation of the secrecy button simply disconnects the A and B lines and the handset from the telephone, and reconnects both to the 'Privacy Unit' which now acts as the telephone, but now with frequency inversion facility.
Jim Foster states that if you were of sufficiently high rank you had the exclusive use of a Privacy Set and all you had to do was to press the button marked 'Secret', and lo! - secret it was. The 'Normal' key was not wired, and simply released the latch on the 'Secret' key, as did replacing the receiver on its rest. If, however, several users were obliged to share a common Privacy Set, then you had to press a button marked 'Engage for Secret' [or, on some versions, 'Hold Scrambler'], and a set of Dolls Eye indicators showed if the Privacy Set was engaged. If it was 'free', then the 'Secret' or 'Scramble' button could be employed. Replacing the handset released any key that had been depressed, but if you wished to revert to 'normal' speech in the middle of a scrambled conversation, a third, central key, unwired, released the 'Secret' or 'Scramble' key. This button was however, marked on some phones, 'Release', but where the telephones were connected to the line via a PBX, 'Recall', so that the user could re-seize the line. To add to the confusion, 'Exclusive' scrambler phones were often installed on multi-use positions, and vice-versa.
"Regarding scrambler telephone hand sets, as far as I have discovered the handset was supposed to be green with matching ear and mouth pieces. But I have in my possession handsets recovered from an airfield here in Cornwall that had ivory handsets painted green with ivory and black mouth and earpieces. Needless to say they were on scrambler telephones. I assume that painted versions overcame wartime and immediate postwar shortages of green components. and nothing more sinister than that." Richard Killen.
Manufactured by: British Ericsson and Plessey, possibly others.
Colours: Black with green handset if used with Privacy Units.
Users: Scrambler version used in government and military establishments. Examples can be seen in the Cabinet War Room museum in London, the Hack Green bunker museum (www.hackgreen.co.uk) and at the Yorkshire Air Museum, RAF Elvington, near York.
Supplied to the GPO by British Ericsson, their Model N8443.
Circuit diagram - N494.
Diagram for the Button labels - N620.
Last revised: December 02, 2018