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Phone
Voice
Scrambler
  
SA 5063/1 →
← SA 5031
  
SA 5063/0
Telephone set used with scrambler phone

Introduced in early 1943 for use on LB (CBS 2 & 3) and Magneto systems, and on for use on long lines with CB/Auto systems. It consists of a Tele. No. 394 with a Key No. 303A switch assembly, a 12-wire line cord ending in a BT No.6 1 connection box, and a No. 164 Jade Green handset [1].

The SA5063/0 was either used on a direct exchange or PBX line, or with the SA5050 Unit Auxiliary device to allow up to three SA5063/0 sets to share a single Frequency Changer unit.

The SA5063/0 has two push-buttons, labelled SECRET and ENGAGE FOR SECRET 2 but the latter (ENGANGE FOR SECRET) has no function if the set was connected to a direct exchange line. A third button could be added to release the other two without placing the handset in the cradle, or for use with various extension plans. In such cases the label was changed accordingly.
  
SA 5063/0 voice terminal

The image above shows a typical SA 5063/0 that is (barely readable) marked as such at the bottom. It also carries the manufacturing code FBA/1, which suggests that it was made at the GPO factory in Birmingham (FB). It is built on a 396 chassis and has a black 164 handset that is (partly) painted lime green. In this case, the bakelite body has no provisions for a third button.

The majority of SA5063 (SA5063/0) units were factory assembled, but they were also occasionally built by engineers in the field from locally available parts. For this reason it is possible that some units are marked Tele. No. 394 on their base and chassis. Due to wartime shortages, the chassis of the Tele. No. 396 was sometimes used as a replacement, as illustrated by the object above.

  1. 20-way box with metal lid.
  2. Later: SCRAMBLE and HOLD SCRAMBLER.

SA 5063/0 voice terminal
Front view
Left view
With handset off-hook
Handset
No. 396 bottom panel with circuit diagram
Close-up of the circuit diagram of the 396
Markings at the bottom
A
×
A
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SA 5063/0 voice terminal
A
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Front view
A
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Left view
A
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With handset off-hook
A
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Handset
A
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No. 396 bottom panel with circuit diagram
A
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Close-up of the circuit diagram of the 396
A
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Markings at the bottom

NOT SECRET
A scrambler system like the Frequency Changer, does not provide any real protection against a professional eavesdropper. All the intercepting party has to do, is reverse the speech spectrum once more to make the conversation intelligible again. This was known by the UK's War Office, of course, but it was believed to be sufficiently secure against a casual eavesdropper, such as the operator in a manually switched exchange, or a service engineer working on the telephones lines.

In order to discriminate scrambled telephone lines from regular ones, circular labels were issued to mark a regular phone as insecure:

SPEECH ON TELEPHONES IS    NOT SECRET

These labels were fitted in the area around the dial or the blanking panel. Although they were intended for regular phones, they sometimes landed on scrambled phones as well. On the majority of scrambler phones it was omitted however. The label on the phone in the image on the right is therefore considered out of place.
  
Close-up of the 'dial' and the push buttons

Note that many 'scrambler phones' that are offered on auction sides such as eBay, carry a circular label that is clearly a (bad) reproduction of the original one. In many cases, a simple typeface like 'Helvetica' or 'Univers' is used, whereas the original one was typeset in 'Gill-Sans'. If you insist on having this label installed on your telephone, you may want to download this reproduction [A].


Telephones and handsets
With the exception of the very early Frequency Changers – that were equipped with a No. 162 — nearly all wartime installations used a voice terminal that was based on the chassis of Telephone No. 394 or 396, both members of the 300-family of GPO telephones that started life in 1937. In all cases, the telephones were given a green handset, so that the voice terminals used with the Frequency Changers (scramblers) could be distinguised from regular (unprotected) telephones.

Two handsets that were used with the Scrambler

The Jade Green version of bakelite handset No. 164 was used for this. The one at the right in the image above is of this type. The same one was used with the earlier Telephone No. 162. In this case the receiver 1 has a black cap rather than a green one, for which there was a good reason.

Standard 164 handsets were fitted with a receiver 2 that was considered of insufficient quality for use with the scrambler system. An engineering directive was therefore given that these should be swapped for alternative ones, 3 but these were only available in black bakelite for most of the war period. A 164 handset with a black receiver cap can also be spotted in the photograph of the Cabinet War Rooms at the top of this page, in which it is held by Royal Navy Captain Richard Pim.

When green handsets were is short supply during the war, regular black 164 handsets (or some­times ivory as well) were painted in a lime green colour that did not match the colour of the Jade Green handsets. The leftmost example in the image above is of this type. The braided cord of the handset could be green or brown, whichever was available.

  1. Also known as earpiece or speaker.
  2. By default, Receiver No. 1L, Diaphragm No. 12, and Receiver Cap No. 18 were installed on handset 164.
  3. The replacement consisted of Receiver No. 2P, Diaphragm No. 25 and Receiver Cap No. 23.

Two handsets that were used with the Scrambler
Jade green GPO handset No. 164
Green handset manufactured in 1935
Green painted handset
B
×
B
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Two handsets that were used with the Scrambler
B
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Jade green GPO handset No. 164
B
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Green handset manufactured in 1935
B
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Green painted handset

Labels
There are also different variants of the metal shield with the text labels that is mounted just below the SECURE and CLEAR buttons. At least four version have been spotted over the years, which are shown below. In all cases, the leftmost button is used to go secure (secret, scramble), whilst the rightmost button is used for clear speech (normal, hold scrambler, engage for secret).


We believe the above label to be the eldest as it does not appear in the 1952 list of labels that was used at the GPO. Furthermore it is present on the phones in our collection that were made in 1938 and 1940 respectively. Other labels that are known to have been issued over the years are:


Depending on the configuration of the telephone set, the user requirements and the presence or absence of a third button at the centre, other arrangements and text labels may have existed. The labels could be engraved or screen printed. If it was screen printed, a condensed variant of the Gill-Sans typeface was commonly used. For a complete overview of the 27 different text labels No. 252 & 253 that were available between 1952 and 1967, please refer to list N620 [14].

 Overview of text labels


Dial
Although it was technically possible to fit a dial to a 394/396 telephone body, the standard issue was without one, as most installations were used on manually switched networks during WWII. In that case the circular hole at the front of the telephone set was covered with Blanking Panel No. 3.

Furthermore, the British Government had its own private network – completely separated from the public switched network – and many of its users, including Churchill, relied on an assistent to set up a call via the exchange operator and initiate a conversation, before handing it over to the user.

If the scrambler was used on networks with automatic exchanges, or on a local PABX that had automatic exchange facilities, the voice terminal could be fitted with a dial, so that the user could select the extension number directly. This is the case with the SA-5030 shown here.
  
SA5030 voice terminal

In post-war systems, most voice terminals did have a dial, as automatic exchanges had mean­while become mainstream in most countries. Nevertheless, the blind telephones sets (i.e. units without a dial) remained in use in many installations, in which case the line terminals of the Frequency Changer were commonly connected in parallel to a regular telephone set (with dial).

 More about the SA-5030 with dial




SA 5063/0 (left) and SA 5063/1 (right)

Restoration
The voice terminals shown on this page are authentic and were used with a Frequency Changer during WWII. According to the stamps, one was made in 1938, whilst the other one is of 1940 vintage. The green handset was made in 1935. The painted one in 1940. The bottom panels are marked SA 5063/0 and SA 5063/1, which means they were issued in 1943 and 1944 respectively.

The problem with these two sets however, was that a previous owner had converted them into house telephones, or intercoms, and used them this way for several years. Obviously he wanted to avoid the use of a small exchange, or PABX, and had converted them for low power use.

The original bell had been removed and its space was used to accomodate two 4.5V batteries: one for the speech loop and one for a small buzzer that was mounted to the chassis. Luckily, the previous owner had applied his modifications in such a way that they could easily be reversed.
  
Lamp fitting mounted to the middle button

A bakelite button had been added to the center of the circular panel that covers the hole of the dial, but this too was easily removed. The bad news was that the batteries were left inside the phones when they were taken out of service. Over the years, the leakage from the batteries had caused considerable damage to the bottom panel and to the small drawer at the phone's front.

Restoration of the phones was started by first taking them fully apart and cleaning the indiviual parts. The modifications were removed and undone and the bakelite body of the unit was washed and treated with bakelite conditioner. 1

The missing parts were then re-mounted to the chassis and the original wiring was restored as per circuit diagram inside the bottom panel. The braided cord of the green bakelite handset was replaced with a high-quality reproduction 2 and a new braided line cord was added to allow it to to be connected to a standard telephone line.
  
Rust caused by leaking batteries

Both phones are now fully restored to their original state as close as possible. The only thing missing right now are two original Frequency Changers to connect them to, so that we can finally demonstrate how Churchill and his staff held private phone conversations during the war.

  1. High-grade bakelite conditioner and other products for restoring bakelite parts are available from a variety of sources, such as this one.
  2. High-quality reproduction cables for old GPO phones, that closely match the original colours and manufacturing properties, are available from Chris Elliot in the UK.

Before restoration
Lamp fitting mounted to the middle button
Bakelite doorbell knob mounted at the center
Telephone No. 394 before its restoration
Bakelite body after restoration
Inside the bakelite body
Rust caused by leaking batteries
C
×
C
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Before restoration
C
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Lamp fitting mounted to the middle button
C
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Bakelite doorbell knob mounted at the center
C
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Telephone No. 394 before its restoration
C
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C
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Bakelite body after restoration
C
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Inside the bakelite body
C
8 / 8
Rust caused by leaking batteries

Documentation
  1. Circular label Speech on Telephones is Not Secret (PDF)
    Crypto Museum, Reproduction, 10 June 2014.
References
  1. Andy Grant, Everthing that you need to know about scramblers but were afraid to ask
    Telecommunications Heritage Journal (THJ), Issue 99, Summer 2017. p. 11—14.
    Reproduced here by kind permission from the author.
Further information
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Crypto Museum. Created: Wednesday 26 May 2021. Last changed: Monday, 07 June 2021 - 07:34 CET.
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