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Phone
Voice
GPO
Scrambler
  
No. 710 →
← SA 5063/0
  
SA 5063/1
Telephone set used with scrambler phone

SA 5063/1 is a voice terminal for the GPO telephone scrambler known as the Frequency Changer or Privacy Set, developed by the UK's General Post Office (GPO) and introduced during World War II, in February 1944, for use on LB (CBS 2 & 3) and Magneto systems, and for use with long lines on CB/Auto systems. It consists of a modified GPO Telephone No.394, fitted with a Key No.303B switch assembly, a 12-wire cord ending in a BT No.6 1 connection box, and a green 164 handset.

The SA5063/1 was used on a direct exchange or PBX line, or – with the SA5050 Auxiliary Unit – to let up to three SA5063/1 units share a single Frequency Changer. The primary difference with the SA5063/0 is the addition of extra security switch contacts, so that other terminals wired in parallel can not eavesdrop on the conversation.

The SA5063/1 had two push-buttons, labelled SECRET and ENGAGE FOR SECRET 2 but the latter (ENGAGE FOR SECRET) had no function if the set was connected to a direct exchange line. These units can not be fitted with a 3rd push-button. 3
  
SA 5063/1 voice terminal

All SA5063/1 units were factory assembled, as the 394 chassis was modified and partially hard-wired to the Key No.303B switch assembly. Due to wartime shortages, the chassis of the Tele. No. 396 was sometimes used as a replacement. The image above shows a typical SA 5063/1 that is marked as such at the bottom. It also carries the manufacturing code FHA/1, indicating that it was made at the GPO factory Holloway (FHA) in North-London. It is built on a No. 394 chassis and was suitable for all GPO scramblers including Frequency Changer No. 6AC and Privacy Set No. 8.

 WWII scrambler phone

  1. 20-way box terminal (BT) with metal lid. Alternatively, an 8-way BT20/8 could be used.
  2. Later: SCRAMBLE and HOLD SCRAMBLER.
  3. The 303/B key assembly misses the plunger for that. There is however record of a label marked PRIVATE - NORMAL - ENGAGE FOR PRIVACY, which suggests that some phones did have a third button. In such cases the unit had to be fitted with a 303A key assembly and the centre button would have been used to release the other two, without the need to place the handset in the cradle.

SA 5063/0 (left) and SA 5063/1 (right)
Left view
Front view
SA 5063/1 voice terminal
With open drawer
Close-up of the push-buttons
Frequency Changer No. 6AC/3 with SA-5063/1 voice terminal
Markings at the bottom
A
×
A
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SA 5063/0 (left) and SA 5063/1 (right)
A
2 / 8
Left view
A
3 / 8
Front view
A
4 / 8
SA 5063/1 voice terminal
A
5 / 8
With open drawer
A
6 / 8
Close-up of the push-buttons
A
7 / 8
Frequency Changer No. 6AC/3 with SA-5063/1 voice terminal
A
8 / 8
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
2 / 4
Jade green GPO handset No. 164
B
3 / 4
Green handset manufactured in 1935
B
4 / 4
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 [3].

 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


Circuit diagram
Below is the internal wiring diagram of the SA 5063/1 telephone set. At the bottom right are the (A) and (B) terminals of the subscriber line. Directly above it, is the wiring to the screw terminal of the Frequency Changer 6x. The make-before-break (MBB) switches KA (1-4) are part of the 303/A Key Unit that is controlled with the 2 (or 3) push-buttons on top of the telephone set. They allow the telephone set to be used for plain as well as secure calls. In secure mode, the speaker and microphone of the telephone's No. 164 handset are routed via the Frequency Changer 6x.


During WWII, the dial armature was often omitted from the SA 5063/0 and SA 5063/1 telephone sets, as the British Government used a private telephone network – completely separated from the public switched network – that was patched manually. The diagram above is also applicable to the SA 5030 and other 300-series telephones that were modified for use with Frequency Changers.


Interior
Getting access to the interior of an SA50xx terminal is quite straightforward. Loosen the four bolts at the corners of the bottom panel (not the rubber feet) and take it away. The bottom panel also holds the small drawer. Note that a circuit diagram should be present under the drawer.

Inside the set is a metal chassis to which all internal parts are mounted. Prominently visible towards the front is the optional bell which takes up most of the space. Apart from the bell, the bottom section also houses the transformer, a large capacitor and a 13-point contact block for connection of the handset and the outside line.

The chassis can be removed from the body of the telephone by loosening three bolts: two at the sides and one towards the rear (behind the capacitor). As most sets have no dial, there are no extra wires that have to be disconnected.
  
Interior bottom view

The top side of the chassis holds the contact strip for the dial (not used here) and a large block with an array of switches. These are the switches that are operated by the two or three buttons that are located on top of the telephone set. In GPO terminology this is the so-called KEY UNIT.

As multiple contacts are needed for switching microphone and speaker, Key No. 303/A is used. It consists of three individual switches with 4, 1 and 4 sets of make-before-break contacts each. This arrangment is also known as 4K-1K-4K 1 , and the complete set is often referred to as 9K.

Towards the front of the key assembly are three plungers that are operated by the buttons on top of the phone. The behaviour of the plungers is controlled by a spring-loaded latch bracket that is fitted at the front. In the current setup, both buttons are latched when they are depressed.
  
Close-up of the frame

Pressing the third button at the centre – when installed – releases the other two. Depending on the function of the centre button — it can be used for example to redial a number, to call the operator, or connect to an extension — it may be configured to latch or to release itself.

At the rear side of the No. 303A switch pack is a 28-point contact block with screw terminals, to which the actual contacts of the 9K (4K-1K-4K) switches are wired. This allows the switch pack to be used for virtually any configuration or application, simply by wiring it up as required.

The line cord and the braided handset cord, are usually fixated to one of the mounting posts of the terminal block, by means of tie knot. Note that the bell was optional on the 394 and 396 phones. It was only fitted when required by the customer. It is present in the phone shown here.
  
tie-knotting of the handset cable

Note that in the images above, the device is fitted with a brown 4-wire cable connection to the subscriber line. This is incorrect, as the device needs 6 to 8 wires for operation with a Frequency Changer (scrambler). In 2021, the device was fitted with a correct cable and rewired for operation with Frequency Changer 6AC and with Privacy Set 8.

 Technical details about 300-series phones

  1. In Ericsson terminology, a single make-before-break set of contacts is known as a 1K springset. Likewise, a double set is known as 2K and a set of four make-before-break contacts is called 4K. More...

Interior bottom view
Interior front/top view
Interior rear/top view
Interior bottom/rear view
Interior bottom/front view
Interior with handset
tie-knotting of the handset cable
Connection block (T) seen from the bottom of the phone
Close-up of the frame
Hook switch engaged (on-hook)
Top view of the 9K switch pack (303/A unit)
Metal plate for 'programming' the behaviour of the three buttons
Terminals for connection of the dial (not used here)
303/A key block (9K switch pack)
Pressing the rightmost button
Pressing the leftmost button
Bottom panel with drawer and circuit diagram
Telephone No. 394LB circuit diagram inside the device
Close-up of the circuit diagram
No. 396 bottom panel with circuit diagram
Close-up of the circuit diagram of the 396
Close-up of the bell
Close-up of the transformer
Capacitor mounting bracket. Note the date stamp on the capacitor (1940).
C
×
C
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Interior bottom view
C
2 / 24
Interior front/top view
C
3 / 24
Interior rear/top view
C
4 / 24
Interior bottom/rear view
C
5 / 24
Interior bottom/front view
C
6 / 24
Interior with handset
C
7 / 24
tie-knotting of the handset cable
C
8 / 24
Connection block (T) seen from the bottom of the phone
C
9 / 24
Close-up of the frame
C
10 / 24
Hook switch engaged (on-hook)
C
11 / 24
Top view of the 9K switch pack (303/A unit)
C
12 / 24
Metal plate for 'programming' the behaviour of the three buttons
C
13 / 24
Terminals for connection of the dial (not used here)
C
14 / 24
303/A key block (9K switch pack)
C
15 / 24
Pressing the rightmost button
C
16 / 24
Pressing the leftmost button
C
17 / 24
Bottom panel with drawer and circuit diagram
C
18 / 24
Telephone No. 394LB circuit diagram inside the device
C
19 / 24
Close-up of the circuit diagram
C
20 / 24
No. 396 bottom panel with circuit diagram
C
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Close-up of the circuit diagram of the 396
C
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Close-up of the bell
C
23 / 24
Close-up of the transformer
C
24 / 24
Capacitor mounting bracket. Note the date stamp on the capacitor (1940).



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

Restoration
The SA 5063/0 and 5063/1 voice terminals shown in the image above are authentic and were used during WWII with a Frequency Changer. 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. According to the markings at the bottom, 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

The two telephone sets phones are now fully restored to their original state as close as possible (2016). After acquiring two original Frequency Changer No. 6AC/3 units in 2021, the telephone sets were internally rewired, to match the wartime configuration. Furthermore, they were fitted with an 8-wire braided cord, so that they could be connected directly to a Frequency Changer.

  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
D
×
D
1 / 8
Before restoration
D
2 / 8
Lamp fitting mounted to the middle button
D
3 / 8
Bakelite doorbell knob mounted at the center
D
4 / 8
Telephone No. 394 before its restoration
D
5 / 8
5 / 8
D
6 / 8
Bakelite body after restoration
D
7 / 8
Inside the bakelite body
D
8 / 8
Rust caused by leaking batteries

Connections
Internal wiring
The table below shows the wiring inside the SA 5030 telephone set, between the contacts of the standard connection terminal (T), the contacts of the key unit (K) and the contacts of the external junction box or block terminal (BT). The contacts are shown in blue in the circuit diagram above.

  • K13
    BR3
  • K16
    BT1
    Line (B)
  • K17
    T9
  • K18
    BT7
  • K19
    M
    Handset, microphone (red)
  • K20
    T4
  • K21
    BT6
  • K22
    BT4, BT5
  • K23
    T5
  • K24
    MR
    Handset, common (white)
  • K25
    BT8
  • K26
    T6
  • K27
    R
    Handset, receiver (speaker) (green)
  • T1
    BT2
    Line (A)
Terminal   T
The drawing below shows the pin numbering of the standard connection block or terminal (T) of the basic telephone set, as seen from the bottom . In the circuit diagram above and the internal wiring table above, these contacts are prefixed with the letter 'T' (e.g. 'T9').  Further information


Connection block T as seen from the bottom of the phone

Key unit   K
Below is the layout of the contact terminals of connection block 9K, also known as Key Unit 303, as seen from the rear of the telephone set. In the circuit diagram and the internal wiring table above, the contact numbers of the Key Unit are prefixed with the letter 'K' (e.g. 'K23').  More


The switches inside the 9K switch pack with their terminal numbers

Block terminal   BT 20/8
In some cases, the SA-5030 was externally wired via Block Terminal BT-20/8. Note that all 8 contacts are used. The Block terminal accomodates the subscriber line as well as the wiring to and from the Scrambler. The exact wiring diagram is shown in the circuit diagram above.

  1. Line (B)
  2. Line (A)
  3. Ground (earth)
  4. Speaker (L) 1
  5. Microphone (L) 1
  6. Microphone (H)
  7. Switched Line (A) - to Frequency Changer 2
  8. Speaker (H)
    Wiring inside the Block Terminal 20/8
  1. Contacts 4 and 5 are shorted. They are connected to the common line of the handset (via KA3).
  2. This line is controlled by switch KA1. It connects the Frequency Changer to the subscriber line when in secure mode.

Crypto Museum standard
To allow the Frequency Changer and suitable telephone sets to be tested, used and demonstrated in various configurations, without altering the fragile vintage wiring of the devices all the time, Crypto Museum has defined its own standard, involving inline 7-pin male/female XLR connectors.


In this standard, an 8-point junction box BT 20/8 is used as the central hub. The SA 50xx voice terminal is fitted to the BT 20/8 via a fixed 8-wire braided cable. The subscriber line is also fitted to the BT 20/8 via a fixed 2-wire or 4-wire braided cable, whilst a fixed 7-wire braided cable with an XLR7/F connector at the end is present for quick (dis)connection of the Frequency Changer.

The Frequency Changer itself is fitted with a fixed 7 or 8-wire braided cable with an XLR7/M connector at the end. This allows the Frequency Changer to be disconnected from the setup without opening it and unscrewing the wires from its terminal block or from the BT 20/8. Below is the pinout of the XLR7/F on the cable that is fitted to the BT-20/8 terminal block.

  1. Line (B)
  2. Line (A)
  3. LB (or unused)
  4. Microphone (H)
  5. Microphone (L)
  6. Speaker (L)
  7. Speaker (H)
    7-pin female XLR socket as fitted to the telephone set (via the BT-20/8)
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.

  2. Robert Freshwater, TELEPHONE No. 394
    BOBs Telephone File (website) Retrieved June 2014.

  3. P.O.E.D. 1 , N 620, Labels 252 & 253, for use with telephones with keys
    Fist issued 20 April 1952. Last updated 19 April 1967. Obtained via [2].

  4. P.O.E.D. 1 , N 264, Telephone No. 164 (handset) diagram of connexions
    First issued 25 January 1935. Last updated 5 January 1968. Obtained via [2].
  1. P.O.E.D. = Post Office Engineering Department.

Further information
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Crypto Museum. Created: Wednesday 26 May 2021. Last changed: Monday, 05 July 2021 - 09:01 CET.
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