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WWII Scrambler Phone
Frequency Changer / Privacy Set - under construction

The Frequency Changer, also known as the Scrambler Phone, was a secure telephone system, developed in the UK during the course of 1939 by the General Post Office (GPO, now: BT). The system consisted of two units: the actual scrambler — known as the Frequency Changer — and a (converted) regular telephone with a green bakelite handset that was used as the voice terminal. 1 It is also known by the post-war names Secraphone, Privacy Set, Privacy Unit and Secrecy Unit.

The scrambler was housed in a wooden or metal enclosure that was usually placed under the table or in an adjacent room. The voice terminal was placed on the desk of the calling parties. It came in several flavours, but was usually based on an adapted standard black 394 or 396 telephone, modified with a 164 handset in Jade Green. Due to shortages at time of war, black handsets were sometimes painted lime green as an alternative.

The image on the right shows an typical SA5063 voice terminal with green 164 handset, that was used with the Scrambler from 1943 onwards [1].
  
SA 5063/1 voice terminal

During World War II, many scrambler systems were installed to prevent occasional or intentional eavesdropping. Initially, the public telephone network was used for this, but the War Office later established its own private network, that was completely independent from public exchanges. 2

The WWII image on the right shows British Prime Minister Winston Churchill behind his desk at the Cabinet War Rooms 3 just 10 feet below street level under the New Public Offices 4 at Whitehall. To his left is Royal Navy Captain Richard Pim who uses the Scrambler Phone on Churchill's desk.

In 1938, after a survey by the Office of Works, this building was thought to be suitable for use as a temporary office in the event of war. It was hastily converted into a reinforced 5 temporary command center and became operational in August 1939, just before the outbreak of WWII.
  
Churchill in the Cabinet War Rooms during WWII, with a staff member operating his Scrambler Phone. Copyright IWM [4]. Click to enlarge.

The Frequency Changer (Scrambler) was developed in 1939 at the Post Office Research Station at Dollis Hill (North-West London). It was first deployed at home (in the UK) and in the field (abroad) in early 1940, with modified 4-wire variants being available for use over radio, and tropicalised versions for areas of high humidity. During the course of WWII, most units were manufactured at the GPO's Holloway factory in North-London (manufacturer code FH). After the war, production was moved to TMC – who marketed the system as Secraphone – and to other manufacturers [1].

  1. In GPO terminology, the voice terminal is commonly known a the telephone instrument.
  2. Most of the War Office's private telephone networks were of the Local Battery (LB) type and were manually switched, which is why the vast majority of voice terminals does not have a dial.
  3. The Cabinet War Rooms — today known as the Churchill War Rooms — are now part of the Imperial War Museum (IWM) and are open to the public. It also houses the Churchill Museum [16].
  4. This building now houses the Treasury.
  5. Although this 'bunker' was reinforced several times during the war, it hardly offered any real protection against a direct bomb hit.

Voice terminals
Telephone No. 162 (1940)
162
Telephone No. SA5030 (1940) Telephone No. SA5031 (1940) Telephone No. SA5063/0 (1943) Telephone No. SA5063/1 (1944) Telephone No. 710 and 740 (1962)
740
 Introduction


Scramblers
Frequency Changer No. 6 (1940) Frequency Changer No. 6A (1940) Frequency Changer No. 6B (1940) Frequency Changer No. 6AA/0 and 6AA/1 (1942) Frequency Changer No. 6AC (1942) Privacy Set No. 7 (1957) Privacy Set No. 8 (1962) Privacy Set No. 9 (1964)
 Introduction



HELP REQUIRED — Crypto Museum are still looking for additional information and circuit diagrams of the above scrambler systems. We are also looking for a wartime valve-based frequency changer and for a post-war Telephone No. 710/740 that was used with Privacy Set No. 8 or 9. If you can provide any of these, please contact us.
Operation
The diagram below shows how the scrambler works. At the right is the 2 or 4-wire line to the exchange or to a radio. At the left is the voice terminal, which can be an individual microphone and speaker, a handset or a telephone. The audio signal from the microphone is first attenuated to reduce its dynamic range. It then passes a low-pass filter, so that only the 20-2000 Hz part of the spectrum is fed to a ring mixer, where it is added to the 2500 Hz signal from an oscillator.

At the output of the mixer, the sum and the difference of the two signals are available, with the difference being the mirrored version of the original signal (here shown in red). This means that low-frequency tones have become high-frequency tones and vice versa. After filtering it again in a low-pass filter, only the mirrored signal is left, which is then amplified and delivered to the line.

Block diagram of the Frequency Changer. Click to see the original drawing [1].

The bottom half of the diagram shows the reception path, which is more or less the same, but in reverse direction. The mixer produces two images again, of which the lower one is the mirrored version of the received signal. After filtering, the original audio signal remains, which is then amplified and delivered to the speaker. The spectrum diagrams should illustrate what happens.

Although the Frequency Changer, or frequency inverter, offered reasonable protection against an occasional (un)intentional eavesdropper, such as the exchange operator or a service engineer working on the lines, it was no match for a professional interceptor. All one had to do, was find the inversion frequency and mirror the spectrum again. A classical case of security by obscurity.

 More about scramblers


Setup
The diagrams below show the different configurations in which the scrambler could be used. In all cases 1 , the push-buttons on top of the telephone set (i.e. the voice terminal) are configured in such a way that in SECRET mode, the microphone and speaker are wired directly to the scrambler.


The diagram above shows the most common configuration, in which a regular (switched) 2-wire subscriber line is used to connect the two parties. In this situation, the fork circuit inside the scrambler unit is used to combine the transmit and receive circuits onto a single 2-wire line.


In addition, when rented 4-wire lines are available, it is possible to avoid the use of a fork circuit and use two separate 2-wire lines for transmit and receive. Although this solution provides the best possible audio quality, 4-wire switched lines were hardly used as they were very expensive.


It was also possible to use the scrambler system over wireless (radio) links. In that case, the units were used in 4-wire configuration, as the transmit and receive lines had to be wired directly to the transmitter and receiver respectively. The diagram shows a full-duplex scrambled radio link.

  1. It was possible to connect a standard (2-wire) telephone set directly to the input of the scrambler, by using another fork circuit (dotted in the block diagram), but in practice this was hardly ever done.



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

Voice terminals
Below is an overview of the various (converted) telephone sets that were used in combination with the Frequency Changers (scramblers). Note that many variations exist, often caused by shortages during the war. In the overview below, we are heavily drawing on the information supplied by Andy Grant in the UK, most of which was published in an article in Telecommunications Heritage Journal in the Summer of 2017 [1]. Many thanks to Andy for permission to use this information.

In the text below, Telephone Set Type Number is abbreviated to Tele.No.

Model Year Tele. Key Wires Block Remark
162 1940 162 - - - + black Tele.No.328 for initiating call
SA5030 1940 328 303A 6 BT20/8 CB/Auto
SA5031 1940 394 1 303A 8 BT20/8 LB (CBS 1, 2 & 3), Magneto
SA5063/0 1943 394 1 303A 12 BT No 6 LB (CBS 2 & 3), Magneto, Direct/PBX
SA5063/1 1944 394 1 303B 12 BT No 6 LB (CBS 2 & 3), Magneto, Direct/PBX
710 1962 710 2 - ? ? CB/Auto
  1. The chassis of a Tele.No. 396 was sometimes used as a replacement for the 394.
  2. Telephone No. 740 was used as an alternative to the 710.

Telephone No. 162   —  wanted
The first scrambler sets that were released in early 1940, used a green bakelite Tele.No.162 as the voice terminal for scrambled conversations. The call would be initiated first with a standard black telephone set of the No. 328 range, that was also connected to the Frequency Changer [1].

Once the call had been setup, both parties would press a button to go secure (i.e. select scrambled speech) and the conversation was continued on the green 162 telephone set. Once the call was finished, both parties would place their handset in the cradle to terminate the connection.

The Tele. No. 162 was actually the GPO version of the Neophone, made by Siemens Brothers 1 of Woolwich. It was introduced by the GPO in 1929 and is nicknamed The Pyramid. 2 The sets were available in black (bakelite), and later also in Jade Green, Chinese Red, Ivory and mottled Brown 3 .
  
GPO 200 series telephone set in Jade Green, which is very similar to the Tele. No. 162. Photograph obtained from www.designc20.com

The 162 had a No. 164 handset, which in this case was also green. The 162 and 328 desktop telephone sets were abandoned in August 1940, when the more familiar combined set was introduced (see below). The image above shows a nearly identical GPO 200-series telephone.

  1. Not to be confused with the German manufacturer Siemens & Halske.
  2. This nickname was introduced in recent years by collectors and was not used at the time.
  3. Although the coloured versions are commonly said to be made of Bakelite, they were actually made of Urea Formaldehyde, which was – like Bakelite – one of the first plastics.

SA5030
Introduced in August 1940 for use on CB/Auto installations. It consists of a Tele. No. 328, fitted with a Key No. 303A switch assembly, a 6-wire 1 line cord ending in a BT20/8 connection box, or Block Terminal, and a green Handset No. 164. The set was normally fitted with two push-buttons – in front of the handset – labelled SECRET and NORMAL (later renamed: SCRAMBLE and NORMAL).

An extra button could be fitted at the centre, to provide RECALL on PABX installations that supported this feature, or for use with various extension plans. In such cases the label would be changed accordingly, for example: SCRAMBLE - RECALL - NORMAL and in most cases a dial would be fitted to allow direct (auto) calls.

Factory assembled SA5030 units are extremely rare (marked SA5030 at their base), as most of them were built by engineers in the field from locally available parts. Such items are generally marked Tele.No.328 on their base and chassis.
  
SA5030 voice terminal

The image above shows a Tele. No. 328 that was converted into an SA5030 by a technician in the field. Due to shortages at the time of manufacturing, a green-painted black bakelite handset is used. The green cord is a high-quality reproduction that was fitted when the unit was restored.

  1. 7 wires if a PABX Recall button is fitted.

SA5030 voice terminal Front view SA5030 voice terminal Wiring Junction box Privacy Set No. 9A with SA5030 voice terminal
A
×
A
1 / 6
SA5030 voice terminal
A
2 / 6
Front view
A
3 / 6
SA5030 voice terminal
A
4 / 6
Wiring
A
5 / 6
Junction box
A
6 / 6
Privacy Set No. 9A with SA5030 voice terminal

SA5031
Introduced August 1940 for use on LB (CBS 1, 2 & 3) and Magneto systems. It consists of a Tele. No. 394, fitted with a Key No. 303A switch asssembly, an 8-wire line cord, a BT20/8 connection box, and green handset 164. It is fitted with two push-buttons, labelled SECRET and NORMAL. 1

An extra push-button could be added at the centre, to provide the RECALL facility of a PABX, or for use with various extension plans. In such cases the label would be changed accordingly.

Factory assembled SA5031 units are extremely rare, as most of them were built by engineers in the field from locally available parts. Such items are generally marked Tele.No.394 on their base and chassis. Due to wartime shortages, the chassis of Tele.No.396 was sometimes used in these sets as a replacement.
  
Example of an SA 5031 voice terminal. Photograph kindly supplied by Andy Grant [1].

  1. Later: SCRAMBLE and NORMAL.

SA5063   SA5063/0
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
B
×
B
1 / 8
SA 5063/0 voice terminal
B
2 / 8
Front view
B
3 / 8
Left view
B
4 / 8
With handset off-hook
B
5 / 8
Handset
B
6 / 8
No. 396 bottom panel with circuit diagram
B
7 / 8
Close-up of the circuit diagram of the 396
B
8 / 8
Markings at the bottom

SA5063/1
Introduced in February 1944 for use on LB (CBS 2 & 3) and Magneto systems, and on for use with long lines on CB/Auto systems. It consisted of a modified Tele.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, which suggests that it was made at the GPO factory Holloway (FH) in North-London. It is built on a No. 394 chassis.

  1. 20-way box with metal lid.
  2. Later: SCRAMBLE and HOLD SCRAMBLER.
  3. The 303B 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.

Left view Front view SA 5063/1 voice terminal With open drawer Close-up of the push-buttons Push-buttons with label Markings at the bottom
C
×
C
1 / 7
Left view
C
2 / 7
Front view
C
3 / 7
SA 5063/1 voice terminal
C
4 / 7
With open drawer
C
5 / 7
Close-up of the push-buttons
C
6 / 7
Push-buttons with label
C
7 / 7
Markings at the bottom

Telephone No. 710 and 740   —  wanted
In the late 1950s, the GPO made the Frequency Changer — meanwhile renamed Privacy Set — to commercial parties. British manufacturer TMC, for example, marketed the equipment under the name Secraphone from the late 1950s to the late 1970s. Others kept using the name Privacy Set.

Privacy Set No. 7, 8 and 9 could all be used with the existing SA5030 voice terminal, but from approx. 1962 onwards, Telephone No. 710 and No. 740 were used as modern a replacement.

The introduction of the 710 and 740 telephone sets more or less coincided with the arrival of Privacy Set 8 and the replacement of manually switched and Local Battery (LB) systems.

For use with a Privacy Set, the No. 710 or 740 telephone set had to be modified. Like with the earlier SA503x terminals, the push-buttons at the top were configured for switching between clear and scrambled speech.

The image on the right shows a genuine No. 740 that turned with a Privacy Set No. 8 up in June 2013 at Gildings Auctioneers in the UK.

  
Telephone No. 710. Copyright Gildings Auctioneers [10].




Overview of the various types of Frequency Changers and Privacy Sets

Frequency Changer mainframes   scramblers
There were several versions of the Frequency Changer — or Privacy Set as it was called from 1951 onwards — depending on their applications and on advances in their technologies. During WWII, all mainframes were known as Frequency Changer, whilst after the war the name Privacy Set was most commonly used. Nevertheless, Privacy Unit and Secrecy Unit also seem to turn up every now and then. British manufacturer TMC marketed the equipment under the name Secraphone [1].

Model Year Case System Valves Remark
Frequency Changer No. 6 1940 Wood CB/Auto CV1732 200-250V AC/DC 1
Frequency Changer 6A 1940 Wood CB/Auto/LB CV1732 200-250V AC
Frequency Changer 6B 1940 Wood CB/Auto/LB CV1732 200-250V AC/DC 1
Frequency Changer 6AA/0 1942 Metal CB/Auto/LB CV1052 200-250V AC
Frequency Changer 6AA/1 1942 Metal CB/Auto/LB CV1052 100-110/200-250V AC
Frequency Changer 6AC 1944 Metal CB/Auto/LB CV1052 As 6AA/1 + 12V DC 2
Privacy Set 7 / 7A 1957 Metal CB/Auto/LB CV138 Improved scrambling
Privacy Set 8 / 8A 1962 Metal CB/Auto CV7005 3 Transistorised
Privacy Set 9 / 9A 1964 Metal CB/Auto CV7005 3 As 8, with extra filtering
  1. Parts of the chassis carry a (potentially dangerous) live voltage.
  2. Uses a vibrator pack.
  3. Germanium PNP transistor.

Frequency Changer No. 6   —  wanted
Introduced in June 1940 for use on CB/Auto installations only. The unit was housed in a wooden case and operated on 200-250V AC or DC mains power. It is built around CV1732 valves (tubes), which are 5-pin triodes designed for signal processing (equivalent to ML4 and VT90/VT129).

No. 6 was short-lived and was succeeded within months by the 6A, 6B, 6AA and 6AC versions, as it had several design issues. The power for the telephone set was derived from the exchange line current, which made it a CB-only device, unsuitable for Local Battery (LB) installations.

Furthermore, as it could be powered from a DC mains source, large parts of the chassis carried a live voltage, making it potentially dangerous for service engineers and maintenance personnel.

At present, no image of this version is available.

  

Frequency Changer No. 6A   —  wanted
This improved version was introduced in August 1940 for use on CB/Auto or LB installations. It was suitable for the 200-250V AC mains only and had an isolated power transformer, which made it much safer for maintenance engineers. Like the No. 6 it is built around CV1732 valves.

There were several other design improvements, but a detailed description of them is currently unavailable. Like the No. 6, the 6A is housed in a wooden enclosure.


At present, no image of this version is available.

  

Frequency Changer No. 6B   —  wanted
Introduced in September 1940, this version was nearly identical to the 6A, but was suitable for the 200-250V AC or DC mains and had therefore a live voltage-carrying chassis (like the No. 6).

This version was intended for use in areas that still had a Direct Current (DC) mains power network, with the ability to run from Alternating Current (AC), once the local mains network had been converted.


At present, no image of this version is available.

  

Frequency Changer No. 6AA/0 and 6AA/1   —  wanted
Introduced in 1942 for use on CB/Auto and LB installations. Unlike its predecessors, it was built around the VC1052 signal processing valve; an octal-based penthode equivalent to the EL32.

The 6AA came in two flavours: 6AA/0 and 6AA/1, differing only in their power supplies. The 6AA/0 was suitable for the 200-250V AC mains, which was used in most parts of the UK.

The 6AA/1 was more flexible and could be powered from the 100-110V AC or 200-250V AC mains. This made the 6AA/1 suitable for use in areas that were still on 110V.

The 6AA was the first Frequency Changer to be housed in a metal enclosure. The image on the right shows a typical 6AA unit with its hinged top lid open, giving a good view of the valves (left).
  

Frequency Changer No. 6AC   —  wanted
Introduced in January 1944, this unit was was nearly identical to the 6AA, but had an even more flexible power supply that was suitable for the 100-110V AC or 200-250V AC mains, or 12V DC.

The 12V DC option used a vibrator circuit, and was intended for situations where the mains power was failing or unavailable.   

Privacy Set No. 7 and 7A   —  wanted
Introduced in November 1957 for use on CB/Auto and LB systems. It is housed in a metal enclosure and was known as Privacy Set rather than Frequency Changer. Rather than being GPO-exclusive, this was the first set to become available through other commercial parties as well.

This version bas built around VC138 penthode valves — equivalent to the EF91 — and offers improved performance and security. It has a modular construction, which made it much more service friendly in case of a fault or repair.

Security was improved by using a different frequency inversion scheme with two stages of modulation — one at 10.0705 kHz and one at 12.570 kHz — thereby minimising the risk of unscrambled speech being delivered at the line in case of a malfunction of the main oscillator. Interestingly, the feature was dropped again on the later transistorised Privacy Set No. 8 and 9.
  
Interior of a Privacy Set No. 7A.

Privacy Set No. 7A was suitable for the 100-110V or 200-250V AC mains, whilst No. 7 came without a mains power supply module, allowing it to be used with other power options, such as a 12V DC input module. The image above the right shows a typical Privacy Set No. 7A.


Privacy Set No. 8 and 8A
Privacy Set No. 8 was the first model that was fully transistorised. It first appears in GPO diagrams from 1962 and was designed for use on CB/Auto systems only. Like Privacy Set 7, is was available via commercial parties. It is arguably the most wide-spread and well-known post-war model [1].

The device is fully compatible with the earlier models and was initially used in combination with the old SA5030 voice terminal. The terminal was later succeeded by Telephone No. 710 and 740, which had a more modern look and feel.

There are variations in the physical construction of this version, as it was made by several UK manufacturers. Refurbished versions of Privacy Set No. 8 have been spotted as late as 1977 [1]. The image on the right shows the interior of a Privacy Set No. 8, that was used as part of a public demonstration set by manufacturer TMC.
  
Privacy Set No. 8 with cover removed

At the bottom — barely visible as it has been repainted — is date code TE 62/1, which confirms that this model is from 1962. The unit shown here is modified with a 6-pin plug and socket for quick connection of the voice terminal and the line. For sales demonstrations, it was used with a transparent version of GPO Telephone No. 328, that is configured as an SA5030 voice terminal.

Privacy Set No 8 Privacy Set No. 8 with cover removed Transparent telephone in front of Privacy Set No. 8 Transparent telephone set and Privacy Set No. 8 Wiring Junction box 6-pin plug Three push-buttons
D
×
D
1 / 8
Privacy Set No 8
D
2 / 8
Privacy Set No. 8 with cover removed
D
3 / 8
Transparent telephone in front of Privacy Set No. 8
D
4 / 8
Transparent telephone set and Privacy Set No. 8
D
5 / 8
Wiring
D
6 / 8
Junction box
D
7 / 8
6-pin plug
D
8 / 8
Three push-buttons

Privacy Set No. 9 and 9A
Privacy Set No. 9 was first mentioned in GPO diagrams in 1964. It is nearly identical to Privacy Set No. 8/8A, but is fitted with improved filtering, possibly to improve performance on poor quality phone lines, but more likely to reduce or avoid leaking of information due to TEMPEST problems.

The image on the right show a Privacy Set No. 9A with its metal cover removed. This version has a flexible mains transformer with various taps, to allow it to be configured for 100V AC to 250V AC mains networks. At the top centre, to the right of the terminal block, are four spare fuses.

  
Privacy Set No. 9A with cover removed

Privacy Set No. 9A Privacy Set No. 9A with cover removed Privacy Set No. 9A with SA5030 voice terminal Privacy Set No. 9A with SA5030 voice terminal Text at the bottom
E
×
E
1 / 5
Privacy Set No. 9A
E
2 / 5
Privacy Set No. 9A with cover removed
E
3 / 5
Privacy Set No. 9A with SA5030 voice terminal
E
4 / 5
Privacy Set No. 9A with SA5030 voice terminal
E
5 / 5
Text at the bottom



Auxiliary equipment used with the scrambler
Auxiliary equipment used with the scrambler


Accessories
Local relay unit for up to 3 voice terminals Indicator No. 401CN (doll's eye) Telephone connection box BT20/8 Telephone connection box BT No. 6
Unit Auxiliary Apparatus   SA5050
The SA5050 was a relay unit, that was introduced in March 1944. It allows up to three SA5063/0 voice terminals, or up to three sets of SA5063/1 terminals wired in parallel, to be connected to a single Frequency Changer. Depressing the ENGAGE FOR SECRET (HOLD SCRAMBLER) on one of the SA5063 terminals, causes the SA5050 to disable the other terminals for the duration of the call.   
Relay unit SA5050. Photograph via Andy Grant [1].

Indicator   401CN
In order to inform the other users on the internal network that the Frequency Changer was in use by someone else and was therefore unavailable to you, Indicators No. 401CN were mounted in the vicinity of the SA5063/1 voice terminals. No. 401CN is a so-called dolls eye status indicator [1].   

Connection box BT20/8
...   

Connection box BT No. 6
...   




Transparent telephone set and Privacy Set No. 8

Secraphone
From 1951 onwards, the Scrambler was no longer known as Frequency Changer but as Privacy Set. Also, from the late 1950s onwards, the devices were no longer made by the GPO but by external contractors, including TMC who marketed the system as Secraphone. The image above shows a Privacy Set No. 8 with a transparent voice terminal that was made for demonstration purposes. In SECRET-mode, the microphone and speaker of the handset are routed directly to the Privacy Set, via a 6-pin plug. The image below shows a TMC advert for the Secraphone [7].

Transparent telephone set and Privacy Set No. 8 Transparent telephone in front of Privacy Set No. 8 Privacy Set No. 8 with cover removed Seen from the left Front view Tranasparent series 300 TMC  telephone set Three push-buttons Wiring
F
×
F
1 / 8
Transparent telephone set and Privacy Set No. 8
F
2 / 8
Transparent telephone in front of Privacy Set No. 8
F
3 / 8
Privacy Set No. 8 with cover removed
F
4 / 8
Seen from the left
F
5 / 8
Front view
F
6 / 8
Tranasparent series 300 TMC  telephone set
F
7 / 8
Three push-buttons
F
8 / 8
Wiring

Advert for the Secraphone by Telephone Manufacturing Co Ltd. (TMC). Via Sam Hallas [6].

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 War Office, of course, but it was considered secure enough to keep an occasional eavesdropper out, 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. 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.



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 stat 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 exchanged for alternative 3 ones, 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 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
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Two handsets that were used with the Scrambler
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Jade green GPO handset No. 164
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Green handset manufactured in 1935
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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.

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.

During the war, this was not common practice however, as most users – including Churchill – would rely on an assistent to establish contact and initiate a (secure) conversation. In post-war installations however, most voice terminals did have a dial, as automatic exchanges had mean­while become mainstream in most countries.



  
SA5030 voice terminal

Real or fake?
Genuine GPO No. 394 telephone sets that have been used as part of a Secraphone secure speech setup, are extremely rare and are very difficult to find. Many collectors have created a mockup by taking an existing multi-button 300, swapping the text shield and adding a green handset to it. If you are a collector and are searching the internet for genuine 394 scrambler phones, there are a few pitfalls to watch out for. Here are some examples of phones that have turned up in the past.

In June 2013, the telephone set shown in the image on the right turned up for sale at Giddings Auctioneers in Leicester (UK). According to the description it is a GPO Telephone No. 394, which had been used as part of a scrambler telephone system, in combination with Security Unit No. 8.

Although this set looks complete and in good condition, we have some reservations about its authenticity. There are strong indications that the set has been put together from spare parts and/or from parts from other telephone sets. First of all, the unit has a dial and, although it is technically possible, this was very uncommon for terminals that were used with the Scrambler.

Secondly, and more seriously, the two buttons, marked NORMAL and SECRET, are labelled the wrong way around. Although various designs of the text label are known, the SECURE button is always on the left and the CLEAR button on the right. This phone therefore seems to be a fake.

  
Telephone No. 394. Copyright Gildings Auctioneers [11].
Another example of a non-authentic scrambler phone is shown in the image on the right. The item appeared on eBay in February 2016, but in this case, the seller made it very clear in his description, that it was a reproduction [13].

According to the description, it consists of a Telephone set No. 328L of 1962 vintage, with a green reproduction handset that was added at a later date. The scramble/normal plate is genuine and was given to the seller in 1981 when he was an apprentice at the GPO/BT, by a former fitter who used to work in the Houses of Parliament.
  
Image of a reproduction Secraphone terminal as sold on eBay in February/March 2016 [14]

On both the above examples, the dials may seem out-of-place, but are necessary if you wanted to operate the Scrambler via a local PABX, like in the second case. If the dials were replaced by a genuine blanking panel No. 3, they would look excellent in any WWII Scrambler display.


Interior
SA5063/1
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

At present, the correct wiring of the switch pack for use in combination with a Frequency Changer is unknown, as the phones in our collection were rewired during the post-war years for back-to-back operation. Nevertheless, all parts are present and wiring it correctly should not be difficult. Note that the line cord shown above is not the correct one as it has only 3 wires. In the original setup, a 6 to 12-wire cable was used between the voice terminal and the Frequency Changer.

 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).
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Interior bottom view
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Interior front/top view
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Interior rear/top view
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Interior bottom/rear view
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Interior bottom/front view
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Interior with handset
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tie-knotting of the handset cable
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Connection block (T) seen from the bottom of the phone
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Close-up of the frame
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Hook switch engaged (on-hook)
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Top view of the 9K switch pack (303/A unit)
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Metal plate for 'programming' the behaviour of the three buttons
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Terminals for connection of the dial (not used here)
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303/A key block (9K switch pack)
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Pressing the rightmost button
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Pressing the leftmost button
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Bottom panel with drawer and circuit diagram
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Telephone No. 394LB circuit diagram inside the device
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Close-up of the circuit diagram
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No. 396 bottom panel with circuit diagram
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Close-up of the circuit diagram of the 396
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Close-up of the bell
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Close-up of the transformer
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Capacitor mounting bracket. Note the date stamp on the capacitor (1940).




Privacy Set No. 8 and 9
As the fully transistorised Privacy Sets 8, 8A, 9 and 9A are all nearly identical, we will restrict the description of their interior to Privacy Set No. 9A, which is shown in the illustration above. The interior can be accessed by removing two bolts (one at the front and one at the rear) and sliding off the metal cover. This reveals a compact metal frame to which all electronic parts are mounted.

The right side of the device gives access to the components, with the power supply unit (PSU) and the terminal block immediately visible, whilst the left side holds the scrambler board.

The PSU is mounted to the bottom panel, which is attached to the main frame by means of six recessed screws at the bottom. After removing these screws, the PSU can be removed as shown in the image above, after which the components of the scrambler board become (partly) visible. In the image on the right, parts of the transmission section plus the 2500 Hz oscillator are visible.
  
Scrambler board (tx section and 2500 Hz oscillator

An extra board, mounted to the frame toward the rear, holds a set of filters. Depending on the model and version, the PSU is suitable for a single predetermined mains voltage, or for a range of voltages, including 100V, 115V, 200V, 220V and 240V AC, as the Privacy Set No. 9A shown here.

The actual scrambler is located behind the other parts, but can be accessed from the left side. It is fitted to a metal frame that is bolted to the case. The image on the right shows the scrambler PCB after it has been dismounted from the enclosure.

The scrambler is built around just five CV7005 1 Germanium PNP transistors, made by Mullard in the UK. According to their date code 2 they were manufactured in late 1963. The rest of the space is taken by some large transformers, capacitors and inductors. Move the mouse over this image to reveal the purpose of the various components.
  
Scrambler board (component side)

The transmission section is at the upper edge of the image, and is shown in red. At the far right is the input transformer (TR1). To its left is the first filter (F1) followed by a mixer, the second low-pass filter (F2) and an amplifier (T1 and T2). At the left is the (telephone) line transformer (TR1).

The line transformer is shown in the image on the right, It is fully balanced and has multiple taps on the primary side, so that it can be wired to match an impedance of 150Ω, 300Ω or 600Ω. When using the scrambler on a 2-wire telephone network, this transformer acts as a fork circuit. Such lines commonly have a 600Ω impedance.

Along the lower edge is the receiving circuit, in the above image shown in green. The line signal is first attenuated (A2) and then filtered (F3), before it is mixed with the 2500 Hz signal from the oscillator, and then amplified (T4 and T5).
  
Line transformer

At the bottom right is the output transformer (TR2) which delivers the signal to the handset of the telephone. It has multiple taps, so that it can be adjusted to match the impedance of the speaker in the telephone's handset, in the same way as the line transformer can be impedance matched.

At the centre of the scrambler board — shown in yellow in the above image — is an oscillator (T3) which is used by both transmitter and receiver. It provides the 2500 Hz signal for the two mixers and must be very accurate, so that it matches to that of the scrambler at the other end of the line.

The actual mixing of the audio signal and the 2500 Hz oscillator signal, is done in a balanced ring mixer that consists of two transformers and four rectifier diodes. The image on the right shows the mixer of the receiver circuit, which is identical to the mixer in the transmission circuit.
  
Mixer, consisting of two transformers and four diodes

Although the circuit of the scrambler is straightforward, aligning this device – which is now more than 50 years old – may be difficult. The device has no potentiometers, but is completely aligned with adjustabe coils. Furthermore the value of the capacitors and the stability of the old CV7005 germanium transistors – and possibly also the OA73 diodes – may have deteriorated over time.

  1. The CV7005 was also made by Philips and is equivalent to the Philips OC71.
  2. This is in line with the marking TES 64/1 at the bottom, indicating that this model is from 1964.

Privacy Set No. 9A Privacy Set No. 9A - interior Privacy Set No. 9A - interior Interior Solder side of the main PCB Scrambler board removed from the case Power supply unit Scrambler board (tx section and 2500 Hz oscillator
Terminal block Filters Scrambler board (component side) Line transformer Mixer, consisting of two transformers and four diodes Attenuator Receiver amplifier Common 2500 Hz oscillator
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Privacy Set No. 9A
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Privacy Set No. 9A - interior
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Privacy Set No. 9A - interior
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Interior
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Solder side of the main PCB
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Scrambler board removed from the case
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Power supply unit
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Scrambler board (tx section and 2500 Hz oscillator
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Terminal block
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Filters
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Scrambler board (component side)
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Line transformer
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Mixer, consisting of two transformers and four diodes
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Attenuator
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Receiver amplifier
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Common 2500 Hz oscillator

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 as close as possible to their original state. 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
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Before restoration
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Lamp fitting mounted to the middle button
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Bakelite doorbell knob mounted at the center
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Telephone No. 394 before its restoration
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Bakelite body after restoration
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Inside the bakelite body
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Rust caused by leaking batteries

YouTube
An excellent documentary about Churchill's Cabinet War Rooms is available on YouTube via the link below. It features a combination of archive film material from the Imperial War Museum's vast collection, and atmospheric dramatisations filmed inside the actual Cabinet War Rooms.




Manufacturers
It is believed that during WWII, Frequency Changers were only made by the General Post Office (GPO), who also developed the unit. The first post-war model to be manufactured by TMC as well was Privacy Set No. 7 [1]. The later Privacy Sets 8 and 9 were made by the following companies:

  • GPO
    General Post Office (now: BT) 1
  • TMC
    Telephone Manufacturing Company 2
  • PYE
    Pye TMC Ltd, later Philips 3,4
  • Landis+Gyr
    3
  • GEC/AEI
    Associated Electrical Industries 3
  • ?
  1. All models.
  2. Privacy Set 7, 8/8A and 9/9A.
  3. Privacy Set 8/8A and 9/9A, albeit possibly with different dimensions.
  4. In the mid-1960s, TMC was taken over by Pye in Cambridge. In 1967, Pye was taken over by Philips.

Connections
Privacy Set No. 8/8A and 9/9A
The table below gives the pinout of the screw terminal block inside Privacy Set 8/8A and 9/9A. This is the lowest row of screws when looking at the device from the right. The first column shows the colours, whilst the second one specified the contact number inside the BT20/8 box.

  1. Green
    BT3
    Line B 1
  2. Black
    BT6
    Line A 2
  3. unused
    Audio in 2
  4. unused
    Audio in 2
  5. White
    BT4
    Microphone
  6. Red
    BT1
    Microphone 3
  7. Blue
    BT1
    Speaker 3
  8. Orange
    BT5
    Speaker
  9. Loop
    wired to 10
  10. Loop
    wired to 9
    Terminal block
  1. In 2-wire configuration the line is connected here. In 4-wire configuration, this is the Audio out line.
  2. Used in 4-wire configuration (e.g. when connected to a radio).
  3. Lines (6) and (7) are joined in the connection box (at point BT1).

BT20/8 Block Terminal
  1. Red
    Microphone/Speaker (common)
  2. Blue
    LINE B (telephone line)
  3. Green
    LINE B (Privacy Set)
  4. White
    Microphone
  5. Orange
    Speaker
  6. Black
    Line A
  7. Grey 1
    Recall (to PABX when present)
  8. Brown
    Ground
  1. The Grey wire was usually identified as SL (Slate).

Glossary
Expressions and abbreviations used on this page:

AC   Alternating Current
Type of current used for most mains networks in the world, typically with an alternating frequency of 50 or 60 Hz. During WWII, most (but not all) of the UK had an AC mains network with a voltage of 110 or 230V. Some regions remained on DC for several years.
BT   (1) British Telecom
Arguably the largest telecom operator of the UK. Previously state-owned and known as the General Post Office (GPO) or the British Post Office (BPO).
BT   (2) Block Terminal
GPO expression for the connection box between a telephone set and the line.
CB   Central Battery system
System in which all the energy needed for transmission and signalling is delivered by the exchange. No local batteries or hand generators are used at the telephone end. (More [3])
CBS   Central Battery Signalling system
Similar to a CB system, except that the mircophone is powered locally by a battery at the telephone end. Power for signalling is provided by the exchange as in a CB system. In the UK there were three types of CBS. (More [3])
DC   Direct Current
Type of current typically used in cars, but not on the mains network. During WWII, parts of the UK still had a DC mains network, whilst the majority used AC.
GPO   General Post Office
The state-owned post and telecommunications operator in the UK, before it was renamed BT and privitised. The GPO was als known as British Post Office (BPO) and simply as Post Office (PO). It is currently known as British Telecom (BT).
LB   Local battery system
System in which a local battery is used for providing the current for the speech circuits.
Magneto   Hand-cranked electrical generator that provides electricity for signalling in an (old) telephone system. In some countries known as inductor, crank ringer, or wake-up unit.
PABX   Private Automatic Branche Exchange PBX
PBX   Private Branche Exchange
Local telephone exchange or switching system, using inside the building of a private organisation, usually connected to the Public Switched Telephone Network (PSTN) via Central Office (CO) lines.
PL   Plessey
Manufacturing code used on the body of the telephone sets and also inside, often stamped on the chassis. The manufacturer's code is also cast inside the bakelite body.
TE   TMC (see below)
Manufacturing code used on the body of the telephone sets and also inside, often stamped on the chassis. The manufacturer's code is also cast inside the bakelite body.
TMC   Telephone Manufacturing Company
British telephone manufacturer. Also one of the manufacturers of the Privacy Sets. Based in St. mary Cray (Kent, UK) and London [7].
Documentation
  1. Circular label Speech on Telephones is Not Secret (PDF)
    Crypto Museum, Reproduction, 10 June 2014.

  2. Original Privacy Set block diagram
    Date unknown. Kindly supplied by Andy Grant [1].

  3. Privacy Set No.8/9 circuit digram, Secraphone Type S2N
    EC526518. TMC London. Date unknown.

  4. Connection diagram SA 5030 to Privacy Set
    Date unknown.
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, 200 Type telephone information
    BOBs Telephone File (website). Retrieved September 2018.

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

  4. Imperial War Museum (IWM), Photograph of Churchill in Cabinet War Rooms
    Retrieved January 2014. Colour version obtained from [1] - September 2018.

  5. Post Office (GPO). Index to E.I.s 1 on external construction and maintenance
    11 September 1959. Retrieved January 2015.

  6. Post Office (GPO). Index to E.I.s 1 on external construction and maintenance...
    4 April 1962. Retrieved January 2015.

  7. Telephone Manufacturing Co Ltd. (TMC), Advert for Secraphone
    Date unknown, but believed to be late 1950s. Retrieved January 2014.
    Via Sam Hallas, website.

  8. PO 2 Telecomms Headquarters, Automatic and Subscribers Privacy Equipment...
    ...Telephone No., 740 and Privacy Set No. 8
    Document number N5164, 12 March 1975.

  9. PO 2 Telecomms Headquarters, Automatic and Subscribers Privacy Equipment...
    ...Telephone No., 740 and Privacy Set No. 8
    Document number N5166, 23 September 1974.

  10. POTDD (GPO), Telephone No. 394
    Document number N494, issue A, 25 January 1968. First released 30 September 1937.

  11. Gildings Auctioneers, Images of Privacy Set No. 8 and telephone sets
    18 June 2013. Retrieved January 2014.

  12. Connected Earth, Wartime communications: Black and green secrecy phone...
    Retrieved January 2015.

  13. Ebay, GPO Black Bakelite 300 Type Phone with Scrambler button and Green Handset
    eBay seller phased_001, item 301880390186. Retrieved February 2016.

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

  15. P.O.E.D. 3 , N 264, Telephone No. 164 (handset) diagram of connexions
    First issued 25 January 1935. Last updated 5 January 1968. Obtained via [3].

  16. Wikipedia, Churchill War Rooms
    Retrieved February 2016.

  17. Louis Meulstee, Wireless for the Warrior
    Website. Retrieved January 2015.
  1. E.I. = Engineering Instructions.
  2. PO or P.O. = Post Office.
  3. P.O.E.D. = Post Office Engineering Department.

Further information
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Crypto Museum. Created: Tuesday 28 January 2014. Last changed: Saturday, 04 May 2019 - 12:49 CET.
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