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Privacy Set No. 8 & 9
Telephone scrambler · 1962-1972 - under construction

Privacy Set No. 8, also known as Secraphone No. 8, was an analogue scrambler for (telephone) voice circuits, developed around 1962 1 by the General Post Office (GPO) at Dollis Hill (UK), and made by a variety of manufacturers [1]. Based on inversion of the voice spectrum, it was the first fully transistorised member of a family of telephone scramblers that started life during WWII as the Frequency Changer. Unlike earlier models, it is suitable for Central Battery (CB) systems only.

The device is fully compatible 2 with all 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.

As the device was manufactured by several UK suppliers, there are variations in the physical construction and in the electronic circuits, but the outer dimensions are always the same. It was in production from 1962 to at least 1972, whilst refurbished versions of Privacy Set No. 8, 8A, 9 and 9A have been spotted as late as 1977 [1].
Privacy Set No. 9A with SA5030 voice terminal

Some manufacturers, such as TMC, incremented the model number each time the circuit diagram was updated, which is how the model numbers 8, 8A, 9 and 9A came into existence. They are all iterations of the same basic design however. Other manufacturers, such as EMI, kept the name Privacy Set No. 8 throughout the lifetime of the product, despite significant changes. Generally speaking, changes can only be tracked by looking at the production code, for example: TES 64/1 means TMC 1964 production batch 1, whilst EMT 72/1 means EMI 1972 production batch 1.

Below is an overview of the different versions and production codes of Privacy Set 8, 8A, 9 and 9A that we have encountered over the years. As there are significant differences between versions and manufactuers, each variant is listed on a separate page, complete with a detailed technical description, a block diagram, detailed circuit diagrams and a description of its interior.

  1. Privacy Set No. 8 first appears in GPO diagrams in 1962. it is intended for use on CB/Auto systems only.
  2. Only when the same carrier frequency is used.

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
Telephone No. 740 and Privacy Set No. 8
Text at the bottom
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Privacy Set No. 9A
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Privacy Set No. 9A with cover removed
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Privacy Set No. 9A with SA5030 voice terminal
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Privacy Set No. 9A with SA5030 voice terminal
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Telephone No. 740 and Privacy Set No. 8
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Text at the bottom

Known versions
Privacy Set No. 8 (TE 62/1), made by TMC in 1962
Privacy Set No. 9A (TES 64/1), made by TMC in 1964
Privacy Set No. 8 (EMT 72/1), made by EMI in 1972
In addition to the model and versions, there were also production variants for various types of applications, commonly identified in GPO terminology with a group number that was added to the model number (e.g. '/3'). The exact meaning of these group identifications is currently unknown. So far, the following production variants have been observed:

  • Carrier frequency
    By default, all Privacy Sets use a carrier frequency of 2500 Hz, so that they are backwards compatible with all earlier models. In some cases, for example when additional security was required, different carrier frequencies were used, such as 3000 Hz and 2000 Hz. This requires a different oscillator, but also different low-pass filters in the transmission and reception path. The Privacy Set No. 9A in our collection uses a 2000 Hz carrier.

  • Power supply
    Most Privacy Sets were supplied with an internal mains power supply unit (PSU), but it was also possible to power it from an external 12V DC supply, such as the battery of a car. In the latter case, the PSU was omitted and the 15 mA current for the carbon microphone (when present) has to be extracted from the subscriber line.

  • 2-wire and 4-wire operation
    Both sides of the device can be configured for 2-wire or 4-wire operation. In most cases, the unit was strapped for a 2-wire subscriber line, and a 4-wire (bare) handset. This is knows as 2-wire to 4-wire operation. When connecting the device to a 4-wire leased line or to a radio, it could be configured for 4-wire operation. This is known as 4-wire to 4-wire operation. With some versions of the device it was also possible to connect a regular telephone set instead of the handset — i.e. behind the scrambler — in which case 2-wire to 2-wire operation was possible. The latter was not recommended however.
Block diagram
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

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All No. 8, 8A, 9 and 9A versions of the Privacy Set had the same external dimensions, regardless the manufacturer. The mounting holes, the power inlet and the line cable cutout are at the same position, so that the devices are interchangeable. The interior however, varies strongly between manufacturers, as shown in the image above. The one on the left was made in the 1960s by TMC and carries the manufacturing code TES 64/1. The one the right is from the 1970s and carries the manufacturing code EMT 72/1. Although this is not certain, it was probably made by EMI. 1

All the devices have in common, are the outer dimensions, the layout of the terminator block at the top left and the electrical specifications, to ensure that they are interoperable. Check out the individual page for each version and variant for additional information, block diagrams, circuit diagrams and a description of the interior.

  1. The manufacturer code EMT is still unconfirmed, but belongs with 99% certainty to EMI. On these pages, we will refer to devices with the EMT manufacturer code as 'EMI-version'.

Privacy Set No. 8   TE 62/1
Made by the Telephone Manufacturing Company (TMC) in London (UK), this is among the oldest versions of Privacy Set No. 8. Built with OC71 PNP Germanium transistors, it is suitable for 2-wire/2-wire up to 4-wire/4-wire operation.

 More information
Privacy Set No. 8 (TE 62/1) with removed cover

Privacy Set No. 9A   TES 64/1
Also made by TMC, this version was introduced in 1964. It is nearly identical to the 1962 Privacy Set No. 8 (shown above), but has several small improvements. In addition, the ability to connect a standard telephone set behind the scrambler was dropped on this model.

 More information
Privacy Set No. 9A with cover removed

Privacy Set No. 8   EMT 72/1
This version of Privacy Set No. 8 was made by EMI in the early 1970s and is built with 1N1303 and 1N1309 PNP Germanium transistors. It is more service-friendly, as the PCBs are hinged and no external parts are used. Furthermore, the electronic circuits are improved in various ways.

 More information
EMI version of Privacy Set No. 8 (EMT 72/1)

  • Type
    Voice scrambler
  • Principle
    Single frequency inversion
  • Manufacturer
    see below
  • Users
    see below
  • Carrier
    2500 Hz or 3000 Hz or 2000 Hz
  • Impedance
    Standard telephone line at 150, 300 or 600Ω
  • Terminal
    Modified conventional analogue telephone set
  • Power
    Internal (mains) or external (12V DC)
  • Mains
    100/115/200/220/240V AC, + 0/5/10V AC
  • Dimensions
    255 × 155 × 90 mm (305 × 155 × 90 mm with mounting flanges)
  • Weight
    4000 grams (EMI) - 4500 grams (TMC)
Privacy Set No. 8 was used by the British Government in the 1960s, 70s and 80s, and also by a number of selected (approved) customers, such as foreign embassies and large corporations and organisations. Below is a non-exhausive list of known users:

Apart from The General Post Office (GPO) there were several commercial parties that were allowed to produce the Privacy Set No. 8 and 9.

  • FH
    General Post Office (GPO), Holloway
  • TE
    Telephone Manufacturing Company (TMC), Dulwich, London (UK)
  • TES
    Telephone Manufacturing Company (TMC/Pye), St. Mary Cray, Kent (UK)
  • GCL
    EMI Communications
  • RAA
    Landis & Gyr, formerly: Aeronautical & General (AGI)
  • EMT
    EMI Telephone Manufacturing 1
  • ?
    Standard Telephones & Cables (STC)
  • ?
  1. The manufacturer code EMT is still unconfirmed, but it most likely belongs to EMI.
 Other GPO manufacturer codes

Known serial numbers
  1. Automatic Systems, Privacy Equipment, DEL & PABX Extension - wiring diagram
    TMC, 1964.
  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: Tuesday, 10 August 2021 - 16:37 CET.
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