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Phone
Voice
Scrambler
No. 8
  
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Privacy Set No. 8   EMT 72/1
EMI version

Privacy Set No. 8, also known as Secraphone No. 8, was an analogue scrambler for (telephone) voice circuits, developed around 1962 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 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.

Privacy Set No. 8 was in production from 1962 to 1972 and was made by several manufacturers. Refubished No. 8 units have been spotted as late as 1977 [1]. The outer dimensions of the device and the position of the cable inlets are identical for all versions regardless the manufacturer, but there are significant differences in the interior.
  
EMI version of Privacy Set No. 8 (EMT 72/1)

This page describes specifically the version of Privacy Set No. 8 that was made in 1972 by EMI Telephone Manufacturing (EMI), as indicated by the marking EMT 72/1 at the bottom of the unit. The circuit is comparable to that of its predecessors, such as the valve-based Frequency Changer 6AC, except that it is made with an early generation of 1N1303/1N1309 Germanium transistors.

A unique feature of the EMI-version is the addition of an improved 2500 Hz carrier oscillator and an extra audio amplifier in the transmission path, fitted on a separate PCB that is mounted on top of the main board. In addition it has a more stable – regulated – power supply unit. This version supports a 2- or 4-wire subscriber line (or radio), and a 3- or 4-wire handset. It is believed that devices from earlier production runs (e.g. EMT 71/1) are identical to this one (EMT 72/1) [3].

Privacy Set No. 8 with line cable
EMI version of Privacy Set No. 8 (EMT 72/1)
Transparent telephone in front of Privacy Set No. 8
Privacy Set No. 9A with SA5030 voice terminal
Telephone No. 740 and Privacy Set No. 8
A
×
A
1 / 5
Privacy Set No. 8 with line cable
A
2 / 5
EMI version of Privacy Set No. 8 (EMT 72/1)
A
3 / 5
Transparent telephone in front of Privacy Set No. 8
A
4 / 5
Privacy Set No. 9A with SA5030 voice terminal
A
5 / 5
Telephone No. 740 and Privacy Set No. 8

Block diagram
Privacy Set No. 8 was originally developed by the GPO, but was made by various manufactuers. Although the circuit is largely the same for all versions, there are notable differences between them. Sub-circuits may be added, changed, or left out altogether. The specific differences are discussed below. For a more general discussion of the Privacy Set's block diagram, look here.

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

At the top is the transmission path. Compared to other manufacturers, the EMI-version has an extra audio amplifier between the modulator (the ring mixer shown in yellow) and the second low-pass filter. At the bottom is the reception path which is identical to the TMC-version. In addition, it has an improved 2500 Hz oscillator and a regulated power supply with extra stabilisation for the oscillator. This makes the device less sensitive for power variations.

 General description of Privacy Set No. 8
 About frequency inversion


Differences with TMC version
Although the Privacy Set No. 8 made by EMI in the 1970s, is interoperable with the 1962 version made by TMC, there are a number of manufacturing changes and changes to the circuit diagram. Below is a list of differences between the EMI version (EMT 72/1) and the TMC version (TE 62/1).

  • Simplified design
  • Secondary fork and bypass circuits eliminated
  • Additional amplifier in transmission path
  • Improved carrier oscillator
  • Regulated power supply units
  • Later generation Germanium transistors
  • Fork (hybrid) made of two transformers
  • Hinged PCBs (much more service-friendly)
  • 4 spare fused (held in rubber grommets)
  • Shielded mains transformer


Click to see more

Interior
The EMI version of Privacy Set No. 8 has the same outer dimensions as the TMC version. The interior can be accessed by removing the two large bolts at the top surface, after which the cover can be lifted off. The 10-way terminal block is at the same positions as in the TMC version.

Inside the device is a mains transformer – bolted to the bottom panel in the bottom left corner – and two hinged printed circuit boards (PCBs): a large one and a smaller one. After removing the screws at the upper edge, both PCBs can be folded down, as shown in the image above.

All transistors inside the device are Germanium PNP types with military markings, indicating that the devices were made to high standards. The small PCB holds the power circuits consisting of two rectifier bridges, capacitors and stabilising circuitry, built around four CV7352 transistors.
  
Power supply board

The CV7352 is the military variant of the 2N1303. The Power supply board is also responsible for providing the 15 mA current that is required for the carbon microphone in the handset of the connected telephone (voice terminal). This current is injected into the microphone wiring.

The large PCB, located at the other side of the chassis, holds the actual invertor. It is similar to the older TMC version, but is built with more modern and smaller components, as a result of which it looks simpler. The Germanium diodes — used in the ring mixers — and the (military) Germanium transistors, are of a later generation.

Several design changes were introduced over the years, indicated by markings like ISSUE 2 and ISSUE 3 on the PCBs. The most remarkable one is the addition of an improved oscillator circuit on a small PCB fitted on top of the existing board.
  
Low-pass filter

It provides a more stable 2500 Hz carrier signal with less distortion, that does not leak out to the rest of the device through the power rail. Incidently, this board also carries a simple one-stage audio amplifier that is inserted in the transmission path, via a short piece of white twisted wiring.

The twisted wires are visible in the bottom right corner of the image above, and are connected to the solder terminals of link LK1 on the invertor board. In an earlier design of the device, this link (which connects the modulator to the low-pass filter) was shorted. The image on the right shows the oscillator board as it is fitted on top of the invertor board. At the right is the single-stage amplifier that has an amplification factor of ~ 4.

If the signal level is too high, the amplifier can be bypassed by desoldering the two twisted wires and shorting LK1 on the invertor board.
  
Oscillator on a sub-PCB

The two large transformers close to the left edge of the invertor board, form the two halves of a fork circuit, or hybrid, in which the transmission path and the reception path are connected to the subscriber line. It also reduces the feedback of the microphone signal into the speaker (sidetone).

Privacy set No.8 from two different manufacturers
Interior (front view)
Interior of the Plessey version
Interior of the Plessey version - inverter board
Terminal block
Power supply board
Inverter board
Low-pass filter
Ring mixer
Oscillator on a sub-PCB
Mains transformer
Line transformer and fork (hybrid)
B
×
B
1 / 12
Privacy set No.8 from two different manufacturers
B
2 / 12
Interior (front view)
B
3 / 12
Interior of the Plessey version
B
4 / 12
Interior of the Plessey version - inverter board
B
5 / 12
Terminal block
B
6 / 12
Power supply board
B
7 / 12
Inverter board
B
8 / 12
Low-pass filter
B
9 / 12
Ring mixer
B
10 / 12
Oscillator on a sub-PCB
B
11 / 12
Mains transformer
B
12 / 12
Line transformer and fork (hybrid)

Circuit diagram
Below is the circuit diagram of the invertor board of the EMI version of Privacy Set No. 8, as taken down at Crypto Museum in July 2021 from the device with serial number 3476 and manu­facturing code EMT 72/1. According to markings on the PCBs, this was the third issue of the design. It is different from the TES-version (TMC), in that it has an improved oscillator and an extra amplifier.

At the right edge are the connections to the outside world – shown in purple – numbered in the order of the contacts of the terminal block. The same numbers are printed in white on the upper side of the PCB. Note that the device has the (+) side of the power supply connected to ground (chassis). If we define the chassis at 0V, this means that the unit is powered by a -12V voltage.

The signal from the handset's (carbon) microphone enters the circuit at the top right. Via a low-pass filter it is fed to the modulator, which consists of a ring mixer (D1-4) with a transformer at either side (T2, T3). The signal is mixed with a 2500 Hz carrier from the oscillator, resulting in upper and lower sidebands. It is then amplified in a single stage amplifier (V3) that is part of the oscillator board. The result is passed through the 2nd low-pass filter – so that only the lower sideband remains – amplified to line level (V1, V2) and fed to the upper half of the fork (hybrid).

EMI version - invertor board (with oscillator board fitted)

The hybrid consists of two transformers (T4, T9), each of which forms one half of the fork circuit. The primary windings of the two transformers are series connected to the subscriber line via terminals (1) and (2). At the secondary side are two windings, one of which is connected to the other transformer via a 600Ω resistor. The other winding is connected to the audio path.

At the bottom is the receiving path. The signal from the lower half of the fork is fed directly to the demodulator, consisting of a ring mixer (D5-8) and two transformers (T7, T8). The 2500 Hz signal from the oscillator is injected into the mixer via the centre tap of T8. This results in upper and lower sidebands. The signal is then passed through a 3-stage low-pass filter – so that only the lower sideband remains – amplified to speaker level (V4, V5) and delivered to the handset.

The oscillator (shown below) consists of an astable multivibrator, built around V11, V12, T5 and C16. It is an improvement over the original single-transistor design, which is probably why it is located on a separate PCB. It has its own power circuit (R13, D17) to make it immune to variations on the -12V power rail and a choke (L9) to prevent the 2500 Hz carrier signal from leaking out.

EMI version - oscillator board

Another improvement over the original design is the addition of a single-stage audio amplifier (around V3), which can optionally be inserted into the transmission path between the modulator and the 2nd low-pass filter. It is fitted on the same PCB as the oscillator, and is connected to the contacts of link LK1. Before the extra amplifier was present, the contacts of LK1 were shorted.

Below is the circuit diagram of the power supply unit (PSU). It consists of a mains transformer with 8 taps on the primary side, which makes the device suitable for a wide range of mains AC voltages. The transformer has two secondary 13.5V AC windings, that are rectified and regulated in two nearly identical circuits that do not share a common ground (i.e. they are fully isolated).

EMI version - power supply board

The lower circuit provides the -12V rail supply for the invertor board on pins (6) and (7). The 0V rail on pin (8) is connected to the chassis (ground). The -12V is protected with a 500 mA fuse that is fitted elsewhere on the chassis. The upper circuit is completely isolated and provides the 15 mA bias current for the carbon microphone of the handset of the connected telephone set.


Restoration
In January 2020, we received two Privacy Set No. 8 units with manufacturing code EMT 72/1, that were recovered in the 1970s from a telephone exchange in Northern Ireland shortly after it had been bombed by the IRA [2]. When we received the two devices, they were in an unknown state.

A first test, in which the device was connected to the local PABX with a known good Privacy Set at the other end, revealed that the reception path was working fine, but that the transmission path was severely muted. Speech could barely be heard through the handset at the other end.

After taking down the circuit diagram (above) it was discovered that the problem was caused by the single-stage amplifier around V3 on the oscillator board. This amplifier had been added to the device in a later iteration of the design, which is why it is not located on the main board.
  
Oscillator on a sub-PCB

Further investigation revealed that the 47µF capacitor (C15) at the base of V3 was fully dried out (measuring just 500 pF), as a result of which it did not pass the signal. This is the only electrolytic capacitor in the device that was made by a different manufacturer (ITT instead of Philips). After replacing C15 in both devices, the Privacy Sets worked like new, with no further adjustments required, which is remarkable for a nearly 50 year old device. The units were also successfully tested against a valve-based Frequency Changer 6AC, confirming their interoperability.

Oscillator on a sub-PCB
Replaced capacitor on the oscillator board
C
×
C
1 / 2
Oscillator on a sub-PCB
C
2 / 2
Replaced capacitor on the oscillator board

Connections
Terminal block
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 side. 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).

Voice terminal   SA 5030
Below is the internal wiring diagram of the SA 5030 voice terminal. At the bottom right are the (A) and (B) terminals of the subscriber line. Directly above it, is the wiring to the terminal block of the Privacy Set No. 8. The make-before-break (MBB) switches KA (1-4) are part of the 303/A Key Unit that is controlled by the 2 (or 3) push-buttons on top of the voice terminal. 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 Privacy Set.


The above circuit diagram is for the SA-50xx voice terminals of WWII, but is also applicable to the post-war modified GPO Telephone Set No. 710 and 740 units. During the war, the dial was often omitted from the telephone set — most exchanges were manually patched — but in the 1960s and 70s most exhanges were automatically switched. In addition, most wartime installations were of the Local Battery type (LB), whilst the post-war systems were generally Central Battery (CB).

Crypto Museum standard
To allow the Privacy Set 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 Privacy Set itself is fitted with a fixed 7 or 8-wire brown PVC cable with an XLR7/M connector at the end. This allows the Privacy Set to be disconnected from the setup without opening it and unscrewing the wires from its terminal block or from the BT 20/8 box. Below is the pinout of the XLR7/M connector that is fitted to the end of the fixed cable of the Privacy Set. The wiring order is identical to the order on the terminal block inside the predecessor: Frequency Changer 6AC.

  1. Line (B)
    green
  2. Line (A)
    black
  3. LB (or unused)
  4. Microphone (H)
    white
  5. Microphone (L)
    red
  6. Speaker (L)
    blue
  7. Speaker (H)
    orange
    7-pin male XLR plug as fitted to the fixed wiring of the Frequency Changer
Specifications
  • Type
    Voice scrambler
  • Principle
    Single frequency inversion
  • Manufacturer
    see below
  • Users
    see below
  • Carrier
    2500 Hz
  • Impedance
    Standard telephone line at 150, 300 or 600Ω
  • Terminal
    Modified conventional analogue telephone set
  • Dimensions
    255 × 155 × 90 mm (305 × 155 × 90 mm with mounting flanges)
  • Weight
    4000 grams (EMT) - 4500 grams (TMC)
Users
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:

Known serial numbers
 Other versions/serial numbers


Documentation
  1. Automatic Systems, Privacy Equipment, DEL & PABX Extension - wiring diagram
    TMC, 1964.
Datasheets
  1. 1N1303-1N1309 Germanium PNP transistor, datasheet
    New Jersey Semi-Conductor Products Ltd. 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. Richard P., Two Privacy Set No. 8 units - THANKS !
    EMT 72/1 version. Received January 2020.

  3. Trevor Cass, Personal correspondence
    Museum of Technology (Spalding, UK), July 2021.
Further information
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Crypto Museum. Created: Wednesday 26 May 2021. Last changed: Tuesday, 10 August 2021 - 18:44 CET.
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