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Scrambler
No. 8
  
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Privacy Set No. 8   TE 62/1
TMC version · 1962

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 World War II as the Frequency Changer. Unlike earlier models, it is suitable for CB/Auto systems only [1].

It is fully compatible with earlier models and was initially used in combination with the SA5030 voice terminal, that was based on a GPO 300-series telephone set. The latter was eventually succeeded by a modified Tele. No. 710 or 740.

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

This page describes specifically the version of Privacy Set No. 8 that was made in 1962 by the Telephone Manufacturing Company (TMC), as indicated by the marking TE 62/1 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 the first generation of OC71 Germanium transistors.

A unique feature of the TMC-version of this device is the presence of a secondary fork circuit, which allows an ordinary telephone set to be connected behind the unit, instead of a modified telephone set (voice terminal) or a bare handset. This is also known as 2-wire/2-wire operation. In addition it has a bypass circuit, which passes the signalling from the subscriber line directly to the connected telephone set and vice versa, without leaking plain voice to the line. The secondary fork and the bypass circuit are omitted from the Privacy Sets of some other manufacturers.

 Versions made by other manufacturers

Privacy Set No. 9A
Privacy Set No. 8 (TE 62/1) with removed cover
Transparent telephone in front of Privacy Set No. 8
Telephone No. 740 and Privacy Set No. 8
A
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A
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Privacy Set No. 9A
A
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Privacy Set No. 8 (TE 62/1) with removed cover
A
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Transparent telephone in front of Privacy Set No. 8
A
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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 Privacy Set No. 8. Click to see the original drawing [1].

At the right is a fork circuit that connects the subscriber line to the voice circuits of the Privacy Set. At the left is the handset with its microphone and speaker. A special feature of this version is the presence of an additional fork circuit, which is combined with the output transformer of the reception path. This 2nd fork can be used to connect a regular 2-wire telephone set directly to the Privacy Set, instead of the bare handset. This is known as 2-wire to 2-wire operation.

Block diagram of the TV-version of Privacy Set No.8., showing the bypass circuit and the secondary fork.

The block diagram above shows how this is done. At the right is the subscriber line, which is connected to the primary fork circuit. At the left is a regular telephone set which is connected to the secondary fork circuit. The bypass circuit ensures that the bell signal is delivered directly to the telephone set and that the dial pulses from the telephone set are passed directly to the line, whilst speech is processed exclusively by the voice circuits of the Privacy Set.

For this to work properly, it is necessary to simulate a regular subscriber line for the telephone set, which is why this version of the Privacy Set has two power supplies: 12V DC for the inverter circuits and 60V DC for the simulated line. The 60V DC power supply, which is actually a current source, also provides the current for operation of the carbon microphone of the handset. It is doubtful though whether this 2-wire/2-wire configuration was used in practice, as it would be difficult to sufficiently suppress feedback (sidetone) of the scrambled audio in the handset. The possibility for 2-wire/2-wire operation was dropped in later versions of the device.

 General description of Privacy Set No. 8
 About frequency inversion



Interior of Privacy Set No. 8, made by TMC in 196. Click to zoom in.

Interior
The TMC version of Privacy Set No. 8 is housed in a brown metal enclosure that measures 255 x 155 x 90 mm and weights 4520 grams. The device has no controls. The interior can be accessed by removing the two bolts at the centre of the short sides, after which the cover can be lifted off. Glued to the top of the frame is a connection diagram for configuration of a suitable telephone set that acts as the voice terminal. This diagram is discussed in detail further down this page.

Inside the device is a standard terminator block mounted in the upper left corner, a mains transformer bolted to the bottom and several supporting circuits mounted to the right side. Hidden behind all this is the actual inverting board which occupies the entire rear section.

The power supply unit can be separated from the device by removing six recessed screws from the bottom of the case, after which the bottom part can be extracted, as shown in the image above. It holds the mains transformer, diode rectifiers, smoothing capacitors and a large choke coil.
  
Mains transformer with voltage selection straps

On top of the transformer, protected by a removable black piece of carton, are the solder straps for selection of the desired mains voltage. By default, it was configured for 240V AC, the (then) mains voltage in the UK. This configuration is now also suitable for mainland Europe (230-240V).

The transformer has two secondary windings of 15V (for the inverter board) and 50V (for the microphone and secondary line current source) respectively. Rectifying diodes and smoothing capacitors are also present on the bottom panel, along with a choke coil for the -12V power rail.

At the right, mounted to the side panel of the chassis, are two choke coils plus a small inter­face board that contains supporting components for injecting a 15 mA current into the handset's carbon microphone. It is also used for the rarely used (optional) 2-wire/2-wire configuration.
  
Microphone and Line interface board

The board also contains two diodes and a number of resistors, that were omitted in later versions of the unit, probably because the 2-wire/2-wire option was no longer supported. At the bottom of this board are three large green 2µF/250V capacitors that are used for a variety of options.

The actual inverter board (the speech scrambler) is mounted behind the visible parts, in the rear section of the chassis. Getting access to its parts is cumbersome and requires the board to be removed from the chassis. This is achived by removing 4 recessed screws from the bottom edges of the chassis and another 4 at the top.

Note that one or more screws at the top may be obscured by the connection diagram. Once the screws have been removed, the inverter board can be freed from the chassis by pushing it to­wards the rear and folding it down as shown.
  
Inverter board removed from the chassis

The various sub circuits are identified in the diagram below. At the top is the transmission path (TX), starting with the input transformer at the right. The path consists of an 11dB attenuator, a low-pass filter, a modulator, another low-pass filter and finally a 2-stage amplifier. The output of the amplifier is supplied to the fork – also known as the hybrid transformer – at the left centre.

Click to see more

At the bottom is the reception path (RX). Starting at the fork, the signal passes a 6dB attenuator and is then applied to the demodulator. The output of the demodulator is taken through a 3-stage low-pass filter into a 2-stage amplifier and then delivered to the output transformer at the bottom right. In early version of the Privicy Set (such as the one shown here) this transformer can be configured as a secondary fork for 2-wire/2-wire configuration, although this was rarely used.

At the center is the 2500 Hz carrier oscillator, which consists of a single transistor (V3) a tuning coil (L22) and two precision capacitors (C25 and C25a). The output of this oscillator is injected directly into the ring mixers of the modulator (transmission) and demodulator (reception).

Privacy Set No. 9A
Privacy Set No. 8 (TE 62/1) with removed cover
Terminal block
Wiring diagram
Privacy Set No. 8 - TMC version - interior
Mains transformer with voltage selection straps
Mains voltage selector
Lift the diagram to find the extra screw
Inverter board removed from the chassis
Inverter board removed from the chassis
Inverter board top view
Primary fork unit (hybrid)
Input transformer (from microphone) and attenuator (CA5)
3-stage low-pass filter
Modulator (balanced ring mixer)
Oscillator
Output ransformer (to speaker)
Microphone and Line interface board
Three large capacitors at the underside of the bypass circuit
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B
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Privacy Set No. 9A
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Privacy Set No. 8 (TE 62/1) with removed cover
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Terminal block
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Wiring diagram
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Privacy Set No. 8 - TMC version - interior
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Mains transformer with voltage selection straps
B
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Mains voltage selector
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Lift the diagram to find the extra screw
B
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Inverter board removed from the chassis
B
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Inverter board removed from the chassis
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Inverter board top view
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Primary fork unit (hybrid)
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Input transformer (from microphone) and attenuator (CA5)
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3-stage low-pass filter
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Modulator (balanced ring mixer)
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Oscillator
B
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Output ransformer (to speaker)
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Microphone and Line interface board
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Three large capacitors at the underside of the bypass circuit

Circuit diagram
Inverting unit
Below is the circuit diagram of the inverter board of the TMC version of Privacy Set No. 8 as taken down at Crypto Museum in July 2021 from the device with serial number 1033 and manufacturing code TE 62/1. At the right edge are the connections to the outside world, as they are numbered on the PCB. 1 Note that this device has the (+) side of the power supply connected to the chassis (ground). If we define the chassis at 0V, this means that the unit is powered by -12V DC.

At the top right is the microphone input. The transformer (T1) is suitable for various impedances. From T1, the signal is fed through an 11dB attenuator (A1) followed by a low-pass filter, before it is applied to the balanced ring mixer around T2, T3 and D1-4. Here the signal is mixed with the 2500 Hz carrier from the oscillator around V3, producing upper and lower sideband products.

The lower sideband is the mirrored image of the original voice spectrum. The signal now passes a second low-pass filter — so that only the lower sideband remains — and is amplified in a two-stage amplifier (V1, V2), before it is supplied to the subscriber line via the fork circuit (hybrid) T4.

TMC version - inverter board

At the bottom is the receiver circuit. From the fork (T4), the signal passes a 6dB attenuator (A2) and is then applied directly to the demodulator, which consists of a balanced ring mixer around T5, T6 and D5-8. Like in the transmission path, this produces upper and lower sidebands. The result is fed through a low-pass filter, so that only the lower sideband remains, and amplified in a two-stage amplifier (V4, V5), before it is supplied to the handset's speaker, via transformer T7.

Note that T7 is a copy of the fork (T4) and has a double function. In the basic configuration (shown here) it acts as the output transformer between the reception circuit and the speaker. In case of 2-wire/2-wire operation however, it can be configured as a 2nd fork circuit, allowing a standard telephone set to be used instead of a bare handset. In practice, this option was hardly ever used, as it was difficult to sufficiently suppress feedback (sidetone) of the scrambled audio.

  1. When taking down the circuit diagram of this device, it was not possible to determine the exact layout and wiring of the hybrid transformer (T4) without damaging the unit. The wiring of the hybrid (fork) is therefore an educated guess, based on the design of the Frequency Changer 6AC. There are several configuration straps to adapt the unit for 150, 300 or 600Ω impedances. It is shown here in the 600Ω configuration.

Power supply unit
Below is the circuit diagram of the power supply unit, which consists of two simple rectifiers with stabilisation circuits, but without a transistor-based regulator. As a result, the voltage on the -12V rail may vary slightly, resulting in a less stable 2500 Hz carrier signal from the oscillator.

TMC version - powre supply unit

The mains transformer (T1P) can be configured for virtually any mains AC voltage, by means of solder straps at its top. T1P has two secondary windings: 15V AC for the -12V DC supply of the inverter board, and 50V AC for the 15 mA current source that provides the bias for the carbon microphone of the connected handset, and the line current in a 2-wire/2-wire configuration.

Note that the design of neither PSU circuit is very sophisticated, as a result of which a rather strong 50 Hz hum is present in the transmitted signal. This is inherent to the design and was fixed in later versions like the Privacy Set 9A. The problem can be fixed in this model, by adding a 22µF/200V capacitor across the output terminals of the microphone current source. In the circuit diagram above, this capacitor is shown in grey (see the description in the Restoration section).


Restoration
When we obtained the Privacy Set No. 8 featured here, in September 2018, it was in working condition. As time progressed however, the audio quality gradually deteriorated, suggesting problems with the electrolytic capacitors. Furthermore, a previous owner had fitted a 6-pin Japanese connector in the space that was originally intended for the strain relief of a fixed cable.

This was probably done as the original wiring was lost, and to allow for quick (dis)connection. In order to bring it back to its original state, the aftermarket connector was removed along with its wiring. It was replaced by original GPO wiring, but due to the fact that the cut-out had been enlarged to accomodate the connector, we had to add the improvised bracket shown here. The bracket is completely invisible from the outside.

At the other end of the cable a 7-pin XLR plug was fitted, wired per Crypto Museum Standard, so that the device can be used in various setups.
  
Restored cable inlet with strain relief

All electrolytic capacitors were swapped for new ones, using the orange jackets of the old ones as camouflage. In addition the four 1µF foil capacitors on the inversion board were swapped for new ones as well, as some of them had attracted moisture. After reassembling the device, the audio quality was much better, but the 50 Hz hum – that was noticable before – was still present.

Further investigation revealed that it was caused by the poor design of the 60V DC power supply unit, that delivers the 15 mA current for the carbon microphone of the handset. Apparently this was a known misfeature, as it was fixed in later models by adding a 10µF/110V capacitor.

As the hum seriously affected the performance of the scrambler, we decided to add a 22µF capacitor to the circuit, by soldering it across terminals 4 and 6 of the connection board on top of the choke coil (L1P). With the capacitor installed, the transmitter is completely clean.
  
PSU with new capacitors

The device is now back in its original state and can be used for demonstrations. It was tested successfully against the wartime Frequency Changer 6AC, to ensure that it is interoperable. The audio quality is excellent and the levels are more than adequate. Nevertheless, manufacturer EMI later added an extra amplifier in the transmission path in its 1972 design of Privacy Set No. 8.

Problems
Restored
Modification
This version of Privacy Set No. 8 exhibits a strong 50 Hz hum in the transmission path, caused by the poor design of the 60V DC power supply. This PSU is responsible for the 15 mA DC current that is required for proper operation of the carbon microphone in the handset of the telephone.

This problem can be fixed by adding a 22µF/ 200V capacitor across the output terminals of the 60V supply, and is best placed between terminals 4 and 6 of the connection block of the PSU, located on top of the choke coil (L1P).

The diagram on the right shows the layout of the connection block and the orientation of the capactor. It has the (+) contact on terminal 4.
  

In later versions of the device, such as the 1964 Privacy Set No. 9A from the same manufacturer, this capacitor is already present at the given location. Privacy Sets from other manufacturers, such as the 1972 version from EMI, dit not exhibit this problem as they had a regulated PSU.

Privacy Set No 8
Privacy Set No. 8 with cover removed
Junction box
Aftermarket connector
Restored cable inlet with strain relief
New, refurbished and old capacitor
Extra capacitor fitted to the solder terminals
PSU with new capacitors
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C
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Privacy Set No 8
C
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Privacy Set No. 8 with cover removed
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Junction box
C
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Aftermarket connector
C
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Restored cable inlet with strain relief
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New, refurbished and old capacitor
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Extra capacitor fitted to the solder terminals
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PSU with new capacitors

Connections
Terminal block
The table below gives the pinout of the screw terminal block inside Privacy Set 8, 8A, 9 and 9A. This is the lowest row of screws when looking into the device as shown below. The first column shows the colours, whilst the second one specifies the contact number inside the BT20/8 box.

  1. Green
    BT3
    Line B 1,2
  2. Black
    BT6
    Line A 1,2
  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 and similar voice terminals. 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 Handset No. 164 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
    TMC
  • Batch code
    TE 62/1
  • Product code
    E526539/1A
  • Carrier
    2500 Hz
  • Impedance
    Standard telephone line at 150, 300 or 600Ω
  • Terminal
    Modified conventional analogue telephone set or bare handset
  • Dimensions
    255 × 155 × 90 mm (305 × 155 × 90 mm including mounting flanges)
  • Weight
    4520 grams
Documentation
  1. Automatic Systems, Privacy Equipment, DEL & PABX Extension - wiring diagram
    TMC, 1964.
Datasheets
  1. OC71 Germanium PNP transistor, datasheet
    Valvo et al. 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.
Further information
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Crypto Museum. Created: Wednesday 26 May 2021. Last changed: Tuesday, 10 August 2021 - 18:39 CET.
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