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Privacy Set No. 9A   TES 64/1
TMC version · 1964 - under construction

Privacy Set No. 9A, also known as Secraphone No. 9A, 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. 9A was in production from 1964 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. 9A with cover removed

This page describes specifically the version of Privacy Set No. 9A that was made in 1964 by the Telephone Manufacturing Company (TMC), marked at the bottom of the unit with the batch code TES 64/1 and production code E526539/3 . The circuit is comparable to that of Privacy Set No. 8 from the same manufacturer. It is made with the first generation of OC71 Germanium transistors.

The device is an improved version of Privacy Set No. 8 with better low-pass filter characteristics and a a notch filter in the reception path. Furthermore, the power supply unit (PSU) has an extra capacitor to reduce 50 Hz hum in the transmission path. The microphone interface board (CD1) is simplified and no longer allows the bias current for the carbon microphone to be taken from the subscriber line. Although the output transformer in the reception path can still be configured as a secondary fork — used for 2-wire/2-wire operation — this is no longer recommended.

This particular production variant is incompatible with the other models, as it uses a 2000 Hz carrier frequency instead of the more common 2500 Hz.

 Versions made by other manufacturers

Privacy Set No. 8 (TE 62/1) with removed cover
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
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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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Privacy Set No. 9A with SA5030 voice terminal
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 description of the operation of the Privacy Set, look here.

Block diagram of Privacy Set No. 8. Click to see the original drawing [1].

With this version it is still possible to configure the output transformer in the reception path as a secondary fork (for connecting a regular telephone set instead of a bare handset), but its use is no longer recommended. In addition, the extra components for extracting the bias current for the carbon microphone from the subscribers line, have been removed. The device has two power supplies: 12V DC for the inverter circuits and 60V DC (or actually a 15 mA current source) for providing the bias current for the carbon microphone in the telephone's handset.

 General description of Privacy Set No. 8
 About frequency inversion


Differences with Privacy Set No. 8
Privacy Set No. 9A is nearly identical to Privacy Set No. 8 from the same manufacturer (TMC), but has a number of manufacturing changes and changes in the circuit diagram. Below is a list of differences between Privacy Set No. 9A (TES 64/1) and the earlier Privacy Set No. 8 (TE 62/1).

  • Improved mains voltage selector (screw terminals instead of solder straps)
  • Improved low-pass filters
  • Extra notch filter in speaker line (CD2 board)
  • 4 spare fuses stowed in the chassis
  • 2-wire/2-wire option no longer recommended
  • Simplified microphone/line interface board (CD1)
  • This production variant: 2000 Hz carrier instead of 2500 Hz


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

Interior
As the fully transistorised Privacy Sets 8, 8A, 9 and 9A are all nearly identical, we will restrict the description of the 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 mounted to the case. The image on the right shows the main PCB after it has been dismounted from the case.

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 the image to reveal the locations of the various blocks.
  
Scrambler board (component side)

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

Circuit diagram
Below is the circuit diagram of the inverter board of the TMC version of Privacy Set No. 9A as taken down at Crypto Museum in July 2021 from the device with serial number 1086, batch code TE 64/1 and production code E526539/3. At the right edge are the connections to the outside world, numbered as on the PCB. 1 Note that this device has the (+) side of the power supply connected to ground. If we define ground as 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 fixed carrier from the oscillator around V3, producing upper and lower sideband signals.

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, 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 signal is then 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.

  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 carrier signal from the oscillator.

TMC version - powre supply unit

The mains transformer (T8) can be configured for virtually any mains AC voltage, by means of solder straps ast its top. T8 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.


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 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
    TES 64/1
  • Product code
    E526539/3
  • Carrier
    2000 Hz 1
  • 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
    4520 grams
  1. Note that this praticular production variant of Privacy Set No. 9A, uses an inversion frequency of 2000 Hz rather than the more common 2500 Hz. As a result, it is incompatible with the 2500 Hz variants.

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 - 18:42 CET.
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