← No. 7
Privacy Set No. 8
The device is fully compatible with the
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 .
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.
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) .
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,
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
➤ 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
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.
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.
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.
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).
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).
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.
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).
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.
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 . 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.
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
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.
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.
BT3Line B 1
BT6Line A 2
unusedAudio in 2
unusedAudio in 2
Loopwired to 10
Loopwired to 9
In 2-wire configuration the line is connected here.
In 4-wire configuration, this is the Audio out line.
Used in 4-wire configuration (e.g. when connected to a radio).
Lines (6) and (7) are joined in the connection box (at point BT1).
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
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).
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
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.
- LB (or unused)
PrincipleSingle frequency inversion
ImpedanceStandard telephone line at 150, 300 or 600Ω
TerminalModified conventional analogue telephone set
Dimensions255 × 155 × 90 mm (305 × 155 × 90 mm with mounting flanges)
Weight4000 grams (EMT) - 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:
Any links shown in red are currently unavailable.
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© Crypto Museum. Created: Wednesday 26 May 2021. Last changed: Tuesday, 10 August 2021 - 18:44 CET.