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Frequency Changer 6AC/3
Secraphone telephone scrambler · 1944-1957

Frequency Changer 6AC/3, also known as Secraphone 6AC/3 or Privacy Set, was a valve-based full-duplex telephone scrambler, developed during World War II (WWII) by the General Post Office (GPO) at Dollis Hill (London, UK). Based on the inversion of the voice spectrum, it was introduced in January 1944 as the successor to the 6, 6A, 6B, 6AA/0 and 6AA/1 models. It belongs to the family of telephone scramblers that were once used by British Prime Minister Winston Churchill.

The system is housed in a metal enclosure in black wrinkle-paint finish, that measures 585 x 360 x 185 mm and weights 41 kg. At the top is a hinged lid that gives access to the connections, the valves and the vibrator pack. The remaining building blocks are in the lower part of the case.

The device was usually installed under a desk or in an adjacent room, and was connected to a converted telephone set – the voice terminal – that was placed on the user's desk. In most cases, the voice terminal had push-buttons to switch between normal and scrambled speech.
  
Frequency Changer No. 6AC/3 with SA-5063/1 voice terminal

The device uses the principle of inversion of the audio spectrum, also known as frequency domain speech scrambling, or speech mirroring. High tones become low tones and vice versa, resulting in unintelligible speech. All the person at the other end has to do, is invert the spectrum once more, to make it intelligible again. Although this method is not very secure — it is not a real encryption device — it was believed to be good enough to protect a conversation against a casual eavesdropper, such as the exchange operator or a telephone engineer working on the lines.

At least 3 variants of the 6AC were made, identified by a suffix to the model number. The one featured here is variant '/3'. The device was introduced in January 1944, and was in production until 1957, after which it was succeeded by Privacy Set 7 and eventually, in 1962, by the fully transistorised Privacy Set 8. The 6AC/3 unit featured here, was manufactured by the Telephone Manufacturing Company (TMC) in London (UK), and was marketed under the name Secraphone.

 Overview of the various models

Frequency Changer No. 6AC/3 with SA-5063/1 voice terminal
Frequency Changer No. 6AC/3 with SA-5063/1 voice terminal
Frequency Changer No. 6AC/3 with open lid
Frequency Changer No. 6AC/3 with SA-5063/1 voice terminal
Interconnection cable
Power receptacle at the top right (only accessible when the lid is closed)
Mains power cable connected
Model and serial number tag on top of the lid
A
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Frequency Changer No. 6AC/3 with SA-5063/1 voice terminal
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Frequency Changer No. 6AC/3 with SA-5063/1 voice terminal
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Frequency Changer No. 6AC/3 with open lid
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Frequency Changer No. 6AC/3 with SA-5063/1 voice terminal
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Interconnection cable
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Power receptacle at the top right (only accessible when the lid is closed)
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Mains power cable connected
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Model and serial number tag on top of the lid

Features
The diagram below provides a quick overview of the features of the Frequency Changer 6AC/3. The device is shown here with open lid and a wartime SA 5063/1 voice terminal placed in front of it. At the upper edge of the right side panel is the mains power receptacle. The mains plug has to be removed before the lid can be opened, to ensure that the user can not touch lethal voltages. The desired mains voltage is selected with a wire strap at the far right. In addition, there are five configuration straps for selection between operation from the mains (M) or a 12V battery (B).

Click to see more

Setup
The wiring to the subscriber line and the voice terminal is fed into the device via a circular hole in the left side panel, and connected to the terminal block at the left. In the simplest configuration, the Frequency Changer is connected directly to the subscriber line, in parallel with a regular tele­phone set with dial, that is used to initiate a call. A simplified telephone set 1 is connected to the voice circuits of the Frequency Changer. It is only used for its handset (microphone and speaker).

Simple configuration

Once the call has been established by means of the regular telephone set, both parties lift the handset of the simple terminal, and put the handset of the regular telephone set on-hook. They can now continue their conversation in secure (scrambled) mode. At the end of the conversation, both parties put the handset of their simple terminal on-hook to disconnect the subscriber line.

In most cases however, a more advanced setp was used, in which an SA-50xx voice terminal is connected to the subscriber line, in parallel to a regular telephone set with a dial that is used for initiating the call. The voice terminal has two buttons, marked SECRET and NORMAL (or similar), so that it can be used for both clear and secure calls. The SA-5063/1 is a good example of this.

Advanced configuration with wartime voice terminal

In clear mode, the terminal can accept incoming calls from the regular telephone set. When switching to secure, the subscriber line is routed via the Frequency Changer, whilst the micro­phone and speaker of the terminal's handset are connected directly to the voice circuits of the Frequency Changer. This routing is controlled by the switch block inside the voice terminal.

When the Frequency Changer was used on an automatically switched network — which was rarely the case during WWII — it was also possible to use a voice terminal that was fitted with a dial, and connect it directly to the subscriber line. The telephone is then used in clear-mode to initiate the call, after which both parties switch to secure. A good example on this website is the SA-5030.

Using a voice terminal with dial

In practice, the Frequency changer and its voice terminal were connected as shown in the diagram below. A junction box, such as the Block Terminal (BT) No. 20/8 or BT No. 6, was used to connect all parts together. The subscriber line is routed to the SA 50xx voice terminal, and the 303/A Key Unit inside the SA 50xx determines whether the line is connected directly to the telephone or via Frequency Changer. In the latter case, the Key Unit disconnects the handset from the telephone and connects its microphone and speaker directly to the voice circuits of the Frequency Changer.


Note that the carbon microphone of the handset is responsible for keeping the subscriber line engaged. When it is connected to the Frequency Changer, the latter 'steals' the DC voltage — required for a proper operation of a carbon microphone — from the subscriber line. This load is enough to signal to the exchange that the line is 'in use'.

  1. Unmarked simplified telephone set, based on the design of Telephone No. 162 or No. 232.

Parts
Frequency Changer No. 6AC/3
Simple terminal (used only for its handset)
Voice terminal (converted telephone set)
AC Mains power cable
AC
Spare parts
Handbook H512/6
Frequency Changer   6AC/3
The bare Frequency Changer (i.e. the actual voice scrambler) is housed in a metal enclosure in black wrinkle paint finish that measures 585 x 360 x 185 m and weights no less than 41 kg.

The unit would normally be installed in a cup­board, under a desk, or in an adjacent room. Once installed, there is no need to have quick access to it, as it has no controls. It is powered from the AC mains or from a 12V battery. At the top is a hinged lid that provides access to the serviceable parts (valves, vibrator, fuses, etc.).

  
Frequency Changer No. 6AC/3 with open lid

Simple terminal   No. 162
In some cases, the unit was supplied with a simple voice terminal, that was based on GPO Telephone No. 162 or 232. The microphone and speaker element of its handset are connected directly to the voice circuits of the scrambler.

In this configuration, the scrambler is connected directly to the subscriber line, in parallel with a regular telephone set that was used to initiate calls. Once the call is established, the 162-unit is used for the secure conversation.

 More information

  
Voice terminal with handset and wiring

Voice terminal   SA 50xx
Instead of the basic voice terminal shown above, Frequency Changer 6AC could also be used in combination with a special (modified) telephone set, such as the SA 5063/1 shown on the right.

This was especially the case during WWII, and allowed the user to switch between clear and scrambled voice calls, using push-buttons on top of the telephone set. These phones had a green handset to show that it was a 'secure' line.

 More information

  
SA 5063/1 voice terminal

Mains power cable
The Frequency Changer 6AC/3 can be powered directly from the AC mains network in most countries. Once the desired voltage has been selected and the five configuration straps are set to 'M', power can be applied to the 3-pin Bulgin receptacle at the top right. The required plug was delivered with the set.  Layout

The image on the right shows a mains power cable that is suitable for the AC mains network in continental Europe.

  
Power cable with Bulgin 3-pin female plug

Spare parts
Depending on the circumstances, each device came with a selection of spare parts, such as spare valves, a spare vibrator pack, light bulbs, power connectors and fuse wire.

It allowed technicians to carry out simple repairs in the field. The image on the right shows a selection of spare parts that were found with the device featured here.
  
Spare valves and vibrators

Handbook   H512/6
In addition to the spare parts shown above, the devices featured here also came with full service documentation, comprising technical details and descriptions of the various building blocks.

The handbook even contains the complete circuit diagram of the three variants or groups: 6AC/1, 6AC/2 and 6AC/3 [A].

 Download the handbook
 View the circuit diagram

  
Handbook No. H512/6 for Frequency Changer 6AC/3

Voice terminal packed in original carton box
Voice terminal with handset and wiring
Voice terminal
Power cable with Bulgin 3-pin female plug
Carton box with spare parts
Spare valves and vibrators
Spare vibrator
Handbook wrapped in greased paper
Handbook No. H512/6 for Frequency Changer 6AC/3
Handbook title page
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Voice terminal packed in original carton box
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Voice terminal with handset and wiring
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Voice terminal
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Power cable with Bulgin 3-pin female plug
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Carton box with spare parts
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Spare valves and vibrators
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Spare vibrator
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Handbook wrapped in greased paper
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Handbook No. H512/6 for Frequency Changer 6AC/3
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Handbook title page

Block diagram
Below is the block diagram of Frequency Changer 6AC/3. At the far right is the 2-wire subscriber line, which is usually connected in parallel to a regular telephone set with a dial. At the far left is the handset of the voice terminal. The audio signal from the microphone is applied directly to a ring mixer (UD), where it is added to the 2500 Hz signal from the central oscillator (VA).

At the output of the mixer, the sum and the difference of the two signals is 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 low-pass filtering it (UC), only the mirrored signal remains, which is then amplified (VB) and delivered to the line.


The bottom half of the diagram shows the reception path, which is more or less the same, but in reverse direction. The mixer (UA) produces two images, of which the lower one is the mirrored version of the received signal. After amplification (VC), it is passed through a low-pass filter (UB), so that only the lower part remains (blue). This is a copy of the original audio signal. The spectrum diagrams illustrate what happens. The inverted (scrambled) audio is shown in red. Note that this only works, of course, when both parties use the same inversion frequency of 2500 Hz.

It is also important that the two low-pass filters (UB and UC) are of a higher order and have a sharp cut-off. This means that the baseband (everything up to the crossover frequency) is passed unattenuated, and that everything above its crossover frequency is sufficiently suppressed. Together with the ring mixers, the low-pass filters are responsible for the overall audio quality.

Although the Frequency Changer, or frequency inverter, offered reasonable protection against an casual (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 classic case of security by obscurity.

 Check out the original block diagram



Click to see more

Interior
Frequency Changer 6AC/3 is housed in a heavy (41 kg) metal enclosure. The interior consists of two sections: the upper 1/3rd that contains the user-serviceable parts, and the lower 2/3rd with the parts that cannot be serviced by the user. The upper section is accessed via the hinged lid.

Note that the lid can only be opened after the mains cable has been removed. Inside the upper compartment are the three CV1052 (EL32) valves (tubes) of the active circuits: the receive and transmit amplifiers, and the 2500 Hz oscillator.

At the far left is a contact block with seven screw terminals, to which the subscriber line and the voice circuits of the handset are connected. At the far right is the power receptacle, which can be configured for the 100-250V AC mains with two wire links. It can also be configured for 12V DC battery use, by altering five metal straps.
  
Three valves (tubes)

Each strap has two possible positions: 'M' for mains and 'B' for battery. Below the straps are four fuses: two (1/5A) for the primary and two (3A) for the secondary power circuit. The fuses consist of a bakelite base with a removable insert that contains a bare fuse wire, which can be replaced.

At the centre is a circular vibrator unit that is used when the device is configured for use with a 12V DC power source. It converts the DC to an AC voltage, which is then transformed to the required HT voltage for the anodes of the valves.

The vibrator unit is installed in a metal enclosure that contains additional filter parts, to prevent harmonics of the vibrator's switching frequency from leaking into the Frequency Changer's audio circuits. A broken vibrator can easily be replaced by the user. To the left of the vibrator, close to the edge of the base panel, is the power lamp.
  
Vibrator pack

When the device is closed, the power lamp is visible through a red lens in the front side of the top lid. It is lit when the unit is powered. The lower section contains the parts that should only be serviced by a qualified enigineer. It is covered by a metal plate that is held in place by two screws.

After loosening these screws (in the upper left and right corners), the front panel can be lifted out, and the remaining circuits are exposed.

The four large grey building blocks are the low-pass filters and the two ring mixers; one for the transmitting side and one for the receiving side. They are shown in the image on the right. The mixers are fitted with potentiometers, that allow adjustment of the balance (i.e. suppress the carrier frequency). Surrounding these four units, are the smaller building blocks, such as the audio transformers and the power circuitry.
  
Filters and mixer blocks

The mains transformer is located in the upper section. It is hidden behind the fuse panel. As the mains receptacle (at the top right) can not be used when the case is open — it is blocked by the top lid — an extra mains receptacle is present for a service engineer. It is located at the far right of the lower section, and is wired in parallel to the regular receptacle. Be careful when using it.

 Check out the circuit diagram

Interior - front view
Terminal block
Three valves (tubes)
Vibrator pack
Vibrator, fuses and power configuration straps
Fuses and power configuration straps
Mains socket (blocked when lid is open)
Filters and mixer blocks
Power circuits
Internal mains socket
Mains power cable connected to the internal receptacle
Mains power cable connected to the internal receptacle
Mains power cable connected to the internal receptacle
Internal serial number on the horizontal mounting panel
C
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Interior - front view
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Terminal block
C
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Three valves (tubes)
C
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Vibrator pack
C
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Vibrator, fuses and power configuration straps
C
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Fuses and power configuration straps
C
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Mains socket (blocked when lid is open)
C
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Filters and mixer blocks
C
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Power circuits
C
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Internal mains socket
C
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Mains power cable connected to the internal receptacle
C
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Mains power cable connected to the internal receptacle
C
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Mains power cable connected to the internal receptacle
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Internal serial number on the horizontal mounting panel

Building blocks
  • UA
    Ring mixer (receiver)
  • UB
    2350 Hz low-pass filter (receiver)
  • UC
    2400 Hz low-pass filter (transmitter)
  • UD
    Ring mixer (transmitter)
  • UE
    Oscillator tuned circuit (adjusted at 2500 Hz)
  • UF
    Oscillator output transformer
  • UH
    Vibrator circuit
  • VA
    Oscillator valve CV1052 (carrier)
  • VB
    Amplifier valve CV1502 (transmitter)
  • VC
    Amplifier valve CV1502 (receiver)
  • XA
    Line transformer
  • XB
    Audio transfomer (input of VB)
  • XC
    Audio transformer (output of VC)
  • XD
    Audio transformer (input of VC)
  • XH
    Mains transformer
Restoration
For many years we had been looking for the war-time version of the Frequency Changer, the device that Churchill used for private conversations. We had found the post-war transistorised version, but older valve-based variants appeared to be unobtainable. Very few were made and even fewer have survived. But in May 2021 our patience was rewarded, when we were offered two complete 6AC/3 units in mint condition, that had been found in an attic in Brussels (Belgium) [1].

First the primary side of the mains transformer was checked. It has a 53Ω DC resistance, which seems about right. On the contacts of the mains receptacle however – which are connected to the transformer – the resistance was infinite.

It turned out that all four fuses in each of the scramblers were broken. This was not caused by a short circuit in the devices, but by galvanic corrosion caused by the potential difference that exists when two different metals are in electric contact with each other. Over time, this had caused the thin fuse wire to corrode and break.
  
Fulse holder with corroded fuse wire

Unlike modern fuses, that can be replaced easily, the fuses of the 6AC/3 contain a short piece of fuse wire that is held in place by two screws. This allows the fuse to be rewired when it is broken. But as bare fuse wire is something of the distant past, it may be difficult to find a suitable supply.

No power cables were supplied with the devices, but fortunately the 3-pin Bulgin plugs were stowed inside the units, so mains power cords could easily be made. The settings of the five configuration straps were checked – all five must be set to 'M' (mains) – and the wire straps of the voltage selector at the right were configured for an AC mains voltage of 250V to avoid damage. 1

In the meantime, a Google search had revealed that fuse wire was still available from a variety of sources in the Far-East, and for very reasonable prices too. We ordered spools with 2A and 3A. 2
  
Power cable with Bulgin 3-pin female plug

When it arrived several weeks later, the four fuses of each device were rewired, and mains power was gradually applied to the two devices by means of a VARIAC. During this process – that is necessary to reform the capacitors – a thermal camera was used to confirm that no parts were overheating. When we were certain that this was not the case, the voltage was increased to 230V.

At this point, a faint 2500 Hz tone started to emanate from the devices, which indicated that the main oscillators were working. Next it was time to hook the devices up to telephone sets and an exchange, so that they could be tested.

As we want the setup to look as authentic as possible, we made a seven-wire braided cord (shown in the image on the right) and connected one end to the terminal block of the Frequency Changer. The other end was fitted with a 7-pin XLR plug, 3 so that it could easily be connected to the telephone line and to the voice terminal.
  
Interconnection cable

As the configuration of the terminal block inside the 6AC/3 is slightly different from the 6AC/1 and 6AC/2, the internal wiring of this block was modified so that it is now compatible with the other models. This involves the removal of the orange wire from terminal (6) and connecting it in parallel with the orange wire at terminal (4). It provides the 3V DC for the carbon microphone. 4

Next, the SA-5030 telephone set from our collection was internally rewired as per wartime instructions, and fitted with an 8-wire braided cord that terminates in a BT-20/8 junction box.

This box is also fitted with a braided line cord, and with a 7-wire braided cord for connection of the Frequency changer. The latter terminates in a 7-pin XLR female connector that mates with the male connector of the Frequency Changer. This way, the Frequency Changer and the telephone set can easily be connected and disconnected. The wiring is shown in the image on the right.
  
SA5030 with 8-wire braided cord and BT-20/8 terminal box

After connecting the Frequency Changer to the wiring of the SA-5030 (via the XLR connectors), the line cord was connected to the local PABX for a first test. The SA-5030 can make outgoing calls and accept incoming calls, whilst a regular POTS telephone set was used at the other end.

In normal mode — the default after initiating a call — speech in both directions was loud and clear, indicating that the SA-5030 was wired up correctly. After switching to scrambled mode — pressing the SCRAMBLE button on the SA-5030 — speech became unintelligible as expected. High tones became low ones and vice versa.

Next, the other one was wired up and connected to a suitable telephone. It was tested against the regular telephone set, but there was a problem. There was a strong residual 2500 Hz tone on the line, even when it was not in scrambled mode.
  
6AC/3 on the workbench

When tried in scrambled mode, both clear and scrambled speech was heard at the same time. Something was clearly wrong, so the device was put on the workbench for further investigation. After studying the circuit diagram and doing some tests, it became clear that the transmission path was fine, but that the ring mixer in the reception path – the demodulator – had a fault.

One half of the mixer didn't work at all, causing an imbalance of the two transformers, resulting in all kinds of side-effects, such as injecting the 2500 Hz oscillator signal back into the line.

After removing the demodulator (block UA) from the device, it was extracted from its enclosure, as shown in the image on the right. Although the demodulator is well-constructed, this particular unit was probably built on a friday afternoon or a monday morning, as none of its screws had been tightened properly. Because of this, both trans­formers could be moved feely by half a cm.
  
Demodulator (UA) interor - right side

In addition to this, the tightly bundled wiring is extremely stiff, and offers little support for movement. As a result of this, two wires that were soldered to the primary transformer (UAE) were broken. After refitting the wires and tightening all screws, the unit was reassembled and externally connected to the device. This time it worked as expected. After mounting it back in place, both 6AC/3 units were completely re-aligned as per service manual and tested – with the original wartime telephone sets – on the local PABX. They now work flawlessly in either direction.

The units produce a clear voice signal, both in clear and in scrambled mode, with an excellent intelligibility. It is truly amazing that all electronic parts – even the capacitors – have all survived.

  1. Although the official mains voltage in Europe in 230V, it is often much higher than that. The mains voltage at our location is 241V and sometimes even higher.
  2. Ordered from AliExpress. 1.5A fuse wire was not available, so we choose 2A wire instead.
  3. Although this is a modern connector, it was choosen for the quality of its contacts, as well as its dimensions. The XLR-7 plug fits through the cable hole at the left side of the Frequency Changer.
  4. The original wiring of the 6AC/3 terminal block is really weird and seems to be illogical. By modifying it as described, the DC voltage for the handset's carbon microphone is taken from the subscriber line.

Cloth silica gel bag, made by Silica Gel Ltd., London
Fulse holder with corroded fuse wire
Power cable with Bulgin 3-pin female plug
Interconnection cable
Terminal block
Interconnection cable
SA5030 with 8-wire braided cord and BT-20/8 terminal box
Wiring inside the BT-20/8 terminal box
Demodulator removed from its enclosure
Demodulator (UA) interior - left side
Interior left side, seen from the rear
Demodulator (UA) interor - right side
Demodulator (UA) interor - right side, seen from the rear
Hybrid (fork) and attenuator
Double transformer (part of the ring mixer)
6AC/3 on the workbench
D
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D
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Cloth silica gel bag, made by Silica Gel Ltd., London
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Fulse holder with corroded fuse wire
D
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Power cable with Bulgin 3-pin female plug
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Interconnection cable
D
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Terminal block
D
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Interconnection cable
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SA5030 with 8-wire braided cord and BT-20/8 terminal box
D
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Wiring inside the BT-20/8 terminal box
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Demodulator removed from its enclosure
D
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Demodulator (UA) interior - left side
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Interior left side, seen from the rear
D
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Demodulator (UA) interor - right side
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Demodulator (UA) interor - right side, seen from the rear
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Hybrid (fork) and attenuator
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Double transformer (part of the ring mixer)
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6AC/3 on the workbench

Problems
  • No power cables
  • All fuses broken due to corrosion
  • Decomposition of the plastic cradles of the voice terminals
  • Strong 2500 Hz tone on the line
Restored
  • Superficial cleaning
  • New power cables made
  • New braided interconnection cables made
  • Fuses rewired
  • Wiring of terminal block slightly modified
  • Demodulator of one unit repaired
  • Both unit completely re-aligned
Tools
When repairing and/or aligning a Frequency Changer 6AC, it may be difficult to inject a tone of, say 400 Hz, from an AF generator directly into the circuit. Likewise, it is difficult to connect a modern oscilloscope directly to the circuit, as it connected to the same ground (earth). In such situations it is recommended to use a standard 600Ω 1:1 transformer as a galvanic separation.

Simple test tools used for testing and repairing Frequency Changer 6AC

The diagram above shows examples of simple circuits that can be used for injection of an AF signal (left) and for measuring (right). Note that the parts shown in grey (C1, R1, X1) are optional. C1 should be inserted when the circuit carries a high DC voltage. R1 should be added when measuring the signal on the line terminals, when the device is not connected to a subscriber line.

In many cases it may be useful to connect a high impedance crystal earphone (X1) in parallel to the output of the transformer, so the the AF signal can be monitored directly. This is particularly useful when the circuit is connected to the line terminals, whilst the balance potentiometers of the demodulator (UA) are adjusted for minimum leak of the 2500 Hz carrier back into the line.

Connections
Power
Power should be applied to the device via the 3-pin Bulgin connector that is located at the top right. It is positioned in such a way, that it can only be accessed (via a hole in the top lid) when the top lid is closed. In the same vein, the plug must be removed before the lid can be opened.

  1. AC (N) Neutral
  2. AC (L) Live
  3. Ground (E) Earth
    Mains power connector pinout when looking into the receptacle
It is important to verify that the correct mains voltage has been configured on the screw terminal block at the right, before applying a mains AC voltage. The device can also be powered from a 12V DC battery, in which case an internal vibrator pack is used. Confusingly however, the same 3-pin Bulgin connector is used for connection of the battery voltage. Furthermore, it requires 5 wire links to be altered. Always check the current configuration before applying mains power.

For service engineers, an extra 3-pin Bulgin connector is available inside the device, that is connected in parallel to the Bulgin connector at the top right. A dummy plug is installed to cover its contacts. It allows the device to be powered when the lid (and the front of the case) is open.

Voltage selector
For connection to the AC mains, the built-in power transformer has a primary winding with a number of taps, that allow it to be configured for virtually any AC mains network in the world. The windings are brought out to a screw terminal block that is fitted in the upper section, to the right of the fuses. By connecting the red and black wire to two of the terminals, the following voltages can be selected. The terminal numbers are shown in brackets (e.g. 1-6 for 250V).

  • 100V (2-3)
  • 110V (1-3)
  • 200V (2-4)
  • 210V (1-4)
  • 220V (2-5)
  • 230V (1-5)
  • 240V (2-6)
  • 250V (1-6) ← our current setting
WARNING — It is important to select the appropriate voltage prior to connecting the device to the mains, as the unit will be activated as soon as the mains is plugged in (there is no power switch). As the power in continental Europe has been raised at the beginning of the century from 220V to 230V AC, an incorrectly configured power transformer may saturate and eventually break down. In practice, mains voltages are often even higher. A mains voltage of 240V is no exception. Whenever possible, select the next higher voltage to avoid saturation.
Line & voice terminal   TA
At the top left of the Frequency Changer is a double terminal block to which the subscriber line and the telephone instrument (i.e. the voice terminal) should be connected. The subscriber line is connected to the lower terminal block (pins 1-2), whilst the microphone and speaker elements of the voice terminal's handset are connected to the upper terminal block (pins 4-7). As follows:

  1. Line (B)
  2. Line (A) 1
  3. LB 1
  4. Microphone (2) 2
  5. Microphone (1)
  6. Speaker (2)
  7. Speaker (1)
The diagram above shows the wiring of the 6AC/1 and 6AC/2 models. The wiring of the 6AC/3 is slightly different (pin 6 is used to pass a DC voltage for the carbon microphone) but it is advised to change it, so that the becomes compatible with the other models. Remove the orange wire from pin (6) and connect it in parallel with the orange wire at pin (4).  See the circuit diagram

  1. In a Local Battery (LB) installation, a 3V battery should be connected between pin 3 of the terminal block (+) and 2 of the BT-20/4 of the instrument (-).
  2. Omitted in a Local Battery (LB) installation.

Voice terminal   SA 50xx
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 Frequency Changer 6AC. 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 Frequency Changer 6AC.


During WWII, the dial armature was often omitted from the SA 5063/0 and SA 5063/1 telephone sets, as the British Government used a private telephone network – completely separated from the public switched network – that was patched manually. An extra push-button (KB) could be fitted (between the other two) for the RECALL feature that was available on some automated exchanges.

Simple terminal   TMC-232X
Below is the circuit diagram of the simplified TMC-232X telephone set. It has been modified for direct operation with the Frequency Changer 6AC, and is used for its handset only. Note that the two contact pairs of the hook switch are used for disconnecting the microphone and also for disconnecting the DC voltage that is needed for the carbon microphone, when it is on-hook.


Note that a different connection pattern is required when using the telephone with the Frequency Changer 6AC/3, as the latter has a slightly different internal wiring of its terminator block.


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

  1. Line (B)
  2. Line (A)
  3. LB (or unused)
  4. Microphone (H)
  5. Microphone (L)
  6. Speaker (L)
  7. Speaker (H)
    7-pin male XLR plug as fitted to the fixed wiring of the Frequency Changer
The XLR7/M plug fits exactly through the cable hole in the left side of the Frequency Changer, allowing the braided cable to be stowed inside the device when it is in storage or in transit.

CV1052 valve   EL32
The CV1052 is an indirectly heated pentode, introduced in 1940 for use in audio circuits. Apart from AC and DC applications, it was also used in car audio systems, either as a class 'A' amplifier or in push-pull configuration. It is equivalent to the EL32. It is also electrically (but not physically) equivalent to the EL2. It was one of the standard valves of the UK's General Post Office (GPO).

 CV1052 datasheet

Pinout as seen from the bottom of the valve

Specifications
  • Years
    1944 - 1957
  • Purpose
    Secure telephony
  • Principle
    Speech frequency inversion
  • Manufacturer
    GPO, TMC
  • Pivot freq.
    2500 Hz
  • Remote
    2-wire telephone line, 600Ω (default or 300Ω (option)
  • Local
    4-wire interface (handset)
  • Circuits
    Oscillator, amplifier (2x), mixer (2x)
  • Valves
    3 x CV1052 (EL32)
  • Mains
    100, 110, 200, 210, 220, 230, 240, 250 V AC 1 (50-60 Hz)
  • Consumption
    50W
  • Battery
    12V, 1.7A DC 2
  • Dimensions
    585 x 360 x 185 mm
  • Weight
    41 kg
  1. Selectable with a wire link.
  2. Configurable with five metal straps.

Known serial numbers
  • ?
    C & E Museum, Kingston, Ontario, Canada
  • U1444/3/1459
    Crypto Museum, Netherlands
  • U1444/3/1467
    Crypto Museum, Netherlands
Documentation
  1. Handbook No. H512/6, Secraphone Type 6AC/3
    Comprising Frequency Changer No. 6AC/3 and Subscriber's Telephone-Set.
    TMC, February 1946 — January 1954.  Circuit diagram

  2. Research Report No. 13631, The performance of Privacy Sets Using Simple Inversion
    Post Office Research Station. Dollis Hill, London, 20 November 1956.
References
  1. Anonymous, Frequency Changer 6AC/3 with accessories - THANKS !
    Belgium, May 2021.

  2. 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: Thursday, 08 July 2021 - 12:45 CET.
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