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Telephone No. 740
Used as Privacy Set voice terminal

Telephone No. 740 was an analogue tabletop telephone set with pulse-dialling, suitable for use on automatically switched networks, introduced in 1968 by the British General Post Office (GPO) as the successor to Telephone No. 710 [1]. The device can be fitted with up to 4 push-buttons and can be configured for use with a Frequency Changer or Privacy Set telephone scrambler.

The image on the right shows a Telephone No. 740 that was recovered from a GPO facility in Northern Ireland together with a Privacy Set No. 8, shortly after the facility had been bombed [2].

The device consists of a black body with a brown handset, and has all four push buttons fitted. 1 The leftmost push-button is brown 2 and is marked SEC'T, which is short for SECRET. When pressed, it connects the subscriber line and the microphone and speaker of the handset directly to the Privacy Set. The other 3 knobs are clear and read MAIN, RECALL and EXTN respectively.
  
GPO Telephone No.740

Telephone No. 740 was introduced in 1968 as the successor to the 1964 Telephone No. 710. It is largely identical, but has an improved enclosure and circuit board. Furthermore, it allows up to four push-buttons to be fitted in the space above the dial. The device shown here has all four push-buttons fitted and carries manufacturing code GEN 73/1 at the bottom, which means that it was manufactured by GEC-AEI Telecommunications in Newton Aycliffe in County Durham (UK).

The modified 740 is compatible with all types of Privacy Set No. 8/8A and No. 9/9A, but also with the earlier wartime valve-based Frequency Changer No. 6. Privacy Sets were used on public subscriber lines until at least 1978, after which they were succeeded by more secure alternatives.

  1. At least two buttons are needed for the Privacy Set. The other two are optional.
  2. Originally, all push-buttons for the 740 had clear caps, so that they could be used with any combination of case and handset colour. The brown SEC'T button is therefore a little out of place, as it stems from the earlier 710 range. It might have been used when clear SEC'T buttons where in short supply.

Telephone No. 740 and Privacy Set No. 8
GPO Telephone No.740
GPO Telephone No.740
GPO 740 with wiring for Privacy Set and subscriber line
At the left is the SEC'T (secret) push-button
Four push-buttons
Block Terminal BT37B
Interior
A
×
A
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Telephone No. 740 and Privacy Set No. 8
A
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GPO Telephone No.740
A
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GPO Telephone No.740
A
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GPO 740 with wiring for Privacy Set and subscriber line
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At the left is the SEC'T (secret) push-button
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Four push-buttons
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Block Terminal BT37B
A
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Interior

Features
The image below show a complete GPO Telephone No. 740, ready for connection to Privacy Set No. 8 or a compatible device. It is fitted with a black 10-wire cable that terminates in a BT37B junction box. The Privacy Set is also connected to the junction box, via a 7, 8 or 10-wire cable.

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Also connected to the junction box, is a 2 or 4-wire subscriber line. In the default situation, the subscriber line is connected to the 740, which acts as a regular telephone set. Once a call has been established and the handset is off-hook, both parties press the SEC'T button on their 740. The device then becomes the voice terminal of the Privacy Set and the conversation is scrambled.

In this situation, the 740 has become the voice terminal of the Privacy Set. The subscriber line is disconnected from the telephone's line circuit and passed to the line input of the Privacy Set. In the same vein, the microphone and speaker of the telephone's handset are disconnected from the telephone circuits and connected directly to the voice circuits of the Privacy Set. By mounting an XLR7/M connector at the end of the brown cable (wired according to our own standard), we are able to demonstrate the 740 in combination with any GPO telephone scrambler in our collection.




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Interior
The interior of the device can be accessed by loosening a single screw at the centre of the rear side, after which the plastic shell can be pulled over to the front, whilst the knobs of the push-buttons (optional) remain inside the shell. The shell catches the bottom panel at the front edge.

The image above shows the interior of the device after the case shell has been removed. All parts are mounted to the metal base. At the bottom are the bell set and a printed circuit board (PCB).

Integrated with the PCB is a U-shaped metal bracket that controls the hook switch (gravity switch in GPO terminology) and up to 4 push-button switches. Mounted between the bell set and the U-shaped bracket is the circular dial. At the rear edge of the PCB are a series of screw terminals to which the handset, subscriber line and any (optional) switch packs are connected.
  
Interior, seen from the rear

The image above shows the interior of the device as seen from the rear. The wiring appears a bit messy, but this is an unavoidable side effect of the modification to make the 740 suitable for use with a Privacy Set. At the rear edge of the bottom panel are the cable inlets for handset and line.

The handset is connected via a brown 4-wire coiled cable, whilst a black 10-way cable is used for connection of subscriber line and Privacy Set. This is different from the usual 4-wire cable.

The difference between a regular 740 device and a Privacy Set voice terminal, is the addition of the 4K switch pack (1/DSP/1244) shown here in the leftmost position (seen from the rear). When the SEC'T button is pressed, this switch is activated and remains locked in position until released by pressing another button. It is also released when the user put the handset in the cradle (on-hook).
  
4K switch (1/DSP/1244)

The 4K switch pack consists of 4 individual changeover contacts, one of which is used to transfer the subscriber line to the Privacy Set. The remaining contacts disconnect the microphone and speaker from the device and connect them directly to the voice circuits of the Privacy Set. The 4K switch is wired to the unused screw terminals on the PCB and to an extra set of 6 screw terminals that it fitted to the side of the U-shaped bracket. From there it is patched to the 10-wire cable.

Interior
Telephone No. 740 interior
Interior, seen from the rear
Switch assemblies inside the 740
Switch locking configuration
Gravity Switch (hook) with up to 4 additional push-buttons
4K switch (1/DSP/1244)
Switch assemblies inside the 740
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Interior
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Telephone No. 740 interior
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Interior, seen from the rear
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Switch assemblies inside the 740
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Switch locking configuration
B
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Gravity Switch (hook) with up to 4 additional push-buttons
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4K switch (1/DSP/1244)
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Switch assemblies inside the 740

Restoration
The Telephone No. 740 featured on this page was recovered – together with this Privacy Set No. 8 – from a GPO facility in Northern Ireland, shortly after it had been bombed by the IRA [2]. When we received it in July 2021, it came with partial wiring and was in an unknown and untested state.

After a first inspection it became clear that someone had previously attempted to rewire the unit – probably for use on the public switched network or on a PABX – as a result of which the junction box (BT37B) was wired incorrectly.

This was easily fixed however, by inspecting the 740 circuit diagram and tracing the wiring from the 4K switch pack under the SEC'T button. Once the wiring was corrected, an 8-wire cable with an XLR7/M connector was added for connection of the Privacy Set, and a 2-wire cable with an RJ11 connector for the 2-wire subscriber line.
  
Block Terminal BT37B

After testing the device on the local PABX with a known good telephone set at the other end, it became clear that the audio in the receiver circuit was rather faint. This was solved by replacing the 4T speaker element in the handset. The unit was tested again and produced clear audio.

The next problem to be solved was the fact that the push-buttons were binding. It was difficult to press them down and some buttons were not always released when another one was pressed.

The image on the right shows the 4 positions of the push-buttons as seen from the rear. There are 4 white nylon plungers that are pushed down by each of the buttons. In this image, only two of them control a switch. The other 2 are only used to release the large 4K switch pack, but as they are unsupported, this does not always work as expected. It would be better if they had a switch.
  
Switch assemblies inside the 740

The binding problem appeared to be caused by ageing of the parts. It was solved by smoothening and greasing the white plungers. Furthermore, a dummy switch was mounted behind the 2nd plunger (connected to nothing), so that it now properly releases the SEC'T button when pressed. Finally, the unit was tested with a compatible scrambler at the far end of the telephone line.

Problems
  • Block Terminal incorrectly wired
  • External wiring incomplete
  • Dial outer ring loose
  • Faint incoming audio
  • Push-buttons binding
  • One push-button knob broken
  • Cover missing from block terminal BT37B
Fixed
Missing
  • Cover for block terminal BT37B
Connections
Crypto Museum standard
To allow the Privacy Sets 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 25-point junction box BT-37B is used as the central hub. The GPO 740 voice terminal is fitted to the BT-37B via a fixed black 10-wire PVC cable. The subscriber line is also fitted to the BT-37B via a fixed 2- or 4-wire cable, whilst a fixed 8-wire cable with an XLR7/F connector at the end is present for quick (dis)connection to the Privacy Set or Frequency Changer.

The Privacy Set itself is fitted with a fixed 7- or 8-wire PVC or braided 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 internal terminal block or from the BT-37B junction box. Below is the pinout of the XLR7/F on the cable that is fitted to the BT-37B terminal block.

  1. Line (B)
  2. Line (A)
  3. LB (or unused)
  4. Microphone (H)
  5. Microphone (L)
  6. Speaker (L)
  7. Speaker (H)
    7-pin female XLR socket as fitted to the telephone set (via the BT-20/8)
Documentation
  1. N840, Telephone No. 740 circuit diagram
    GPO, 18 February 1969 - 12 Actober 1972. Issue A (1 page). 1

  2. N4700, Auxiliary Units and Additional Parts for 700-type Telephones
    GPO, 12 August 1963 - 15 May 1977. Issue C (4 pages). 1

  3. N848, Auxiliary units for Telephones, Nos. 710, 711, 740, 741 & 746
    GPO, 21 January 1969 - 9 May 1977. Issue J (4 pages). 1

  4. Press-Buttons for Tel. No. 710, 711, 740, 741 & 746
    GPO, 10 February 1969 - 28 July 1980. (2 pages). 1
  1. Document obtained via [1].

References
  1. Robert Freshwater, Telephone No. 740
    Bob's Telephone File (website). Retrieved August 2021.

  2. Richard P., GPO Telephone No. 740 - THANKS !
    GEN 73/1 version, modified for use with Privacy Set. Received July 2021.
Further information
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Crypto Museum. Created: Monday 09 August 2021. Last changed: Tuesday, 10 August 2021 - 13:58 CET.
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