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SA 5030
Telephone set used with scrambler phone · 1940

SA-5030 was an analogue voice terminal for the Frequency Changer, also known as the Privacy Set or the Scrambler Phone, introduced in August 1940 by the British General Post Office (GPO) and made by various parties, including TMC and the GPO itself. Intended for use on CB/Auto installations, it consists of a Tele. No. 328, fitted with a Key No. 303A switch assembly, a 6-wire line cord 1 ending in a BT-20/8 junction box, or Block Terminal, and a green Handset No. 164.

The set has two push-buttons labelled SECRET and NORMAL (later: SCRAMBLE & NORMAL), just above the dial. Only the SCRAMBLE button is latched. It is released when the other button is pressed on when the handset is placed on hook.

An extra button could be fitted at the centre, to provide RECALL on PABX installations that supported this feature, or for use with various extension plans. In such cases the label would be changed accordingly, for example: SCRAMBLE - RECALL - NORMAL, and in most cases a dial would be fitted to allow direct (auto) calls.
SA5030 voice terminal

Factory assembled SA5030 units (marked SA5030 at their base) are extremely rare, as most of them were built by engineers in the field from locally available parts. Such items are generally marked Tele. No. 328 on their base and chassis. The image above shows such a Tele. No. 328 that was converted into an SA-5030 by a technician in the field. Due to shortages at the time of manufacturing (World War II), a lime-green-painted black bakelite handset is used. The green handset cord is a high-quality reproduction that was fitted when the unit was restored. 2

  1. 7 or 8 wires if a PABX Recall button is fitted.
  2. Restoration by Andy Grant [1]. Green reproduction cord obtained from Chris Elliot.

SA5030 voice terminal
Front view
SA5030 voice terminal
Junction box
Privacy Set No. 9A with SA5030 voice terminal
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SA5030 voice terminal
2 / 6
Front view
3 / 6
SA5030 voice terminal
4 / 6
5 / 6
Junction box
6 / 6
Privacy Set No. 9A with SA5030 voice terminal

Circuit diagram
Below is the internal wiring diagram of the SA 5030 telephone set. At the bottom right are the (A) and (B) terminals of the subscriber line. Directly above it, is the wiring to the screw terminals of the Frequency Changer 6x. The make-before-break (MBB) switches KA and KB are part of the 303/A Key Unit that is controlled with the 2 (or 3) push-buttons on top of the telephone set. 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 6x.

During WWII, the dial armature was often omitted from the SA-5030 telephone sets, as the British Government used a private telephone network – completely separated from the public switched network – that was patched manually. The diagram above is also applicable to the SA-5063 and other 300-series telephones that were modified for use with the Frequency Changer.

Internal wiring
The table below shows the wiring inside the SA-5030 telephone set, between the contacts of the standard connection terminal (T), the contacts of the key unit (K) and the contacts of the external junction box or block terminal (BT). The contact IDs are shown in blue in the diagram above.

  • K13
  • K16
    Line (B)
  • K17
  • K18
  • K19
    Handset, microphone (red)
  • K20
  • K21
  • K22
    BT4, BT5
  • K23
  • K24
    Handset, common (white)
  • K25
  • K26
  • K27
    Handset, receiver (speaker) (green)
  • T1
    Line (A)
Terminal   T
The drawing below shows the pin numbering of the standard connection block or terminal (T) of the basic telephone set, as seen from the bottom. In the circuit diagram above and in the internal wiring table above, these contacts are prefixed with the letter 'T' (e.g. 'T9').  Further information

Terminal block (T) as seen from the bottom of the phone

Key unit   K
Below is the layout of the contact terminals of connection block 9K, also known as Key Unit 303, as seen from the rear of the telephone set. In the circuit diagram and the internal wiring table above, the contact numbers of the Key Unit are prefixed with the letter 'K' (e.g. 'K23').  More

Terminal block of the Key Unit (K) as seen from the rear of the phone

Block terminal   BT 20/8
In most cases, the SA-5030 was externally wired via Block Terminal BT-20/8. Note that all 8 contacts are used. The Block terminal accomodates the subscriber line as well as the wiring to and from the Scrambler. The exact wiring diagram is shown in the circuit diagram above.

  1. Line (B)
  2. Line (A)
  3. Ground (earth)
  4. Speaker (L) 1
  5. Microphone (L) 1
  6. Microphone (H)
  7. Switched Line (A) - to Frequency Changer 2
  8. Speaker (H)
    Wiring inside the Block Terminal 20/8
  1. Contacts 4 and 5 are shorted. They are connected to the common line of the handset (via KA3).
  2. This line is controlled by switch KA1. It connects the Frequency Changer to the subscriber line when in secure mode.

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 5030 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- 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/F on the cable that is fitted to the BT-20/8 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)
  1. Circular label Speech on Telephones is Not Secret (PDF)
    Crypto Museum, Reproduction, 10 June 2014.
  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: Monday, 05 July 2021 - 09:00 CET.
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