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Telephone No. 162   No. 232
Telephone set used with scrambler phone - this page is a stub

The first scrambler sets that were released in early 1940, used a green bakelite Tele.No.162 as the voice terminal for scrambled conversations. The call would be initiated first with a standard black telephone set of the No. 328 range, that was also connected to the Frequency Changer [1].

Once the call had been setup, both parties would press a button to go secure (i.e. select scrambled speech) and the conversation was continued on the green 162 telephone set. Once the call was finished, both parties would place their handset in the cradle to terminate the connection.

The Tele. No. 162 was actually the GPO version of the Neophone, made by Siemens Brothers 1 of Woolwich. It was introduced by the GPO in 1929 and is nicknamed The Pyramid. 2 The sets were available in black (bakelite), and later also in Jade Green, Chinese Red, Ivory and mottled Brown 3 .
GPO 200 series telephone set in Jade Green, which is very similar to the Tele. No. 162. Photograph obtained from

The 162 had a No. 164 handset, which in this case was also green. Shortly after its introduction the 162 became a popular model and was re-released in the Tele. No. 232. The version without the dial was known as 232CB. None of the 162/232 models has a built-in bell. If a bell was required, an external bell unit could be fitted elsewhere, or (optionally) at the bottom of the unit.

  1. Not to be confused with the German manufacturer Siemens & Halske.
  2. This nickname was introduced in recent years by collectors and was not used at the time.
  3. Although the coloured versions are commonly said to be made of Bakelite, they were actually made of Urea Formaldehyde, which was – like Bakelite – one of the first plastics.

Simple terminal   TMC-232X
Between 1944 and 1957, long after the 162 and 232 models had been succeeded by the 300-series, the British Telephone Manufacturing Company (TMC) supplied a simplified variant of the 232 with its Secraphone No. 6AC/3 Frequency Changer; a World War II telephone scrambler.

The image on the right shows the device that was a supplied with the Frequency Changer 6AC/3 in our collection. It has a bakelite body, no dial, and as the drawer fitted at the bottom. It is fitted with a bakelite No. 164 handset. 1 The cradle is made of bad (deteriorated) plastic. 2

As the unit is only used for its handset — the microphone and speaker elements are connected directly to the voice circuits of the Frequency Changer — it is not fitted with an induction coil (transformer). The hook switch in only used to interrupt the connection to the microphone.
Voice terminal

It can not be used to make calls, nor can it be used to accept calls. According to the instructions on the circular Secraphone label, the call had to be initiated on a regular telephone set (that was connected in parallel with the Frequency Changer. Once the call was established, the user could pick up the unit's handset, after which the handset of the regular phone was replaced. As the unit has no markings or stamps at its bottom whatsoever, we have given it the provisional name TMC-232X, indicating that it was made by TMC and that it was used for a special application.

 More about Frequency Changer 6AC/3

  1. Instead of the normal No. 164 marking, it has the letters 'TMC'.
  2. The cradle is made of a poor quality plastic, and had become brittle.

Circuit diagram   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.

  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.

  2. BritPhones, Coloured GPO 162s came in a variety of materials...
    Retrieved May 2021.
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Crypto Museum. Created: Wednesday 26 May 2021. Last changed: Sunday, 06 June 2021 - 08:21 CET.
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