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Privacy Set No. 7
Telephone scrambler · 1957-1962 - this page is a stub

Introduced in November 1957 for use on CB/Auto and LB systems. It is housed in a metal enclosure and was known as Privacy Set rather than Frequency Changer. Rather than being GPO-exclusive, this was the first set to become available through other commercial parties as well.

This version bas built around VC138 penthode valves — equivalent to the EF91 — and offers improved performance and security. It has a modular construction, which made it much more service friendly in case of a fault or repair.

Security was improved by using a different frequency inversion scheme with two stages of modulation — one at 10.0705 kHz and one at 12.570 kHz — thereby minimising the risk of unscrambled speech being delivered at the line in case of a malfunction of the main oscillator. Interestingly, the feature was dropped again on the later transistorised Privacy Set No. 8 and 9.
Interior of a Privacy Set No. 7A.

Privacy Set No. 7A was suitable for the 100-110V or 200-250V AC mains, whilst No. 7 came without a mains power supply module, allowing it to be used with other power options, such as a 12V DC input module. The image above the right shows a typical Privacy Set No. 7A.

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Crypto Museum. Created: Wednesday 26 May 2021. Last changed: Wednesday, 28 July 2021 - 05:38 CET.
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