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Mieco 25A
Code Phone Scrambler

Mieco 25 and 25A were analogue scrambler phones produced by Mieco (USA), a subsidary of Polarad Electronics Corporation (USA) in the 1970s. The phone uses a simple frequency inversion scheme (commonly known as a scrambler) with 25 settings, and was interfaced to a standard telephone set by means of an acoustic coupler. It was sold well into the 1980s.
The image on the right shows a typical Mieco model 25A in action. It consists of a handset, an acoustic coupler and some controls, housed in a rectangular metal case. The device is operated from either a 4.2V or 9V battery, and two dials with 5 settings each are used to set any of 25 possible 'codes'.

The handset of a regular telephone set is placed on the acoustic coupler inside the Mieco 25. The difference between model 25 and 25A, is that the acoustic coupler of the latter is adjustable to accomodate phone handsets of different sizes.
Operating the handset of the Mieco 25A

The initial version of the Mieco 25, was powered by a TR-133 Mallory 4.2V battery. Later versions with a serial number of 4584 or higher, such as the one shown here, used a standard 9V block battery, such as the Eveready 522. Being a scrambler, the Mieco 25 and 25A offered limited - if any - security, and could easily be broken by the avarage eavesdropper.
Closed Mieco 25A Opened Mieco 25A with instruction sheet Opened Mieco 25A Operating the handset of the Mieco 25A The battery inside the battery compartment Closing the battery compartment The Mieco 25A connected to a standard telephone set Code table inside the top lid

Operating the Mieco 25 was rather straightforward. After opening the case and taking out the black handset, the handset of an ordinary telephone set was placed on the acoustic coupler.
An elastic strap with a lock at one end, was used to keep the handset in place during the call, and also to prevent ambient sound from 'leaking' into the conversation. This was further aided by a rubber gasket around the circumfere of the speaker of the acoustic coupler.

The image on the right shows how a standard European (Dutch) telephone set was used with the Mieco. As the model 25A had an adjustable microphone, it could easily be adapted for different size handsets. The Mieco 25A is shown here with the handset of an Ericsson T-65.
The Mieco 25A connected to a standard telephone set

Once the handset was in place, the two rotary switches at the front were used to set up a suitable 'code'. As each rotary switch has just 5 settings, only 25 'codes' are possible. Once set, the power switch is used to turn the unit on. A table inside the top lid of the unit, shows how each handset should be configured for each possible setting.

A conversation would usually be initiated in clear, after which both parties would agree on a suitable setting. If the settings were agreed before the start of the conversation, a call could be made directly in scrambled mode.
Operating the handset of the Mieco 25A The Mieco 25A connected to a standard telephone set Fixating the handset Fixing the strap Adjusting the microphone Setting the 'code' Switching ON the Mieco 25A Code table inside the top lid

Inside the Mieco 25A is a well layed-out out single-sided PCB, with first-class electronic components. Most of the circuit is built in duplicate, allowing full duplex calls. The actual scrambler is based around a Motorola MC1496L balanced modulator/demodulator. High quality filters are used to determine the frequency inversion point. The two rotary switches, which 5 settings each, are used for changing the carrier frequency and, hence, the inversion point.
The PCB is located at the bottom of the unit. Only a hand full of wires are used to connect the PCB to the speaker, the microphone, the handset and the controls. The two rotary switch each contain a network of resistors (R) and capacitors (C) that are used for altering the carrier frequency.

According to production dates on various components, the Mieco 25A shown here was built around 1980. The Motorola MC1496L modem was discontinued around 1988.
Close-up of the PCB of the Mieco 25A

Interior of the Mieco 25A Close-up of the PCB of the Mieco 25A The PCB of the Mieco 25A Close-up of the Motorola MC1496L Close-up of a filter unit

Philips Security
Despite the fact that the Mieco 25A is rather well built, it provides only limited security. Using a frequency inversion scrambler with a single carrier, makes decoding rather easy for an interceptor. Furthermore, the user has to be very careful as to avoid clear voice data to 'leak' directly into the handset on the acoustic coupler (TEMPEST). A well-trained interceptor can sometimes 'interpret' the scrambled voice data, even without specialized equipment.

Nevertheless, a pair of Mieco 25A units was bought by Philips Security Office in the early 1980s, and used for top-level conversations between high-ranking officials of Dutch electronics giant Philips. It was thought at the time that a voice scrambler would provide sufficient security for confidential information.

Philips Security Office was not aware that Philips had its own cryptographic industry, that was known as Philips Usfa (later: Philips Crypto). When the latter discovered the fact that their superiors used inferior technology, the Mieco 25A was replaced by more secure Philips crypto phones such as the Spendex 30/35 and - later - the PNVX series.
  1. Mieco, Model 25 and 25A Operating Instructions
  2. Motorola, MC1496L datasheet

Further information

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Crypto Museum. Last changed: Saturday, 22 December 2012 - 11:05 CET.
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