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CIA
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Maico R
CIA probe microphone amplifier · 1954

Maico Model R was a transistorised hearing aid, developed in 1954 by The Maico Company Inc. in Minneapolis (Minnesota, USA). It was used — in a modified form — by the US Central Intelligence Agency (CIA) as a (pre)amplifier for wall-probe covert microphones [1]. The device was probably known at the CIA as Amplfier #216, and was used in combination with Acoustic Probe #6143 [A].

The device consists of a black plastic body and has a stainless steel front panel, embossed with the name 'MAICO'. At the left side is a socket for an ear­piece. At the right side is the volume knob with built-in on/off switch. It is powered by an R-401 mercury battery of 1.4V, 1 that should be installed behind a hinged panel at the back. 2

The device is basically a standard Maico Model R from which the internal microphone is removed and a 3 mm jack socket has been added at the top. The metal clip, that was used to attach the device to the clothing, has also been removed.
  
Maico Model R modified by/for the CIA, showing the 3 mm jack socket at the top. Viewed from the top right.

It is likely that the Maico Model R was modified for CIA use in 1954 or shortly thereafter, as the Model R was succeeded in 1955 by newer designs, such as the models S, V and W [2]. It is known from an internal CIA memorandum, that by July 1957, the device was internally referred to as 'the standard Maico probe' [3]. It is also known from surviving CIA Operating Instructions [A], that the device had the CIA designator Amplifier #216, and that it was used to amplify the sound picked up by Probe #6143. The latter is currently unknown, but was probably a condenser microphone.

Maico is a commercial hearing aid brand, founded in 1937 in Minneapolis (Minnesota, USA) by Leland A. Watson. It was one of the first companies in the world to introduce an all-transistor hearing aid in 1953, shortly after the invention of the transistor (1947). 3 The Model R was one of its successors, and was built around three CK718 transistors, made by Raytheon in the US [4]. Maico Diagnostics still exists today, and has offices in Minneapolis (USA) and Berlin (Germany).

  1. Ray-O-Vac R-401, Beltone BP401R, Duracell RM-401H, Eveready E401E, Zenith ZM401. Note that mercury batteries are no longer produced today, because of the environmental impact of mercury.
  2. It should be noted that the battery lid is (manually) engraved with the text Mallory TR-152 2.5V, which is almost twice the required voltage of 1.4V. It is possible that the CIA used the higher voltage.
  3. Although the principle of the transistor was invented in 1947 at Bell Labs (USA), it was first produced in large quantities by Raytheon (USA) in 1952 – especially for use in hearing aids – followed in 1954 by Texas Instruments (USA) and later Sony (Japan) for use in consumer appliances [4][5].

 Maico Model R modified by/for the CIA
Maico Model R modified by/for the CIA, showing the 3 mm jack socket at the top. Viewed from the top right.
Modified Maico Model R with earphone
Earphone plug and socket
CIA-modified Maico Model R hearing aid
CIA-modified Maico Model R hearing aid, with earphone connected.
Earphone used with the modified Maico Model R
Rear side
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 Maico Model R modified by/for the CIA
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Maico Model R modified by/for the CIA, showing the 3 mm jack socket at the top. Viewed from the top right.
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Modified Maico Model R with earphone
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Earphone plug and socket
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CIA-modified Maico Model R hearing aid
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CIA-modified Maico Model R hearing aid, with earphone connected.
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Earphone used with the modified Maico Model R
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Rear side

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Interior
The Maico Model R consists of a black plastic body with a stainless steel front panel, that is held in place by three screws: two at the sides towards the top, and one at the bottom. After removing these screws — don't lose them — the front panel can be lifted off, and the interior is exposed.

Towards the bottom is a battery compartment that accepts a 1.4V type 401 mercury battery, to be inserted behind a hinged lid at the rear of the device. The rest of the case is filled with a brown pertinax printed circuit board (PCB) on which the electronic components are fitted. The volume control knob can be operated from the side.

The empty space at the centre of the PCB is where the internal microphone was originally fitted. It was removed by the CIA and replaced by a 3 mm jack receptacle, in order to use it as a pre-amplifier for external (probe) microphones.
  
3 mm jack socket (modification)

The circuit consists of three amplifier stages, coupled by means of transformers. Each stage is built around a CK718 transistor, that was first made in large quantities in 1952 by Raytheon (US). They were developed especially for maintaining Raytheon's dominant position in the hearing aid industry, for which they had previously produced special subminiature vaccum tubes. The CK718 is a Germanium Alloy Junction Transistor [4]. The Maico Model R was in production until 1955.

Modified Maico Model R with front panel taken off
Interior
Interior
Volume potentiometer, CK718 transistor and adjustable resistor
3 mm jack socket (modification)
With open battery compartment
Model and serial number at the rear
Inscriptions on the cover of the battery compartment
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Modified Maico Model R with front panel taken off
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Interior
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Interior
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Volume potentiometer, CK718 transistor and adjustable resistor
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3 mm jack socket (modification)
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With open battery compartment
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Model and serial number at the rear
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Inscriptions on the cover of the battery compartment

Documentation
  1. Operating instructions for Probe 6143
    Date unknown, but probably 1954 or 1955.
References
  1. Pete McCollum, CIA standard Maico probe - THANKS !
    Received September 2020.

  2. Hearing Aid Museum, Maico Model R Transistor (Body) Hearing Aid
    Retrieved September 2020.

  3. CIA, Current Status of the Testing & Evaluation Program
    Memorandum for Chief, TSS/ED, 9 July 1957. Page 3.

  4. The Transistor Museum, Raytheon CK718 Transistor
    Retrieved September 2020.

  5. Thayer Watkins, History of the Transistor
    San José State University. Retrieved September 2020.
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Crypto Museum. Created: Saturday 19 September 2020. Last changed: Thursday, 24 September 2020 - 08:53 CET.
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