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Mk. 328
Clandestine receiver

The Mark 328 (Mk.328) is a solid-state miniature receiver for clandestine operations, developed by HMGCC at Hanslope Park (UK) and introduced in 1970. Contrary to its 1954 predecessor, the valve-based Mk.301, it was one of the first fully transistorized receivers for special purposes. It no longer needed plug-in coil packs and was distributed in a Tupperware TM style box. The receiver is also known as Radio Receiver 328R or as the complete radio station UK/TRR-328.
 
The receiver has a rather unusual 'modern' look and feel. It is completely self-contained and is powered by two internal 6.75V Mercury cells (Mallory TR165) or by an external DC power source that can deliver 10.5 to 16V at 13-18mA.

The image on the rights shows a typical Mk.328 ready for operation. A telesopic antenna is fitted to a threaded socket at the far corner and a pair of headphones is connected at the front. The receiver delivers approx. 1mW audio into 2Kohm headphones. RF sensitivity is 3-25µV at 1mW AF output, depending on the frequency band in use.
  

On top of the receiver, just above the headphones socket, is the 22K high-impedance line output, which allows a (tape)recorder or an amplifier to be connected. At the front left is a film-type frequency scale behind a rectangular plastic window, that can be tuned by two rigged knobs at the side. The desired frequency band and the modulation type are selected by two slide switches at the right, just in front of the telescopic antenna. In between these two switches are two snap-in terminals for the connection of an external antenna and ground, giving far better reception.
 
The polythene box The polythene box Inside the box Close-up of the contents of the box The receiver with antenna and headphones The bare receiver Close of the frequency scale and the controls Band selector switches
Receiver, antenna, headphones and accessories The receiver with antenna and headphones Transparent plastic box with the accessories Contents of the accessory box Close-up of the bottom of the telescopic antenna Inside the receiver Inside the receiver Inside the receiver

The Mk.328 is a single conversion receiver, with switch-selectable band-pass filters before and after the RF stage, a local oscillator, a mixer, a 3-stage IF amplifier, a detector, an AF amplifier and a BFO oscillator. The frequency scales can be calibrated using the built-in 250kHz/1MHz reference oscillator. The receiver is suitable for all frequencies between 2.5 and 30 MHz, divided over five colour-coded ranges:
  • 2.5 - 5MHz (orange)
  • 5 - 10 MHz (green)
  • 10 - 15 MHz (yellow)
  • 15 - 20 MHz (red)
  • 20 - 30MHz (white)
The receiver was distributed with some accessories, such an antenna, headphones, instruction booklet, etc. The complete kit was packed inside a polytene box, such as the one shown below.
 
Inside the box, the various units are held in place by polystyrene foam. The blue receiver is visible in the right, whilst the left compartment holds a transparent plastic box with the additional accessories. The high-impedant headphones (2Kohm) are placed on top of the accessory box.

In the middle of the box is the telescopic antenna that has a rather unusual M4 thread screw at the bottom, allowing it to be bolted onto the receiver (see the pictures below).
  

The user had the option to use the telescopic antenna (screwed into a terminal at one of the corners of the receiver), or a supplied wire antenna. In the latter case, the receiver also needed a proper connection to the gound (earth).
 
The following accessories were present:
  • Receiver
  • Telescopic antenna
  • 2-way headphones (in-ear)
  • 2 Mallory mercury cells (6.75V each)
  • Instruction booklet
  • External battery lead
  • Earth lead
  • Wire antenna
  • Ear clip
  • Jumper clip (?)
  • Two different recoring cables (Lemo and Uher)
  


 
Download
References
  1. HMGCC, Mk 328 Receiver User Handbook
    UK, National publication No. 249. Part of the Mk.328 package.
    Date unknown, but probably 1970.

  2. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN 0952063-36-0, September 2004

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