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Mk. 301
Clandestine receiver

The Mark 301 (Mk.301) was a miniature valve-based receiver for clandestine activities. It was designed in 1954 by SG Hart (VK5HA) when working for HMGCC at Borehamwood (UK). It was intended as a replacement for the war-time MCR-1 and was built around 5 subminiature valves.
 
The Mk.301 is suitable for all frequencies between 500 kHz and 18.5 MHz, divided over four ranges. Unlike the MCR-1, that needed four different coil packs, the Mk.301 comes with a single coil pack that can be inserted in four different ways, marked with the numbers 1-4.

The image on the right shows a typical Mk.301 receiver with detached coil pack. The case of the receiver is constructed in such a way that the coil pack fits inside the 'missing corner'. The number in the top right corner corresponds with the band number (1-4) that is being used.
  
Mk.301 with detached coil pack

The coil pack in the image above is positioned in such a way, that band number 2 will be used when the coil is plugged-in. A battery pack or an external power pack should be used to supply the necessary 1.5V (LT) for the filaments and 67.5V (HT) for the valves. It is connected to a 4-pin socket at the bottom of the receiver (here visible at the front). A suitable (wire) antenna and earth lead (ground) can be connected at the top of the receiver (not visible in the image above).

The receiver came with an accessory box of approx. the same size as the receiver, that was also used as the battery pack. Alternatively, the optional Mk.301PP Power Pack could be used as a mains power supply. It could be fitted inside the battery box. The complete kit (without the power pack) was distributed in a small carton box that also contained the user instructions.

The receiver was intended for several services that were controlled by Hanslope park (UK), but it was later adopted for use by the Army's Special Forces as well. The Mk.301 was replaced in 1970 by the fully transistorised Mk.328 that had a band selector rather than a separate coil pack. A nice article about the Mk. 301 by Wim Kramer (1992, Dutch) is available for download below [3].
 
Mk.301 in original carton Opening the package Mk.301 with detached coil pack Mk.301 with coil pack attached Mk.301 receiver and accessory/battery box Accessory box with removed lid Tuning the Mk.301 in band 4 Small enough to fit inside a pocket

 
Controls
The image below shows the controls of the Mk.301 receiver. The receiver has only three adjustments: volume, BFO and tuning, and is extremely easy to operate. At the right is the coil pack that can be mounted in four different ways, each representing one of the frequency bands.


Connections are at the front (power), top (earphones) and rear (antenna and ground). The receiver is turned on as soon as power is supplied to the 4-pin socket at the front. Turn the volume control to an appropriate level and use the frequency dial to select the desired frequency.
 
Receiver
Weighting just 1kg and measuring just 17 x 9 x 3.5 cm, the Mk.301 is one of the smallest valve-based receivers ever built. Five subminiature valves are used to create a superheterodyne receiver with great sensitivity. For frequency adjustment, the designer (SG Hart) invented a revolutionary 2-section side-by-side tuning capacitor with spiral scale, that fitted the slimline case.
 
The receiver is tuned by rotating the large black dial with one finger. It has a linear scale and is very smooth in operation. A set of tables inside the top lid of the battery box converts the linear scale into the actual receiving frequency.

Although the receiver can use a single short wire as antenna, it works best with a longer wire plus a suitable earth (counterpoise). The antenna inputs and the earth connection are at the top of the receiver. The sockets are visible as three small holes. For the reception of strong signals, a separate attenuated antenna input is available.
  
Mk.301 with coil pack attached

A suitable wire antenna and earth wire with the appropriate plugs were supplied with the kit. They were usually stored inside the accessory box. The antenna wire is 17m long and has to be as free as possible. The earth wire is 3m long and has an alligator clip at the other end, allowing it to be connected to, say, the water pipe. The AF output amplifier delivers a 1mW signal into a pair of high-impedant (15kΩ) earphones, similar to the ones that were used for hearing aids.
 
Mk.301 with detached coil pack Mk.301 with coil pack attached Tuning the Mk.301 in band 4 Coil pack 4 in operation Power and earphones connected to the Mk.301 Antenna and earth wires Standard antenna socket (red) and earth (black) Using the attenuator

 
Accessory box
The Mk.301 came with a metal box of the same size as the receiver itself. The box contains the accessories, such as the earphones. When empty, the accessory box could be used as the battery box, taking a single 1.5V battery for the filaments and a 67.5V DC battery for the HT voltage.
 
The contents of the accessory box can be accessed by removing the top lid. The inside of the lid contains four tables, one for each coil. The table converts the linear scale of the Mk.301 into the actual frequency, similar to the MCR-1.

The image on the right shows the accessory box with its top lid taken off. Some accessories are packed in thin white paper. The empty accessory box can be used to create an LT/HT battery box. A colourful fixed 4-wire cable attaches the battery box to the receiver. It connects to the 4-pin socket at the bottom of the receiver.
  
Accessory box with removed lid

A separate power cable allows the Mk.301 receiver to be connected to an alternative power source, such as a pair of (LT/HT) batteries or an external power supply unit. Also in the accessory box is a pair of high quality earphones, similar to the ones that were used in hearing aids of those days, and a piece of cardboard with the antenna and earth wires.
 
Opening the accessory/battery box Accessory box with removed lid Contents of the accessory box Battery box connected to the Mk.301 receiver Earphones Antenna and earth wires External power cable Battery plug and socket

 
Power Supply Unit Mk.301PP
Some Mk.301 units were supplied with a drop-in power supply unit (PSU) allowing the Mk.301 to be powered from the mains rather than from batteries. When present, the Mk.301PP AC mains adapter could be placed inside the accessory box and fitted precisely in the space that was normally used for the batteries. The mains power cable leaves the case through a hole in the side.
 
The image on the right shows a good-looking replica of the original Mk.301PP power pack. It was built to the original specifications and sizes.

It has a rectangular shape with a 'blob' at one end. The blob replaces the 1.5V filament battery and has a contact at either side, that directly mates with the contacts inside the battery box.

The two 'flying' contacts for the 67.5V HT battery should be connected to the sockets on the top surface of the blob. To the right of the two HT terminals is the mains primary fuse.
  
Mains Power Supply Unit (PSU)

When the Mk.301 PP is mounted in the way described above, the case can be closed again. The mains lead can be fed through a hole in the right side of the battery box, together with the 4-colour wiring to the receiver. It connects to the mains by means of two banana-type plugs.
 
Mains Power Supply Unit (PSU) Power Pack placed inside the battery box Closed battery box containing the Mk.301PP Close-up of the 'blob' showing the fuse and the HT terminals Close-up of the connected HT leads

 
Manual
The Mk.301 receiver came with a small booklet with instructions on how to use and maintain the receiver. The booklet also contains the full circuit diagrams of both the receiver and the (optional) Power Supply Unit Mk.301 PP.

Two variants of the manual are currently known: issue 2, that came in the brown envelope shown on the right, and issue 3, that had a blue cover. Apart from a few minor differences, this manual is identical to the earlier issue 2 release.

Issue 2 is available for download below [1].
  
Operator's Manual (issue 2)

 
Operator's Manual (issue 2) Rear view of the Operator's Manual (issue 2) Operator's Manual (issue 3) Close-up of the Operator's Manual (issue 3)

 
Interior
The Mk.301 receiver can be opened by removing 8 small bolts at the sides and a further 2 at the rear. The outer case shell can then be removed completely, whilst all components stay together in the main frame. The image below shows the interior of the Mk.301 after removing the outer shell.
 
At the front right is the famous two-section side-by-side tuning capacitor. The two sections are driven simulataneously by the frequency dial via a set of cog wheels (not visible here).

At the left rear is a small sub-frame holding the five subminiature valves and some adjustable inductors. A piece of foam at the rear prevents the valves from falling out of their sockets when the receiver is transported. The remaining passive components are mounted at the bottom of the sub-frame, visible here at the front left, with the output transformer at the corner.
  
Mk.301 interior

The coil pack can be attached in one of 4 ways, to the contact pins behind the tuning capacitor. Inside the coil pack is a set of coils and adjustable capacitors that have been calibrated for the appropriate frequency bands. The coil pack consists of two halves that each hold the components for two bands. The adjustable coils and trimmers are accessible through holes in the case.
 
Mk.301 interior Mk.301 interior Close-up of the frame holding the five valves Mk.301 interior detail Cog-wheel coupled 2-section side-by-side capacitor and spiral scale Interior of the coil pack Front view of one side of the coil pack Rear view of one side of the coil pack

 
Valves (tubes)
  • 1 x DK96 (local oscillator mixer)
  • 2 x DF72 (IF amplifier and BFO)
  • 1 x DAF70 (Detector, AFC, AF stage)
  • 1 x DL75 (Audio output amplifier
Frequency ranges
  1. 500 - 1250 kHz
  2. 1.20 - 3.25 MHz
  3. 3.10 - 7.77 MHz
  4. 7.75 - 18.5 MHz
Technical specifications
  • Principle: 4-valve super-heterodyne receiver, with BFO for CW and Phone reception
  • Frequency range: 500 kHz - 18.5 MHz
  • IF frequency: 465 kHz
  • AF output: 1mW into 15kΩ
  • Power supply: 1.5V LT (125mA) and 67.5V HT (12mA)
  • Mains power supply (optional): 100-250V AC 50/60Hz
  • Antenna: 17m wire
Contents list
  • Receiver
  • Coil Box Assembly
  • Battery Box Assembly
  • Battery Lead Assembly
  • Earth Lead Assembly
  • Areal Lead Assembly
  • Aerial & Earth Lead Spool
  • Phones Complete
References
  1. HMGCC, Instructions and Service Book for the MK.301 Receiver
    Original booklet, issue 2. Date unknown, but probably late 1954.

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

  3. Wim Kramer, De MK 301 Spionage-ontvanger
    RAM Magazine 134, August 1992. pp. 16-19 (Dutch). 1

  1. Reproduced here by kind permission from the author Wim Kramer.

Further information

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