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This is my Marconi CR100/2 communications receiver. I bought it from a chap in the New Forest several years ago. He’d refurbished... No, he’d rebuilt it to a very high standard. It now has a product detector which is great for SSB reception.
I bought my first CR100 from GWM Radio Ltd, an army surplus store in Portland Road, Worthing. I was in my early teens and, owning a proper communications receiver, I could finally call myself a real shortwave listener! Once I’d set it up in my bedroom and connected the aerial, I don’t think I switched it off for several days. I heard comms between army units on manoeuvre on Salisbury Plain. I heard fishing vessels and coastguard stations. I heard a test transmission from Tel Aviv. I heard licensed amateurs chatting about aerials and transmitters. And I heard pirates! Needless to say, I was in my element.
Sadly, those days have gone. Youngsters these days who are interested in radio communication buy ready made units. Sling up a piece of wire for an aerial, plug in your Chinese box, and you’re on the air. Having said that, there are some newcomers to the hobby who are keen to learn and build their own equipment. But, I do believe they are few and far between.
The Marconi CR100. A British Wartime Communications Receiver
During the Second World War, reliable radio communications were essential to Britain's armed forces. Messages had to be received over considerable distances, often under difficult conditions and in the presence of interference.
One receiver that played an important part in this work was the Marconi CR100, a substantial valve communications receiver developed around 1939–1940 by Marconi's Wireless Telegraph Company.
Unlike the domestic wireless sets of the period, the CR100 was designed specifically for professional communications. It offered excellent sensitivity, adjustable selectivity and the ability to receive both speech and Morse code over a wide range of frequencies.
The receiver was used by the Royal Navy, Royal Air Force and Army, with several versions produced to meet the requirements of the different services.
After the war, many CR100 receivers entered the surplus market. They became popular with radio amateurs and short-wave listeners, who appreciated their rugged construction and performance.
A Receiver Built for Wartime Service
The Marconi Company, based in Chelmsford, Essex, had considerable experience in the development of communications equipment. By the outbreak of war in 1939, radio had become an essential means of communication between ships, aircraft, military headquarters and shore stations. The CR100 was designed to operate under demanding service conditions. Its heavy steel cabinet, substantial chassis and carefully screened circuits were intended to provide reliability and stability.
The naval versions were particularly well known and received the Admiralty designation B28. Some were provided with radar interference suppression facilities, intended to reduce interference from radar equipment operating aboard the same vessel. Other versions were adapted for RAF and Army communications, including facilities for operation alongside a transmitter.
Weighing approximately 37 kg, the CR100 was certainly not a portable radio. It was intended for installation in a communications room, shore station or other fixed operating position.
The CR100 Receiver Circuit
The CR100 employs the well-established superheterodyne principle.
Incoming radio signals are amplified before being mixed with a locally generated oscillator signal. This converts the received signal to an intermediate frequency of approximately 465 kHz.
The receiver has two RF amplifier stages, a separate mixer and local oscillator, and three intermediate-frequency amplifier stages. This is quite an elaborate arrangement, but it provides considerable advantages when receiving weak signals.
The two RF stages improve sensitivity and help to reduce image-frequency interference. The three IF stages provide most of the receiver's gain and selectivity. After IF amplification, the signal passes to the detector, where the recovered audio is amplified and delivered to headphones or an external loudspeaker. A beat frequency oscillator, or BFO, is provided for receiving Morse code transmissions.
Frequency Coverage
The CR100 covers frequencies from approximately 60 kHz to 30 MHz in six bands.
| Band | Frequency coverage |
|---|---|
| 1 | 60–155 kHz |
| 2 | 155–420 kHz |
| 3 | 500–1,400 kHz |
| 4 | 1.4–4 MHz |
| 5 | 4–11 MHz |
| 6 | 11–30 MHz |
The wide coverage enabled operators to receive long-wave, medium-wave and short-wave transmissions. There is a deliberate gap between 420 and 500 kHz, which includes the receiver's intermediate frequency. The lowest bands were particularly useful for certain maritime services, while the short-wave bands provided long-distance communications.
Selectivity and the Crystal Filter
One of the most useful features of the CR100 is its variable selectivity. When receiving a strong AM transmission, a reasonably wide bandwidth is desirable because it allows better audio quality. However, when trying to receive a weak Morse signal close to another station, narrow selectivity becomes much more important. The CR100 offers several passband settings, nominally 6,000, 3,000, 1,200, 300 and 100 Hz.
The narrower IF settings employ crystal-filter arrangements to discriminate between closely spaced signals. The narrowest, 100 Hz, setting additionally uses a sharply tuned audio-frequency filter centred at approximately 1 kHz. This arrangement was especially valuable when receiving weak CW signals through interference. It is an interesting example of how engineers achieved high selectivity using valves, transformers, quartz crystals and carefully designed tuned circuits.
The Valves Used in the CR100
The standard receiver employs eleven valves, including the mains rectifier.
| Valve type | Function |
|---|---|
| KTW62 | First RF amplifier |
| KTW62 | Second RF amplifier |
| X66 | Mixer |
| KTW62 | Local oscillator |
| KTW62 | First IF amplifier |
| KTW62 | Second IF amplifier |
| KTW62 | Third IF amplifier |
| DH63 | Detector, AVC and first audio amplifier |
| KT63 | Audio output |
| KTW62 | Beat frequency oscillator |
| U50 | Mains rectifier |
Operating the Receiver
The CR100 has a large front panel containing the tuning scale and various operating controls. These include the main tuning and fine-tuning controls, waveband selector, aerial trimmer, RF gain, AF gain, passband selector, BFO adjustment and operating mode switch. The separate RF and audio gain controls give the operator considerable flexibility.
For AM reception, automatic volume control can be used to compensate for changes in received signal strength. For Morse reception, the operator can select manual gain control and adjust the BFO to produce a suitable audible beat note.
The aerial trimmer is another useful feature. Correct adjustment helps to obtain maximum signal transfer from the aerial system, particularly on the higher frequency bands. Although a modern communications receiver is much easier to operate, the CR100 provides a fascinating insight into the skills required of wartime radio operators.
Variations of the CR100
Several versions of the receiver were manufactured.
- The CR100/1 was associated mainly with Admiralty service and was known as the B28.
- The CR100/2, also designated R1297, was supplied for Army and RAF applications and included facilities for working alongside a transmitter.
- Other versions introduced changes to aerial connections, audio outputs and interference suppression.
- The CR100/6, also known as the B35, was adapted for naval direction-finding applications.
- The CR100/7 incorporated additional noise-limiting facilities and an IF output connection.
- The CR100/8 included changes to the headphone and loudspeaker arrangements.
Although the receivers are similar in general appearance, there can be significant differences in their external connections and internal circuitry. For this reason, anyone servicing a CR100 should establish the exact version before carrying out electrical work.
The CR100 After the War
Following the end of the Second World War, large quantities of military communications equipment became available to civilian buyers. For radio amateurs and short-wave listeners, this was an opportunity to obtain professional equipment that would previously have been beyond their reach.
The CR100 was a particularly attractive proposition. It was capable of receiving amateur transmissions, international short-wave broadcasting, maritime communications and many other signals. During the 1950s and 1960s, examples could be found in amateur radio stations throughout Britain. Although its size and weight were disadvantages, the receiver offered good performance and a wide range of operating facilities. The CR100 is still remembered as one of Britain's classic wartime communications receivers.
Manual in my manuals and circuits library.