The Marconi CR300 is a British valve communications receiver, designed primarily for maritime use. It is a substantial piece of British radio engineering, dating from the Second World War period, and is particularly interesting to vintage radio collectors, radio amateurs and enthusiasts of maritime wireless history.
It was designed in the early 1940s by G. J. McDonald and was manufactured by the Marconi Company. The receiver was intended for use aboard merchant ships and naval vessels, where reliable reception across a very wide range of frequencies was essential.
Technical specifications
| Specification | Details |
|---|---|
| Manufacturer | Marconi |
| Model | CR300 |
| Circuit | Superheterodyne |
| Frequency coverage | 15 kHz–25 MHz |
| Wavebands | 8 |
| Valves | 8 |
| Audio output | Approximately 2 watts |
| Internal speaker | Yes |
| Weight | Approximately 55 lb (25 kg) |
| Power requirements | 24V and 250V DC |
The receiver covers an impressive frequency range, extending from very low frequencies right through to the shortwave bands.
The two versions
| Version | Crystal calibrator | Notes |
|---|---|---|
| CR300/1 | 500 kHz | This version was widely used by the Royal Navy. |
| CR300/2 | 690 kHz | A later version of the receiver. |
The circuit
The CR300 uses an eight-valve superheterodyne circuit, incorporating:
- RF amplification.
- Frequency changing (mixer/oscillator).
- Two intermediate-frequency stages.
- Second detector and automatic gain control (AGC).
- Beat-frequency oscillator (BFO) for Morse reception.
- Audio amplification and output stage.
- Built-in loudspeaker.
It uses two intermediate frequencies, approximately 98 kHz and 570 kHz, depending on the tuning range. One particularly interesting feature is its variable selectivity, which can be adjusted down to approximately 100 Hz according to contemporary technical literature.
Power supply
The receiver requires:
- 24V supply for its valve heaters.
- 250V DC for its high-tension supply.
The Marconi Type 889 power supply allowed operation from 230V AC mains, as well as various DC supplies. This is an important consideration when restoring one today.