The Eddystone 358X is a very old receiver. It uses plug-in coil packs for each band, quite a novelty these days!
Introduction
The Eddystone 358X is a fascinating example of British communications receiver design from the Second World War. Introduced around 1940, it was a high-quality, general-coverage superheterodyne receiver designed for demanding professional and military applications.
Manufactured by Stratton & Co. Ltd. of Birmingham, the receiver was built at a time when reliable radio communications were essential to Britain's war effort.
Unlike the ordinary domestic wireless sets of the period, the 358X was intended to receive weak and distant signals, often under difficult conditions. Its robust construction, excellent mechanical engineering and sophisticated tuning arrangements made it a serious communications receiver.
The history of the Eddystone 358X
The Eddystone name was already well established before the Second World War, particularly among radio amateurs and professional radio engineers. The company had earned a reputation for manufacturing quality radio components, including variable tuning capacitors, slow-motion drives and precision mechanical assemblies.
The 358 series was developed in 1940 as a commercial venture, although wartime production was largely directed towards priority customers. Approximately 5,000 examples of the series are reported to have been produced, with the 358 and 358X being the most common versions.
Production was not without difficulties. During the Birmingham Blitz of 1940, Eddystone's manufacturing facilities suffered extensive damage. Production was subsequently transferred to a former open-air swimming establishment at West Heath, known locally as the Bath Tub.
Despite these extraordinary circumstances, Eddystone succeeded in resuming production of the 358 series, supplying communications equipment for Britain's war effort.
Wartime use
The 358X became associated particularly with Royal Navy communications, although Eddystone also supplied radio equipment for other government and military requirements.
In naval service, receivers were required to maintain reliable communication over considerable distances. High-frequency radio made it possible for ships to receive signals from shore stations and other vessels far beyond the range of ordinary line-of-sight communications.
The 358X could receive both amplitude-modulated speech and continuous-wave Morse transmissions. One reported difficulty with the receiver in naval service was radiation from its local oscillator. Excessive oscillator leakage through the aerial system could potentially be detected by enemy direction-finding equipment. This was a serious consideration aboard wartime ships, where radio emissions could compromise a vessel's position.
Contemporary restoration research records that the Marconi CR100/B28 replaced the 358X in some seagoing applications, with displaced receivers finding use at naval shore stations.
Frequency coverage and tuning
One of the most distinctive features of the Eddystone 358X is its interchangeable coil-pack system.
Instead of using an elaborate wavechange switch, Eddystone employed removable coil units containing the tuned circuits appropriate to particular frequency ranges. The complete range of ten packs provided coverage from approximately 40 kHz to 31 MHz, with a small gap around the receiver's intermediate frequency. The usual supply included coil packs B, C, D and E. Other packs could be obtained for additional frequency coverage.
This system had several advantages. Each coil unit could be designed and aligned for its own frequency range, reducing the compromises associated with complex waveband switches. It also permitted a very wide frequency range while retaining good sensitivity and stability.
There was, of course, one disadvantage. Changing bands required the operator to physically remove one pack and fit another. The system bears a resemblance to the famous American National HRO receiver, which also used interchangeable coil drawers.
Nevertheless, the arrangement suited professional communications work, where an operator might spend long periods monitoring a particular band rather than constantly changing frequency.
The receiver circuit
The 358X is a seven-valve superheterodyne receiver, employing a fairly conventional arrangement of RF amplification, frequency conversion, IF amplification, detection and audio amplification.
The incoming signal first passes through a tuned aerial circuit before reaching an EF39 variable-mu pentode. This valve provides radio-frequency amplification ahead of the frequency changer, improving sensitivity and helping to reduce unwanted responses. The amplified signal is then fed through another tuned circuit to the frequency changer.
An ECH35 triode-heptode valve performs the frequency conversion. The triode section operates as the local oscillator, while the heptode section mixes the oscillator signal with the incoming RF signal. The resulting intermediate-frequency signal is then passed to the IF amplifier stages.
The receiver employs two EF39 IF amplifier valves, operating at a nominal intermediate frequency of 450 kHz. These stages provide much of the receiver's overall gain and selectivity. Conventional double-tuned IF transformers are used between the stages.
The crystal filter is one of the most interesting features of the 358X. In normal reception, the receiver uses the conventional tuned IF circuits. When the crystal filter is selected, a twin-crystal arrangement provides a much narrower passband. This is particularly useful when receiving CW signals in the presence of interference from nearby stations. The narrow selectivity helps the operator separate wanted signals from unwanted transmissions. The presence of the crystal filter is the principal distinction between the ordinary 358 and the 358X.
An EBC33 valve provides signal detection, automatic gain control and initial audio amplification. The recovered audio is passed to an EL32 output pentode, which drives the output transformer. Connections are provided for headphones and an external loudspeaker. The receiver also incorporates a tone control, useful when adjusting the audio response to suit reception conditions.
For Morse reception, the operator switches on the beat-frequency oscillator, or BFO. This generates a signal close to the intermediate frequency, producing an audible beat note when mixed with the incoming CW transmission. The operator can then adjust the BFO for a comfortable Morse tone. The BFO is housed in a separate unit mounted behind the front panel.
The front-panel meter
An unusual and very useful feature of the 358X is its built-in meter. Rather than functioning solely as a conventional signal-strength indicator, it can be switched to measure the anode currents of individual receiver stages. This allows the operator or service engineer to check the operating condition of the valves.
A substantial change in anode current may indicate a weak valve, incorrect bias conditions or a component fault. The meter can also provide a rough indication of received signal strength by observing changes in RF amplifier anode current caused by the AGC system.