The Eddystone 909A is a 7 tube AC/DC single conversion communications superhet receiver having a single stage of RF amplification and two IF stages designed specifically for marine use in the band 1.6 to 4.7 Mcs. This coverage is split into two switched ranges and rapid selection of the HF Distress and Calling Channel (2182 Kcs) that may be relayed via a low impedance output to a monitor speaker located on the ship's bridge.

The RF section, embedded into a cast alloy chassis (see the 4th photo above), employs a 12BA6 vari-mu RF pentode from the aerial via a bandpass coupled tuning circuit which is composed by two ferrite inductors. The primary inductor has the antenna coil and the input signal tuning coil, the secondary inductor has the output signal tuning coil. This doubly tuned signal is injected to the RF amplifier. This type of coupling provides an extremely high degree of image attenuation, likely to be used in close proximity to stations radiating strong signals on or near the image frecuency. Precautions have been taken to ensure that the aerial will not become "live" through direct connection to the chassis, while static "build-up" in the aerial during electrical storms is fed to earth via the two static "leaks", the resistors R1 and R2 (2.2 Mohms each one). AGC is applied to Antenna and Mixer stages, both form the RF section.

IF section: Output from the Mixer stage is taken via the first IF transformer to the first of the two IF amplifiers. 12BA6 vari-mu RF pentodes are employed in theses stages both of which are fed from the AGC line. The input to the first IF tube is switched to introduce a crystal bandpass filter which provides good adjacent channel selectivity when reception is marred by strong signals close to the desired channel. The second IF stage feeds the two diodes of the tube 12AT6 doublediode-triode. One diode is used as signal detector in conjuction with one metal semiconductor rectifier which functions as a series type noise limiter that may be taken out of circuit when noise limiting is not required. The other diode is fed via the plate of the last IF amplifier rectifies the incoming carrier to provide AGC bias, that will be delayed by the voltage drop across the cathode resistor of the triode portion used as an AF preamplifier.

AF and PS section: Output from the signal detector circuit is applied to the AF Gain control which feeds the triode portion of the 12AT6 which is resistance capacity coupled to the tube 19AQ5 that loads the output transformer. The power supply circuitry is basically conventional for an AD/DC receiver, having the usual half wave rectifier and series heather chain. However, to allow the receiver to be operated from external HT and LT supllies, a link system is incorporated so that the heaters may be changed to a series/parallel network, and at the same time the internal HT rectifier/smoothing filter are taken out of circuit to simplify connection of the external HT supply.

Mods & restorations (mouse-over to zoom)

This unit is model 909A/2 that uses a transistorized power supply (Pics #1 & 2) fitted internally to allow direct operation from a 24/28 VDC ship line voltage source. For home use the radio was restored to the standard model 909A by removing this power supply, also the 8-way Alpha connector was replaced by a normalized octal connector and a power switch installed aside (pic #3 and #5). Filaments are connected in series via the voltage link board of the radio, voltage to filaments is reduced by means of a 968 ohm ballast resistor (pic #3). HT voltage is obtained directly from the 230 VAC mains, a silicon diode followed by a 68 ohm resistor (pic #4) reduces B+ voltage to about 250 VDC at the input of the smoothing filter, giving 230 VDC at the output, also this resistor protects the rectifier diode of the input electrolytic capacitor peak charge. A 100 Kohm load resistor (pic #4) was added to provide a fixed load in order to control the peak voltage accumulated into the output electrolytic capacitor while filaments are still cold. Two leaky paper capacitors were replaced. The receiver manages very well voltages from 220 to 240 VAC, and this restores the 909A standard AC/DC mains system without voltage selector, useless nowadays.

This receiver is now back to life, but its usefulness is minimal, it is a simple AM receiver of the old marine frequencies for fishing boats equipped with a cristal filter and noise limiter to be used mainly on MCW (CW code tones AM modulated) and phone communications. For collectors only, but it is very interesting to analyze its circuital design because of the way in which the band filter systems have been added, both in the front-end and in the intermediate frequency, very straightforward. And like all the classic Eddystone models, bomb-proof construction, you will never see this type of construction again.