Eddystone model EC10 Communications Receiver

serial No C01769


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Introduction

The model EC10 receiver was one of Eddystone's very early solid state communications receivers, which was first introduced in 1963. It employed ten Germanium transistors (OC171 X7 OC83 X3) and three diodes. It is compact, well built and intended for operation from its own internal battery supply or from a mains-operated power supply unit. Five ranges give continuous coverage from 550 kHz to 30 MHz, and included are the medium wave broadcast band, the marine band from 1500 kHz to 3000 kHz, all the short wave broadcast bands and six amateur bands from 160 metres to 10 metres.

The receiver accepts normal AM signals and CW telegraphy, a special filter being provided to increase selectivity in the latter mode. Although not designed for single sideband operation, signals in this mode can be received by appropriate setting of the BFO for carrier insertion. Features standard to all Eddystone receivers are incorporated. The flywheel-loaded tuning knob controls a finely engineered gear drive with a reduction ratio of 110 to 1, resulting in smooth precise tuning. The main scales occupy a length of nine inches and are clearly marked directly in frequency to an accuracy of calibration within 1%. Tuning to a given frequency is therefore relatively easy and an auxiliary logging scale permits dial settings of preferred stations to be recorded for future reference.

Alternative aerial sockets are provided allowing the use of a really good aerial system where circumstances permit, or of a short rod or wire where nothing better can be arranged. An internal speaker is fitted and a telephone headset can be used where preferred.

Power is derived from six U2 type cells housed in a separate detachable compartment. Current drain and hence battery life is dependent on the audio output. For long life, it is recommended that the HP2 heavy duty type of cell be flitted.

(OC171 X7 OC83 X3)


I have had this radio for nearly 20 years and have switched it on only very rarely. Recently (2020) on powering it up I discovered that a couple of the OC171s were playing up resulting in great crackles and eventual near silence. I have now snipped all the transistor screen leads and it now plays daily in my workshop.

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Underside RF and oscillator stages

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Upper board IF BFO and output stages

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Power supply Cat No.924 delivers 9 Volts at up to 200 mA

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Amended First stage RF amplifier as suggested Chris Arthur VK3CAE

Replacing the 1st RF Amp (TR-1) with a Silicon Transistor

Chris Arthur VK3CAE tried an experimental modification as a temporary replacement for the dead OC171 connected in Common Base configuration. This was to make best use of the OC171's somewhat limited Hfe characteristic which falls off at higher frequencies. In using a Silicon transistor over a Germanium device there will be an improvement in signal to noise but the DC conditions will be a little different. He used a 2N2907 PNP Si device and changed the bias resistors as follows:
Resistor R1 (68KΩ) with a 33KΩ. Resistor R3 (470Ω) with a 220Ω.

I used a 2N2907 as suggested and shunted the existing resistors with a 68KΩ and a 470Ω rather than removing them from the board to achieve the same result.


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