Eddystone EC958

The eddystone EC958 is a high stability high performance communications receiver first manufactured in 1972. There are a number of variants becoming more sophisticated as time went on. Here I am going to describe the most basic version (which is still fairly complex).

Overview

The receiver covers the frequency range 10kHz to 30MHz in ten switched bands. The upper four (30MHz down to 1.6MHz) are special in that they employ incremental tuning. Two modes can be used selected by a switch under the tuning dial:

Continuous Tune -- the main tune and incremenental tune work as a conventional pair, the reading on the incremental dial is relative to that of the main tuning.
High Stability -- the main tuning locks to 100kHz separated points (locking being indicated by illumination of the `High Stability' lamp). The incremental tuning is then absolute with reference to the main tuning position.

On the lower bands (i.e. 10kHz up to 1.6MHz) the tuning is continuous and the incremental dial does nothing.

Variant Models

The basic set described here is the EC958, but Eddystone produced some variants. This list should start to describe them. Click on the images for a larger version of the photograph.

**EC958 Models**
Model	Description	Block Diagram
EC958	Basic Model	Block Diagram
EC958	Basic Model (slight fascia changes)
EC958/1	40db Attenuator
EC958/7	Digital LED kHz display, 40db Attenuator	Block Diagram
EC958/9	ISB version of EC958/7

Circuit Description

The following information is based on drawing out diagrams of the set as (until recently) I didn't have a handbook.

Alignment

Turret Alignment

Each setting has two parts, a capacitor and an inductor. The trimmers are adjusted at the high frequency end and the inductors at the low frequency end. First remove the cover from over the turret (with the disc names marked on it). For discs other than A, the trimmers are accessible at an angle of about sixty degrees to the vertical. For disc A there is a hole at the back of the turret disc enclosure through which a tool may be passed. The order of adjustments is as follows:

Main Tune Oscillator (disc D)
Signal Frequency Circuits (discs A to C)
Harmonic Selector Circuits (discs E to F)
Tuning of Harmonic Selector L1

*Alignment Frequencies*
	Alignment Frequency		Adjustments
Range	Trimmer (MHz)	Core (MHz)	Disk "A"	Disk "B"	Disk "C"	Disk "D"	Disk "E"	Disk "F"
1	29.0	20.1	C380/L36	C400/L44	C420/L54	C451/L64	C480/L74	C490/L78
2	19.0	10.0	C383/L37	C404/L45	C424/L55	C453/L65	C484/L75	C494/L79
3	9.6	4.1	C386/L38	C405/L46	C425/L56	C456/L66	C485/L76	C495/L80

Spice Models

When testing a faulty receiver I used some spice models to locate the fault by comparing theoretical predictions against actual observations.

Main Tune Oscillator

This appears to produce wonky waveforms on the low switch ranges! However this is exactly how it should be, but unfortunately Eddystone never included waveforms in the Workshop manual, only DC voltage measurements. The waveforms can be observed on the rear pannel socket (especially if the synthesised working link is in place). The diode in the circuit biases the MOSFET by rectifiying the waveform and cutting the MOSFET further off as the amplitude of the oscillation increases. The SPICE model of the oscillator, using a 1N4448 diode (as per the original design), can easily be modified to show that there is no perceptible change in using a 1N914 (readily available from maplin).

Photographs of Scope Traces

These photographs were kindly sent to me by Barry Smith (G4IAT), who took them using a digital camera. They illustrate the distortion observable in the sine wave.

*MTO Waveforms*
Range	Similar Ranges	Scope Trace
Range 3	Range 1,2 and 4
Range 6	Range 5
Range 7	Range 8
Range 9	Range 10

Injection Level into RF compartment

These measurements were made from a correctly working EC958 set by G4IAT. They agree with the values I measured after carring out the necessary repair work on my set.

**MTO Injection Voltage**
Range	Voltage (Peak-Peak)
1	0.3
2	0.65
3	1.3
4	3
5	3
6	5
7	3.5 (top clipped)
8	4 (top clipped)
9	6 (flat top and bottom)
10	6 (flat top and bottom)