Eddystone 940
Written by Bryce Ringwood   

The Eddystone 940 was (probably) designed for English gentlemen to listen to foreign broadcasts from the colonies. They may also have been used as marine receivers in some branches of the UK maritime services. It is a straightforward single superhet receiver with a the following valve line up:

ValvesEddystone 940

ECC189 - 1st RF Stage
6BA6/EF93 - 2nd RF Stage
ECH81 - Mixer
EC90/6C4 - Local oscillator
6BA6EF93 - 1st and 2nd IF
EK90/6BE6 - Product Detector
ECC82/12AU7 - 1st Audio
EB91/6AL5 - Detector / AGC
EL91 / Approximately 6AM5 - Push Pull Audio
GZ34 - Rectifier 
VR150/30 - Regulator

Controls

Tuning             - Combination friction/gear drive giving a 110:1 turns ratio. Flywheel assisted.
Wavechange - 5 Ranges .5 to 1MHz, 1 to 2.4MHz, 2.4 to 5.5 MHz, 5.5 to 12.7 MHz and 12.5 to 30MHz
RF Gain and AF Gain (Volume) controls
BFO Pitch for CW or Single Sideband reception
Selectivity       - Max (12kHz), Min (7kHz), Crystal
Crystal Phasing - In the crystal position, provides a movable "notch" to filter out unwanted CW signals
Noise Limiter
AGC ON/OFF
AM/SSB switch
Standby (For use with a transmitter. Quiets the set when someone else is talking to you)
Carrier Level meter - calibrated 0 to 10
Logging scale

I purchased this set because it looked as if it was in good condition, but when I got it home received an unpleasant surprise. Here's the background to the story. 

On removing the cabinet from the chassis, I saw that all the IF transformers had been "beheaded". I removed each one to find that someone had tried to implement a strange "variable coupling" scheme using what (sadly) looked like Eddystone EC10 transformer windings linked to threaded rods. 

I suppose I should have returned the set and demanded my money back. What I actually did was to attempt to restore the IF transformers back to their original state by disassembling some transformers in my junk box and gluing the formers on to the plastic base of the original coils. In their original design, Eddystone had used very large values of capacitor to tune the coil of the transformers to the correct frequency. This resulted in comparatively few turns (about 120). In theory, then, making coils to tune to the intermediate frequency should not have been particularly difficult. There are, however a few complications, such as, how tightly were the coils coupled ? – plus Eddystone had also put tertiary windings in the coils to broaden the bandwidth in the 'Min' position. As can be imagined, getting all these adjustments correct to bring the set back to its original specification is well-nigh impossible, but I have managed to make the set operate, probably something like it originally did (I will never know) and certainly it works very well.

As a matter of design, I can only guess that having such physically small coils makes it easy to get a higher 'Q' (quality factor). I used really good silver mica caps for tuning - something to ponder.

The only other problems with the set were a defective 100k AGC resistor (a common fault) and parasitic oscillations of the first RF stage – cured by putting anode stopper resistors in the offending ranges. This latter fault needs some re-examination – maybe the the grid bias voltage is not correct, but another one of my Eddystone's had this problem from new. Sometimes its a problem with the aerial matching. A previous owner had also replaced the ECC189 vari mu triode with an ECC88. The RF cover was missing and I made a new one from aluminium.

I can see that the owner may have wanted to experiment with this radios IF stage, because it is said to have poor "skirt selectivity" - in plain english, this means that for strong stations, the bandwidth of the receiver will appear to be much broader than that stated in the specification. There was an article in "Short Wave Magazine" on how to improve the 940. I really didn't like that article one bit. This article also suggested removing the ECC189 and replacing it with the ECC88. All you will achieve is degraded AGC performance.

So, here is an example of a radio that actually works very nicely, but because of all the modifications has absolutely no value whatever. It does look good though.

Review

It is not possible for me to give a fair review of this set, but it has some very good features. Because it is a valve set, I can keep it plugged in, since it won't get too badly damaged by nearby lightning strikes.. The dial scale is just about the easiest to read of any set and the radio is very sensitive up to about 28MHz, where a problem (maybe unique to this set) causes a rapid fall off.  Reports on the audio quality vary, but I would side with those who see it as "adequate", rather than good. The noise limiter distorts the audio quite markedly. The receiver has the "expected" amount of frequency drift, but it will resolve SSB on 15 and 20m. (No doubt aided by the excellent tuning mechanism.) The two RF stages are effective at preventing image interference. This radio is in daily use because of its simplicity.

If you are considering buying one of these sets, please be aware that it is likely to have been modified to an even greater degree than my unfortunate purchase. You need to inspect it very thoroughly first. The RF section is rather difficult to get at, but other than that, servicing is straightforward.


 

 
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