The Eddystone Model EC10

by Chris Arthur VK3CAE

The Model EC10 was one of Eddystone's very early solid state communications receivers, it employed all Germanium transistors and single conversion. Coverage was 550 kHz to 30 MHz in five ranges with a BFO making CW and SSB reception possible, a further compliment was a selective audio filter for CW. The EC10 was available here in Australia from around 1966, with the "slightly" improved EC10 Mark 2 being released around 1970. The EC10 had several variants during these years, most of which will not be found here in Australia. Some of these variations were minor cosmetic changes, however, options such as Long Wave or a crystal marker can also be found.

The later EC10 Mark 2 is easily recognized, it has an aluminum look decal panel and additional fine tuning control, there's also a carrier level meter mounted in the tuning dial. The fine tuning control on the Mark 2 was a quite an improvised modification, a varicap diode was added to the heterodyne oscillator with the tuning pot placed in the former EC10 head phone jack position. Obviously the head phone jack had to be moved, unfortunately the rear panel was the most likely position, this did make things a little difficult for head phone users.

As a communications receiver, the EC10 in comparison to previous Eddystone vacuum tube type receivers rated rather poorly. The main problem was due to the low hFE and signal to noise ratio of Germanium transistors, this gave way to poor sensitivity and receiver overload problems.  Another problem with the EC10 was that it lacked additional Pre-selection and being a single conversion receiver this resulted in imaging problems above 22 MHz. One also finds the recovered audio to be a little on the class B side, this is particularly notable during MW broadcast reception. The EC10 does however have most other Eddystone traits, the tuning mechanism is a joy to use with its flywheel like response, other controls are well laid out and the quality of finish was excellent. The analog tuning scale is very good, it has a rated error of 1% or 100 kHz in 10 MHz but I've found that most properly tuned units will display no more than 0.5% error across the whole dial.

To take the EC10 home in 1967-68 you would need to have parted with more than $200 Australian, rather a  hefty sum when you consider the UK price at the time of around 50.  I should at this point mention the Eddystone EB35, 36 and 37 models also fully transistorised, which at a glance could be mistaken for the EC10. These were a broadcast reception model and a closer look will reveal some of the following control differences. A power switch replaces the RF gain, the BFO is replaced by a tone control and the push button switch bank for filter, BFO and AGC selection was not included. The EB3x'ers also had different frequency ranges, with the inclusion of Long Wave from 150 - 350 kHz and a HF limit of 22 MHz however, there were also variations and options as with the EC10.

Basic Specs for the EC10 and EC10 Mark 2:

                                    Frequency coverage - 550 kHz to 30 MHz (in five bands)
                                    Intermediate Frequency - 465 kHz
                                    Sensitivity - <5uV for 15dB s/n ratio
                                    Image rejection - 50dB at 2.0 MHz, 20dB at 20 MHz

Download the EC10 and EC10 Mk2 Circuit Diagram in GIF format here.

Improvements and experimental modifications for the EC10...

One thing we should keep in mind is that the EC10 is now a vintage receiver, it has collector appeal.  Any-thing we do should really be carried out in a manner that preserves the original state, most of the following adheres to this. Things like replacing all of the Germanium transistors with Silicon devices could be done but this would be considered a radical modification. I have however, carried out some tests using a Silicon RF transistor as a replacement for the original Germanium transistor in the 1st RF amplifier... More on this later.

The first and most basic thing you should do is replace all of the electrolytic capacitors, as many older types of electro's are notorious for leakage. Make sure you observe operating voltage and polarity.

Poor BFO response
The BFO circuit was only intended for use with CW and even here a strong signal can cause problems, there's just not enough coupling. To fix this problem and make SSB reception possible, you need to change C-67 on the IF / AF board from 1pF to 10pF, preferably polystyrene. The result will be a greater level of BFO carrier coupling into the IF. You will need to use the BFO and tuning controls together as to much BFO carrier will swamp weaker signals, (keep AGC on) the fine tune on the later Mk-2 becomes quite useful here.

Mains power supply
A distinct disadvantage of the EC10 is the lack of an internal mains power supply, there is enough space to add your own but a few points need to be made. Current consumption with the lamps on is around 200 to 250 mA maximum at 9 Volts and regulation of this voltage is recommended as the internal zener regulator is slightly sensitive to input variations. I recommend the use of a small toroidal mains transformer, conventional I-E type transformers are larger and suffer EM field leakage that may cause "Hum" problems. To mount a transformer and supply circuit within the EC10, you need to make a replacement panel for the battery compartment. A point to note is that the EC10 is Positive Earth, this is not really a problem unless you try to run other Negative Earth devices off the same power supply, if the separate Earths come in contact, one has a short across the common supply. It's best to run separate Isolated (non-eathed output) power supplies, you can then share a common ground or "0 Volt point" without any problems.

Replacing the 1st RF Amp (TR-1) with a Silicon Transistor
After finding my EC10 without RF amplification I tried an experimental mod as a temporary replacement for the dead OC171. If you look at a circuit diagram of the EC10, you will find that it employs a rather unique common Base configuration for the first RF Amp. My guess is that this was done in order to make best use of the OC171's somewhat limited Hfe characteristic, in any case sensitivity still falls off at higher frequencies. If you intend to do some SWL-DX  and don't care about maintaining originality, you can pep things up a little on the higher frequencies by replacing TR1 (OC171) with a later Silicon PNP RF transistor.   In using a Silicon transistor over a Germanium device there will be a instant improvement in signal to noise but we just can't pull out the old and stick in the new. We need to consider Bias and other DC conditions, a Germanium transistor does of course function in the same manner as a silicon device in that it amplifies current but the DC conditions are a little different. We need to rework the resistor values to suite our replacement silicon device.

In this case I used a "2N2907" PNP Si device, simply because I had about a dozen of them in my collection. You can get an equivalent  "PN2907" from Dick Smith Electronics here in Australia without any problems...

Resistor changes are as follows:

        Replace Bias resistor R1 (68 K Ohm) with a 33K 1/4 Watt resistor.
        Replace Emitter degeneration resistor R3 (470 Ohm) with a 220 Ohm 1/4 Watt resistor.

On completion and testing you should notice an immediate improvement in sensitivity at higher frequencies. You can further increase gain be reducing R3 to 150 Ohms but this will more than likely result in AGC overload at lower frequencies. There is a trade off here, with increased RF gain you also increase the likelihood of imaging problems above 20 MHz. It would be better to obtain gain in the IF and to do the job properly all of the Germanium transistors should be replaced. Additional Pre-selection would reduce the imaging problem considerably but so far I haven't gone there, if you have please let me know how it went... A final note is that I found another OC171 in original packing and returned my EC10 Mk2 to the all Ge line-up.

Chris VK3CAE (Ex VK3JEG)