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Eddystone 870A radio (1961)

Not knowing what it was, I bought this interestingly different radio in 2000. Though it was heavy it did not contain a mains transformer. I fixed it and found out about Eddystone, a brand of communication receivers and lesser shipboard radios.
In The Netherlands, you don't see that many English valve radios. In the valve era, most of the radios sold here were Philips or one of the brands they owned. So I was pleased to find an English radio to add to my tiny collection. According to the label on the back it has been manufactured by Stratton & Co, Ltd in Birmingham. The model indication is “cat. no. S870A”. It has 5 bands that cover the whole frequency range from 150 kHz to 24 MHz.

inside view

A view the inside the Eddystone 870A.

The radio has been built quite compactly. On the front, there are three controls: one for volume/on/off, a larger tuning knob and a band switch. It has a simple design: an ordinary superhet, 110 V AC/DC supply with series resistors for 210 and 240 V operation. The valves are all miniature (7-pin) types: The valves all have 150 mA heater current and are series connected. Here is the schematic diagram I have drawn for it.

At first, this radio puzzled me. Although it is an AC/DC set, it is not a cheap radio. Usually, radios are built as AC/DC sets to save the cost of the mains transformer. But this is a solidly built radio with a good tuning mechanism. It looks like a piece of professional communication equipment with its two handles at the front and its heavy metal case. If you look closely at the tuning dial, you see that the 5 bands give complete coverage from 150&bsp;kHz to 24 MHz, not just the broadcast bands. The tuning mechanism has a gear mechanism that allows precise tuning with a vernier scale in the center to note the exact position of a station. The valves are kept in place by springs and clips, indicating that the designer expected some shocks and vibrations. Because, unlike normal household radios, its exterior design seems to largely ignore trends and fashion in furniture design, it was a bit hard to estimate its age.

bottom view

A view from the bottom.

On the left, you see the trimming capacitors of the various frequency ranges. On the bottom, there are the two mains fuses, the mains filter coils and capacitor. You can also see the fly-wheel behind the tuning knob.

The explanation why a professional radio would be built as an AC/DC set was given to me by Gerard on the Dutch Forum on Old Radios. He has an Eddystone model 670 in his collection and explained that these radios were designed for shipboard use, where 110 V DC on-board power was quite common. Hence the need to design the set for DC power. This also explains its sturdy construction and professional look. He pointed me to some information on Eddystone on Malcolm F. Bennett's Vintage Radios UK pages. Looking for more information on these radios I found the Eddystone Classic British Receivers pages by Alan Clayton, a fan and radio amateur. Here I found the information to date this radio. Later on, in 2008, I found there is also an Eddystone User Group web site. that also contains a lot of interesting information and links to my site. I decided to return the favour.

The metal case is dangerous on an AC/DC radio, although the frame is isolated from the case by pertinax and rubber mounting parts. Imagine a small metal object would fall into the radio and short the frame to the case... electrify your family members! Unlikely, though, because the circulation grids in the case have very small holes. A safety earth must be to connected to the metal case to safely operate this radio. It would not have been approved in Holland at the time. Well, it was meant for Britain, of course, where electric safety standards since many years require an earth connection on all sockets and electric equipment, unlike the Dutch home wiring standards that put the emphasis on splendid isolation.

I bought this radio on an Internet auction. It looked a bit used, some of the paint was chipped and some lettering on the tuning dial had dropped off. The owner told me he it had been stored on a damp attick for many years. This also explains the rust spots on the case and the inside. He told me he thought this had been a radio for shipboard use. At first I doubted this but this but I changed my mind after learning more about Eddystone.

I was told the radio did not work. As I opened the case, one obvious cause immediately came to light: the 12AT6 had white getter and it had a crack. I found a new 12AT6 and replaced the faulty one. This brought life to the radio. I was able to get a number of stations but the sound was distorted. Checking the output stage pointed out that the coupling capacitor between the AF pre-amplifier stage and the output stage was leaky. Using my DVM on the grid I got a reading of as much as 6 V!

Replacing the capacitor improved the quality of the sound. But it seems that the output valve is soft or worn out. Its cathode bias voltage is only 4 V now so it has less than 20 mA of anode current. Maybe extended use with the leaky capacitor has caused an exhausted cathode. Anyway, the sound is acceptable as I do not need a loud radio.

When trying the radio on the SW bands, I was not impressed by its performance. I only got a few strong stations. Their position on the dial was off, too, so I concluded that the radio needed trimming. The AGC voltage was only 2 V on even the strongest stations.

First I tried some replacement valves for the mixer and IF amplifier. This made no difference. Listening more carefully, started to realise that a BBC station I received on the LW band was out of place, and was actually an image of the 648 kHz BBC transmitter. So my Eddystone was really out of callibration!

Looking closely at the IF transformers, I saw that the cores were damaged. Somebody must have tried to "trim" this radio with the wrong tools. I decided to first measure the IF passband. This was bad: there were two peaks instead of one.

So I tried to align the IF stages again. But this didn't work out. The tuned circuit of the detector transformer didn't have a clear maximum. Was it because of the damping on the circuit? At one stage, one of the cores fell out and I knew why the alignment was so hard: it was broken and half of it was missing. So there wasn't enough core material to tune it! Searching through my junkbox, I found some old IF transformers that had cores the same size, only different thread. But they fitted. I tested them in a spare coil, to know which cores had the same effect on the coil's inductance. After fixing the IF, I set the oscillator and preselector circuit coils and the radio was playing well again. I've been listening to it from time to time since then, and it's still working allright.

Copyright © 2000-2003 by Onno's E-page         published 2000-10-31, last updated 2008-10-05