Eddystone Communications Receivers

 The Eddystone Model 670A

 

  I bought this very heavy all-metal receiver from a chap in Christchurch. The model is the "A" version dating from 1955 and is not to be confused with the basic 670 which appeared in 1948. The set is interesting as it's designed for use on AC or DC mains and does not use an isolating transformer. I imagine this "feature" must have resulted in many mishaps over the years as the internal chassis can be at mains potential. A study of the mechanics reveals that the outer case is isolated from the inner chassis by numerous fibre washers and it's highly unlikely that this type of design would pass muster now because of todays much more stringent electrical safety standards.

See the earlier Eddystone 670

The mains plug is a two pin affair which can easily be wrongly wired or inserted and woe betide the careless engineer connecting a signal generator ground connection to the chassis as there could easily be 240 volts AC between them.

With the case removed, leaning one's hand on the chassis could have unexpected results!

In my example the loudspeaker cone was damaged so I fitted a nice new unit from an old Hitachi speaker. Unfortunately there wasn't enough room for the magnet to fit easily so I bent part of the chassis an eighth of an inch or so to make it fit. I then realised that the speaker coil is at mains potential but the metal speaker frame is at front panel potential. As there was only a gnat's whisker space between the speaker frame and the chassis the new arrangement was decidedly dodgy so I've resolved to repair the old cone and refit the original speaker.

When I connected the receiver to the mains via an isolating transformer it slowly came to life. Checking with my signal generator revealed that the IF transformers weren't tuned too far out, and after resetting the trimmers on each of the four bands, performance was quite lively.

As the set has been stored in a damp environment the condition of the case has deteriorated somewhat so proper restoration will be shelved for the time being. To fix the paint finish to its diecast front panel and case may require sandblasting before repainting.

   

 The pictures above show the clearance between the speaker magnet and chassis between which may be 240 volts AC. Note the "Westector" type HT rectifier and the ballast resistor for handling unwanted surplus heater voltage.

Below: Showing the trimmer adjustment holes for the four bands and right, stuck to the underside of the case is most of the original receipt from Webbs Radio showing that in 1955 it cost about £15. I paid £10 in July 2000 and at todays "values" the original price would have been something like £300.

 

 
 

 

The Eddystone S740

 

 

 

 This finely built receiver dates from 1950

Although Eddystone produced many designs of receiver from before WWII to after the end of the valve era all had the hallmark of good mechanical engineering design. All had superb slow motion tuning dials with beautifully smooth flywheel action. Not long after this model was produced the dial was redesigned into a full-width horizontal shape. In use this is a more pleasing type to use but was more complicated as it meant that a dial cord and pulley system was needed as well as a geartrain. The fine-division logging scale in the newer version was moved to a more convenient central position instead of being an offset dial as in this model

 

Unlike several Eddystone receivers, this model uses a mains transformer rather than being designed for AC/DC use.

Coverage:

Three short wavebands plus medium waves

10.5MHz-30.6MHz; 3.7MHz-10.6MHz; 1.4-3.8MHz

and 205-620 Metres (which is wider in coverage than usual to include the low frequency shipping band)

 

 I think someone replaced the front escutcheon plate, hence no markings under the knobs?
 

 

 Rear view through the top cover towards the rear

 

 View showing the anti-backlash gears apparently made from Tufnol or Bakelite

 

 View towards the front panel through the top

 

 See the instruction manual for the Eddystone S740

 Eddystone EC10MkII

 

 Compact little communications receiver with the usual high quality tuning arrangements always found on Eddystone sets.

It covers the following bands: 550-1500KHz; 1.5-3.5MHz; 3.5-8.5MHz; 8.5-18MHz; 18-30MHz

with Filter, BFO and AGC switch to aid reception

Click to see the EC10 brochure

Click to see the Eddystone EC10 circuit diagram

 

 The outer case is detached by removing four chrome-plated screws and pulling it away. You can't pull it off completely without first unplugging the rear-mounted power supply module seen further down this page. Above you can see the very neat layout of tuning coils (an aligner's dream).

 

 There are two main circuit boards. The six RF transistors might pose a problem because this type, the OC171 can develop gradually worsening internal shorts. Other transistors used are an OC71 and three OC83. The detector/AVC diode is an OA90 and there's also an OA70 diode and an OAZ203 zener diode.. all diodes and transistors are germanium-based with the transistors all PNP requiring therefore a negative supply to their collector circuits.

 

 Above you can see the power connector. This is spectacularly poor, having two mains connections (for the on/off switch) plus DC output for powering the set, but no polarising so it could be inserted the wrong way round by a careless dabbler. How does one know which way round it should go? Simple.. there's a yellow sticker on the power supply which corresponds with the end yellow wire.
 

 A BNC aerial socket is now fitted

 

 My example of the EC10 has a mains power supply, fitted at the rear of the receiver case. It uses an SEL M160P selenium rectifier and conveniently the transformer is tapped for 110 volts, although not a modern type which saves on copper by paralleling two windings. This one just has a tap half way down the primary winding meaning that twice the capacity of current is accommodated than that really needed. You can see below a zener diode (OAZ203) between the two blue condensers with the wirewound resistor serving to reduce hum as well as limiting the zener current. Very basic, which I like, as there's less to go wrong. The power supply is the Achilles Heel of equipment newer than this, being the most unreliable part, not to mention their designer's blatant disregard for a noise-free radio spectrum.

The circuit diagram shows the battery-powered version of the EC10. That has the same 4-way connector and the designer's clearly didn't consider all eventualities in making the transition between battery and mains. What could have been done to make things foolproof? Looking at the circuit.. the on/off switch in battery mode connects the battery positive output to chassis and the battery negative output to the set's supply rail. In mains operation the PSU positive is connected to chassis via a 500mA fuse and the mains transformer primary return via a 50mA fuse to the mains supply. The result of turning on a set with a reversed connector now depends on how the mains plug is wired. You can see below that mains earth connects to the PSU chassis which is fine so long as the mains plug has this wired. Live mains connects to the receiver chassis via the 50mA fuse and this will blow if mains safety earth is present. The mains return through the transformer will connect to the set's power supply rail. If the on/off switch is not in the on setting the sets chassis will now be at mains potential if the safety earth is missing. If the safety earth is OK the fuse will blow immediately. If the on/off switch is already on and the safety earth is missing an unsuspecting user will get a shock under some circumstances but the fuse should blow before he expires. It's interesting to look at the white mains cable below. Is this the original lead or has it been fitted retrospectively? If it's a new lead, was the original a two-conductor red/black mains lead? If so then of course there never was a mains safety earth. Maybe another EC10 owner can shed some light on this?

 
 I decided to just reassemble this receiver rather than check the alignment because I had other things to do. Firstly I discovered one of the two captive nuts for attaching the power supply had been glued into place and attempting to fit a screw removed the captive nut. I found a large washer and a screw and nut to match the single knurled screw that was in good order and refitted the power unit to the case. Then I discovered the internal power lead was only just long enough to fit once the outer case was fitted. Thinking laterally I lifted both the case and the chassis and found by offering the two at right angles I could just fit my hand into the space and was able to fit the plug (yellow wire to yellow marker), then holding the case in one hand and rotating the chassis with the other was just able to slide the chassis into the case. I'd previously plugged in the mains lead and the set seemed to pick up lots of medium wave stations so hopefully the old OC171 transistors are OK...

 

 See the Eddystone 770R Receiver

 See the Eddystone S640

 

 See the Eddystone 870A

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