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Old 4th Jan 2023, 7:31 pm   #1
Join Date: Jun 2020
Location: Manchester, UK.
Posts: 115
Default Eddystone S640

I restored an Eddystone S640 communications receiver last year and thought I’d finally better write it up as there were a few interesting points. I have some photos too which I’ll add if I can figure out how.

It all started when I spotted a tatty looking Eddystone S640 on eBay. It looked a bit sorry but the ‘640 was the first receiver I ever owned when I started as a short-wave listener at the age of eleven so I just had to get it. You know how it goes…

Anyway, before bidding I magnified all the photos provided and scrolled around carefully to check all the valves and major components were present and looked okay – a good tip. Apart from a couple of the valves looking like the metallic coating was flaking off everything seemed as I remembered it.
To my amazement nobody else was interested and I got it. I wonder if they all somehow knew how much work it would be...

Once the set arrived I had a good look at it. The plastic tuning scale had at some point been varnished and this was now all flaking off and looked horrible. That would have to be taken out and cleaned up. Also the crystal phasing switch and variable capacitor had for some reason been disconnected, the wires were still there but not soldered on – odd. Hard to see clearly what was going on here but the front panel was going to have to come off anyway to sort the dial out so I decided I’d have a look later.

First off I tested the mains transformer, choke and output transformer for continuity and insulation. All seemed ok so I powered up with the rectifier removed and confirmed the heaters and dial lights worked ok. So far, so good.

I fitted new HT smoothing caps beneath the chassis, leaving the old can in place for appearance, and also replaced the few other electrolytics I could see – there aren’t many in the set.

Next up was the audio sub-chassis. Nice of them to fit the audio section on a small removable sub-chassis – that you can’t actually remove without unsoldering and unthreading a bunch of wires that go all over the rest of the set. Anyway, with the sub-chassis out I replaced all the paper caps, another electrolytic and a couple of resistors that had gone high. THAT capacitor (the one that feeds the grid of the AF output valve) was a big chunky mica one and it tested fine so I left it.

Having checked there were no shorts on the HT line I dug out my lamp limiter and variac and gently woke the set up. It sort of worked – I could receive stuff on all three ranges but not particularly well, and there was a heterodyne with every signal that I assumed - wrongly - was the BFO that was stuck permanently on. Also cranking the RF gain up caused the set to oscillate. I decided – again wrongly – this was probably due to the failed metallic coating on a few of the valves.

Anyway, it was a worker, so off came the front panel, dial glass, pointers and tuning scale. I now know this is not trivial! If you’re going to do this, do a search on the forums and online, make notes and take photos. Taking things off the wrong way will bite you later and getting it all back together properly is even more fun. Anyway, the varnished tuning scale cleaned up okay with a combination of IPA, a toothbrush, a pencil rubber, my fingernail and a fair bit of patience and care.

With the front off I could now see that the crystal filter had been completely bypassed with a wire jumper. Hopefully that wasn’t a sign that the crystal itself was damaged or missing but we’ll see. Even if it was, the set would still work with the switch set to ‘out’ so I removed the bypass wire and reinstated all the other connections.

It was then that I realised the heterodyne I heard earlier definitely wasn’t the BFO, because the BFO capacitor was knackered - the plates were touching. There are two circlips that hold the shaft in the proper position against the spring and the front one was missing. After rooting fruitlessly through all my junk I made one from the bottom of a tin can and it actually worked.

After putting the front panel back on I tried the set again and got similar results plus the BFO didn’t work at all. Took the BFO can off, a few waxy caps to change in here and a burnt out anode resistor, maybe a result of that shorted BFO capacitor? Sorted all that out and the BFO now worked but the set was still unstable.

On to the main chassis then. Swapped all the waxy paper caps I could see and a handful of dodgy resistors. That didn’t fix it. The only remaining place was inside the coil box, and in there were a couple of waxy caps I could easily get to and three more that were properly buried way down the bottom under other stuff and would be pigs to change. I swapped the two easy ones and hoped I wouldn’t need to do the others. Wrong again.

I still had an unstable set. With the RF gain control way down and a strong signal from my sig genny it worked normally. With the control above about a third I got heterodynes each side of the carrier and with it up beyond half way the set took off and howled like a banshee.

I tried swapping the valves with the suspect metallizing into less critical locations and then wrapped them in tinfoil. This helped slightly but didn’t fix it, so it was time to bite the bullet and do the coil box.

The only way to reach those caps buried way down in the guts was to carefully disconnect and/or remove the components above, and to use haemostats, long pliers, wires with hooks on and a big light and a magnifier. There were three caps to do, and the third and final one – a screen grid decoupling cap - revealed the problem. While disconnecting one end of the screen grid resistor with the intention of moving it aside, to my surprise the entire resistor fell out along with the pin from the valve holder. Examination revealed the wire had never been soldered to the tag – just pushed through the hole with some solder around it that had failed to flow. It must have been like that right from the factory. Replaced the capacitor, put the pin back in, soldered it all properly and bingo! All instability was gone.

Next to see if the reconnected crystal filter works and check the alignment. It did work, but the alignment was WAY off, particularly in the IF. Everything was becoming clear now, this set had always been unreliable and previous owner(s) had tried mods and twiddling but never got to the root of the problem because it was buried deep in the coil box where even I had not wanted to go.

With the set aligned it now works really well, much more sensitive than the one I remember having. It’s a joy to use, and with the case resprayed and the knobs and switches cleaned up it looks and feels almost like new.

If there’s a moral here it would be don’t give up – and don’t make assumptions!
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Old 4th Jan 2023, 8:29 pm   #2
G6Tanuki's Avatar
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 12,444
Default Re: Eddystone S640

Nice work! Good to hear you got to the bottom of the instability-problem.

I guess Eddystone's QC was a bit questionable at times: I've got an 840A here [bought at the Newbury rally a decade back] that didn't work on the highest-frequency range; investigation showed that the LO coil had one winding where the soldered termination of the winding to the tag on the end of the former still had the enamel on the end of the winding.

It could _never_ have worked on that band! So how did it pass any sort of alignment/test?

Not complaining though, its upper-HF-bands uselessness probably explained why I got it cheap!
"Acht Nul Noyyun Zwo Funnuf" -Magdeburg Annie.
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Old 4th Jan 2023, 10:04 pm   #3
Join Date: Jun 2020
Location: Manchester, UK.
Posts: 115
Default Re: Eddystone S640

Agreed. I've learned now that although things are built right most of the time you never can rule out a manufacturing defect, even if the set is older than you are!
Here's a photo of the offending tag.
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Old 6th Jan 2023, 2:05 pm   #4
kan_turk's Avatar
Join Date: Mar 2014
Location: Dublin, Ireland
Posts: 378
Default Re: Eddystone S640

Great job - fair play for your perseverance - If only we could always follow our own advice

Happy New Year to all

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