Eddystone 870 radio question.

[turretslug]

Originally Posted by stuarth

I can see both sides to this.

As to the suggestion that the OP buys an AC only Eddystone, I would point out that the 870 is much smaller than most Eddystones, and has LW for Radio 4.

This is so- years ago, I more-or-less fortuitously acquired a nigh-on immaculate 750 from someone living close to me- fortuitously as in not really having considered Eddystones before -and was quite impressed at the neat and sturdy construction. I thought that it would be nice to see if Eddystone did a basic-controls, "all-wave" AM valve broadcast set that included LW as the casual listening/AM tuner source but quickly realised that there was a choice between the bare-bones AC/DC sets or, at the other extreme, the heavy, deep-case, collectible and complex 830/4! The latter not including MW to boot, nor the also heavy, deep-case and collectible 850- nice radio but definitely a one-trick pony.

Luck threw up a 670A nearby, apparently the owner had advertised it every now and then for years with no joy, even reducing the asking price a couple of times. I don't know how it had spent most of its existence, but my guess would be on a damp carpet in a leaky shed.... The case bottom and a couple of inches up around sides and rear were heavily rusted and pitted, nearly through in a couple of places, the diecast front had bubbled-up paint and powdery corrosion all along the bottom, inside was filled with cobwebs and dead insects, there was even cobwebbery between tuning scale and front glass! The lower half of the speaker cone was mouldy and disintegrating and the front panel inside face was also considerably corroded with a general inner distribution of nasty harsh silt. I had heard that the 670 series has relatively few survivors- yet it seemed no-one else was prepared to pay £20 (less than a typical round of drinks at the time) for this "has potential" specimen and apparent antique. On those grounds, I don't really see that I have de-valued it any further- especially when any set has a minimum value based on valves (especially output), output transfomer, PSU components, and, in the end, its weigh-in component (Eddystones tend to be heavy!). Maybe the seller could have got more on eBay or at a vintage event, but wherever Eddys appear for sale, there seems to be an effective grapevine anyway, so I would say to someone who thought that I was doing wrong- "Why didn't you go and buy it then if it's so significant?" I rather think I saved it from the bin-men.

Adding a transformer PSU makes it far more couth in my eyes, besides running cooler, and the lucky find of an identical electrically and foot-print wise IFT in a rally rummage box meant that I could add a second IF stage in the place where the 840A model accomodates BFO coil and valve. As well as the worthwhile extra gain, the skirt selectivity is markedly improved- e.g. night-time 900 and 918kHz are pleasantly free of any 909kHz splatter, making for a satisfying tuning-around experience.

I have hung onto original PSU parts, Paxolin rear panel etc, so that it could be returned to original, but the front appearance aspect is unchanged- my conscience is clear and I feel that I've given the excellent tuning drive and sturdy mechanics something closer to the electronics that they deserved in the first place, the sort of thing that the designer might have wanted without accountants leaning over them.

[AdrianH]

Well work is in progress on the radio, the front panel has been removed, partially grit blasted, primed and top coated,.. The dial movement is obviously off and will get a wipe down and a bit of light oil on moving parts.
I have identified all the tubular paper caps (HUNTS) that I will be replacing.

But, there is a problem with the mains filter or chokes that are fitted on the chassis. (See the attachment in 2nd post)

These looked to have been cooked and are really a dark chocolate brown in colour, they still give continuity and measure 1.8mH per coil, but each coil has a resistance of 144 Ohms.
The sheets I have do not give any values for these but to my calculations this is way to much for what was an expected mains current of 190 milli-Amps.

With my simple calcs using I x I x R it would be 5.2 watts per coil 10.4 watts over all and a total voltage drop of 27 Volts across each coil!

I can only guess that the chokes are made from stranded Litz wire and there are very few strands still left in each coil.
Now OK using a transformer on the mains input I will reducing the mains current roughly by half down to approx 90 mA, but leaving what I believe is a faulty component in place is not acceptable. A quick look on the Internet has not come up with replacements, so I think the basic IEC mains input socket will be replaced with one containing a mains suppression filter.

Adrian

[Radio Wrangler]

Those chokes don't sound good, but one caution: Canned mains filters even those with integrated IEC receptacles contain X-rated capacitors across the mains and will inevitably do the pop-bang-smoke thing at some time in the future. Discrete filtering might be more easily fixed whenever it does it.

David

[AdrianH]

This is true, they do contain the caps. I guess in the original circuit, it was a case of mains socket - fuses - On/off switch - chokes and cap - radio. Where as with an IEC filter unit the mains is always across the filter cap. I could always use a 1 Amp common mode choke (CPC FT00010), mount it and a cap on a small PCB to replace the existing choke assembly.

Adrian

[turretslug]

I must dig out the original double mains choke that was in the 670A and check that- it's a fetching wave-wound stack that I measured at around 2mH per section on its air core, implying lots of turns, but the failing Litz aspect is a good point.

Those integral IEC C14 filters are appealingly convenient and found in allsorts, but the "always live" aspect chews at my worry bones- as an alternative, there are what I colloquially think of as "oval filters", typically in a short oval epoxy-sealed aluminium tube with either welded fixing tab or separate clamp and available in a wide range of current ratings and either wire-ended or solder tag options. I used one of these in the 670A power conversion outlined, wired post-switch and mounted under the transformer tray overhang. If anyone's ever bothered to dig open a package filter and observed its iffy butt-soldered connections, minimal clearances and budget-looking capacitors, you'll understand my misgivings about permanent connection- even if they are adorned with an international multitude of approval logos. Out of sight, out of mind and all that....

I certainly appreciate Phil G4SPZ's misgivings about increasingly rare vintage items, but I'm sure that there will be sufficient in the hands of collectors (and Eddystones have a dedicated and enthusiastic following) that there will always be near-concours examples as representatives of the art as-was. What I, AdrianH, stuarth and many others are doing is respecting the original item by keeping it in active service as an example of a vintage item that still has day-to-day relevance and usage when many of the breed might otherwise fade away in the proverbial sheds and lofts, only to be discarded down the line by the unappreciative- after all, the majority of the population would simply regard much of what we enthuse about as "old rubbish getting in the way".

Colin

[AdrianH]

The choke you describe in the 670A sound identical to this one. If you do dig it out I would be grateful if you could let me know what each coil resistance is?



Adrian

[Leon Crampin]

Radiospares (as was) sold an almost identical double mains filter choke - worth a look.

If you are using a mains transformer with an earthed electrostatic screen between the primary and secondaries, I would suggest that the mains filter components are redundant. I certainly don't like capacitors across the permanently live mains input, and even those which are only live with the set don't enjoy good reliability. Fit a 1A fuse to BS1362 in the mains plug.

I'm assuming of course that your mod entails a double wound isolating transformer. I wouldn't personally fit an autotransformer - that amount of work needs to be "rewarded" with an isolated chassis.

Leon.

[AdrianH]

As the set was originality a AC/DC one it has 500 mA fuses in both live and neutral legs which will be staying in place.

I have had a quick look through RS and not seem similar to the old choke in style, then again there are a few thousand available.

Adrian

[Restoration73]

The two power line chokes are there to deal with the supply line noise which was
inevitable with ships' 110V dc supplies.

I have both 870 and 840C and if you don't use an isolating transformer then you must
ensure DC negative goes to mains neutral. If not the control spindles become live
and the knobs don't offer the insulation of those used on many domestic ac/dc sets.

[G6Tanuki]

There are 'filtered' IEC leads available - essentially an IEC plug one one end and a trailing IEC-socket-with-built-in-filter the other; I prefer these over filters fitted inside the radio because they take-out the mains-borne noise a metre or so away from the radio rather than conducting it inside the radio to there be filtered-out.

They are also easier to replace if the capacitors in the filter release their Magic Smoke!

For my Eddystone 840A I buit - in a diecast box - a line-filter using one of the 'oblong' types from RS (I think it was actually made by Schaffner) - the box has a 3-core incoming lead bgut a 2-core output that goes to the radio. That's the closest approximation I can get to the Eddystone "Cat. 732" filter that would have originallyu been specified for solving mains-borne interference.

https://www.radiomuseum.org/r/eddyst..._unit_732.html

[turretslug]

My archive system of labelled Lurpak tubs has come up trumps- the input choke ex the 670A looks identical but measures at 8.4 ohms per section. One caveat (I'm probably teaching you to suck eggs) is that some DMMs can get fazed by inductive components and come up with weird results, but otherwise that choke does sound sick.

That's a very significant warning re. the knobs, they are a hazard with minimally-recessed grub-screws. My 670A had evidently been "worked on" in the past, but had been lethally mis-re-assembled with chassis and case brought into metallic contact in not one but two places as a result, at the volume control bush and one of the coilbox-to-front-panel securing screws. It really is essential when getting hold of one of these DC-capable sets to thoroughly check it before applying power. I'd be reasonably content to accomodate a classic AC/DC radio with wood or plastic casing and rational degree of ingress protection, but this style of metal casing with umpty-ump insulating bushings and uncertain past with weakest-link syndrome is another matter. The 750 and 670A here have near-as-dammit identical tuning-and-scale-pointer drives, but whereas the 750 uses a stranded bronze wire to drive the pointer, the 670A uses a domestic-style stranded nylon string- I'm sure that this to avoid the hazard of a snapped tuning cord bridging chassis-case isolation, but I'm also sure that a stray screw/washer/wire-clipping could achieve the same result.

I take the point re. whether a mains filter is strictly necessary (I used a current-manufacture split-bobbin transformer) but the fusspot in me likes a radio to be devoid of signals without the aerial connected and I've been surprised at what "gets in" through the mains connection with other sets in the past.

[AdrianH]

Even the brackets on that choke look the same. So I will replace with a common mode choke.

OK about all the comments on isolation, this is why I am going the isolated transformer route.

Adrian

[AdrianH]

The tubular paper caps have now been replaced with the BVWS Metallised Polyester film type. Two of the Electrolytics re stuffed and one replaced with a smaller type, replaced the volume control, it was 100K and the circuit said 500K. I will need to turn up a Nylon insulator for the mounting of the pot into the front panel as it is missing.

The 0.05uF cap that was on the output side of the two mains chokes metered 0.8 Ohms on the DVM, so definitely failed at some point in the past, that will be replaced with an X2 ?

I have cleaned and spray painted the front aluminium case/panel having cleaned the dial movement etc.

Need to get some 1mm steel to build the inner panel, make up the mains filter on a PCB and still to do the rear case.

Things are progressing slowly, just hope the vales are good at the end of this.

Adrian

[AdrianH]

Internal side panel done and fitted, the transformer fitted and have rewired the mains side to the new transformer. The series heater resistor and thermistors refitted in a different location. A faulty Long wave osc coil repaired, a quick tune up and I am listening to the radio now.

The bands only really seem to come alive after dark. The selectivity of the radio seems a little wide.

There is a 180 K resistor across the 1st IF secondary and I wonder if making this a bit higher or removing it would make it that little bit sharper.

But it is a basic 4 valve plus rectifier single superhet radio. Still to complete some cosmetic work on the radio, but pleased I can listen to it now.

Adrian