Eddystone 840C

[Sideband]

Hi Guy's, I don't think I've ever posted in the amateur radio forums before. I have acquired an Eddystone 840C. Not actually had one of these before so I thought it would be interesting to get it working. I've downloaded a manual which has a good copy of the circuit. I don't think it will give outstanding performance being only a single superhet but it seems well built and will no doubt be better than trying to use a domestic shortwave receiver with a signal generator as a BFO!

So having got inside it, there is previous work very obvious although the critical RF/IF stages look pretty much untouched. One thing immediately obvious is that the mains dropper has been removed and a mains transformer has been fitted. It still has all its U range valves and a valve rectifier so it's not been modified for parallel heaters. Curiously a small 6V transformer has been fitted to provide dial illumination. There is a 32/32 electrolytic hanging in the wiring underneath and another electrolytic squeezed in there as well. These probably just replace the original smoothing.

I'll need to investigate further around all this to see exactly what has been done.

The speaker is well past its best so I might have to look for a replacement. I think there is supposed to be a headphone socket on the front. This appears to have been moved to the back and a pot is fitted on the front where the phones socket should be. Oddly, the pot is not connected although it looks like it might have been at some time. Maybe this had been wired as a volume control for the phones only.

The main concern is the grafted in mains transformer. Was there a standard mod for a mains transformer if the dropper failed? It looks to have been fairly well done but I'll need to check around it all before I even think of applying mains.

Apart from all that, the IF stages look pretty much untouched. There are loads of Hunts Mouldseal capacitor and the red/yellow Plessy electrolytics which will all get changed.

If nothing else, it will give me something to do during the winter months and make a change from the run-of-the-mill radio's and TV's!

[turretslug]

Not a bad basic general coverage set, but IMHO there's a bit of an imbalance between the rock-solid coil-box and front-panel castings and smooth, good resolution tuning drive, and the more hum-drum electronics- at least it has an RF stage to its credit. ISTR this final incarnation of the 840 series got a linearising mechanism in the tuning drive and a DM70 tuning indicator.

One thing to check (apart from its basic safety!!) is that the revised power arrangements don't compromise the warm-up protection associated with the delicate DM70 filament.

Good luck with it, I have a 670A, essentially the AM-only forebear and I came to like its straightforward but effective nature,

Colin

[Aub]

Have tried the Eddystone user group? The mod you've described must have been done a few times over the years.

Good luck with it


Aub

[Restoration73]

Sounds like it might be a line output mod.
Note if you are using a single wire aerial the jumper plug linking A1 to G must be fitted.
The original dropper arrangement was great if you wanted a room heater.
I have a near mint 840C here, with a 940, 870 and EC10, the latter transistorised.
As stated, easy to reinstate and still just about useable for SSB.

[Silicon]

I assume that the standard modification would be to install a 240V to 110V mains isolation transformer.

An auto-transformer may have been installed.

[G6Tanuki]

I have an 840A here as my bedside radio, it's OK for listening to the lower HF bands late at night , but it's got image problems above 15MHz and the lack of stabilisation of the HT line to the local oscillator is a pain because the RF gain control shifts the frequency by quite a bit which is annoying when listening to SSB transmissions.

Mine only cost me 20 Quid though, it was a bit of a heap with a BY127 diode wired across the rectifier, the smoothing choke replaced by a totem pole arrangement of old TV dropper resistors and a 63V rated part fitted as 'that capacitor '....

So a few hours of work were needed to get it going.

[G6fylneil]

In 1990 I replaced most of the resistors in my 840C as they had all gone high, this improved the sensitivity slightly and brought the HT down a bit. It's still got all its Hunts capacitors, apart from the brown "tone" one on the output anode. (these sets have a 350V tubular ceramic as "that capacitor"). I replaced the carbon rf gain pot. with a wire-wound one, as it had a high end resistance. The rf amp. cathode resistor had burnt out and been shorted, not an easy part to access. The main smoother looks to have been replaced at some point, but it's dated 1966 and still ok.

[Sideband]

This one still has all its Hunts but most are cracked so they will get the heave-ho! Mind you there are some literally buried underneath all the coils. I have NO idea how to get to those unless there is another way in...

I will have to look at the fitted transformer and do some meter checks. I didn't think about it being an autotransformer....Hmmm.

Thanks Aub....yes I found the User Group. I might have a look on that site first and see if there is anything about a possible transformer mod.

All good fun!

[Sideband]

It's not an autotransformer. Having removed the transformer and done a few resistance checks, it's such a bodge-up I've decided to remove it and use a capacitive dropper which I can mount in place of the original dropper above chassis. There is also an additional 32/32 cap wired in parallel with the existing cap and taped up in the corner. I'll remove it all and replace as required...no idea at this stage if the original smoothing is OK or not.

I notice the HT is quite low.....shown as 115V in the manual.

[G6fylneil]

If the capacitors are cracked they obviously need replacing, my 840C has the later "Dipseal" capacitors, the only mouldseal was the easily accessible one for the tone control.

[Sideband]

Originally Posted by Sideband

It's not an autotransformer. Having removed the transformer and done a few resistance checks, it's such a bodge-up I've decided to remove it and use a capacitive dropper which I can mount in place of the original dropper above chassis. There is also an additional 32/32 cap wired in parallel with the existing cap and taped up in the corner. I'll remove it all and replace as required...no idea at this stage if the original smoothing is OK or not.

I notice the HT is quite low.....shown as 115V in the manual.

First stage completed....well not the final component but a temporary dropper cap made up of parallel X caps just to test the operation. Firstly I took out the mess of transformer, wiring and wirewounds that had been grafted into the circuit along with the extra smoothing cap that was wobbling about in the wiring. I've not connected the rectifier anode yet, I just wanted to check that the heaters worked with the new dropper. Well they do. The value of the cap needs tweaking upwards slightly to get the correct heater voltage (it's about 5 volts low at the moment) but the basic idea works and just needs refining. I have a good quality motor run capacitor on order that I should be able to fit in place of the original dropper.

The original mains input socket had also been removed to make room for the transformer together with one of the fuseholders. I intend to fit a standard IEC socket and reinstate the missing fuseholder. I can't do anything about the missing voltage selector but that is not important nowadays anyway. The IEC socket won't be original of course but a lot better than a knotted mainslead hanging out of the back!

Slowly but surely I'm breathing life back into this old Eddystone.

[Restoration73]

I think the low HT (seems to extend valve life) is due to being able to run on 110V DC
ships supply.
Whilst some caps may need replacement, there are also some carbon composition
resistors, of the higher power types, that may have changed value and can be upgraded.

Note this receiver's original price was around £65 and sold well.

[Sideband]

£65 in 1960 (or was it earlier than that) was quite an outlay! Must have been regarded as a very expensive receiver.

[G6Tanuki]

It does look expensive when you consider that a military surplus AR88D in grade-1 condition would set you back only a quarter of the price of the Eddystone.

[turretslug]

It's a shame the previous owner hadn't put a bit more effort into making a couth job of what was presumably a well-intentioned impulse to make an expensive possession safer. I ran my 670A from an external 1:1 mains isolating transformer for a long time but in the end decided to make it a self-contained and unambiguously Class 1 device with metal (1/8" aluminium) rear apron and internal mains transformer. A step I had ummed and aahed about for some time, finding a big lucky dip box of valves at a vintage event that included lots of EAF42s for 10p each was the clincher.

[Sideband]

I wonder if someone could take a photo of the IF section under the chassis? There is some rather untidy wiring and what looks like an added tagstrip. Not sure what is going on. It might just need tidying up but melted insulation with a soldering iron just looks so messy...and it looks like he used plumbers solder! If I can see some good close-up photos, it might help me understand what is going on. I don't think anything has been modified going through the circuit but it's just messy.

[Sideband]

Originally Posted by turretslug

One thing to check (apart from its basic safety!!) is that the revised power arrangements don't compromise the warm-up protection associated with the delicate DM70 filament.


Colin

Good point Colin. Well the thermistor had been removed when the transformer was grafted in so there was no protection from the instant voltage applied. I've no idea (yet) if the DM70 is good or not but I'm sure I have a spare somewhere. Anyway now that I've decided to use a capacitive dropper, the switch-on surge is not a problem. The series/parallel heater arrangement requires about 70 volts at 200mA and last night I monitored the voltage at switch-on. It ramps up over about 12 seconds to the required voltage (well it's the right voltage now since I added another 0.22uF in parallel with the existing 2.7uF) so I don't think there is any danger to the DM70. It will be worth checking the voltage across the 8 ohm shunt resistor to make sure its at or near 1.4V.

[turretslug]

In the absence of any other photos of the IF underside/tagstrip area, I found a couple of previous threads on the 840C with photos that might provide guidance;

https://www.vintage-radio.net/forum/...d.php?t=156774

https://www.vintage-radio.net/forum/...d.php?t=191864

[Sideband]

Thanks Turretslug. The first link gives some good underchassis views. Not sure what has been going on around the BFO and detector so with a bit of luck and a fair wind hopefully I can sort it out with the aid of those pictures. One thing is dead certain....there is no tagboard near that first I.F. There are two germanium diodes in mine that are very obviously not original. They appear to be connected to the original detector diode in the UAF42, in series to the IF transformer with a 0.1 from the junction of the diodes to somewhere....!. No idea what that is for so they will come out and the circuit put back to original.

Still some work to do before I apply some HT.....

[Sideband]

I think the User Group has provided the answer to some of the 'modificatons'. It would seem that the 840C did not have a noise limiter...I think it had been removed with the 'C' version. It would seem that the previous owner has attempted to re-introduce it....very badly. I'm going to return the circuit to normal anyway. There are other methods of noise limiting without being invasive.