Eddystone 840C

[Sideband]

Well tonight I aligned the rest of the bands. I'm going to go through it again now that I'm used to what does what but as suspected all bands (except band 1 for some reason) were miles out. I had to repeat adjustments 3 or 4 times on bands 2, 3 and 4 to get the maximum (and adjusting the wrong trimmer for the wrong band which meant I had to go back and do it again) but we now have a working radio. Just spent half an hour or so tuning around the amateur bands. They don't seem to be as lively as they used to be. Best thing is that I can now resolve SSB without resorting to using the sig genny! Found a couple of really strong DK's on 40 metres...not exactly DX but there is so much QRM some of the fainter amateurs are difficult to hear. Top band here is just as noisy as MW and 80 is only slightly better. 20, 15 and 10 seem dead although I know 10 is very much dependent on the sunspot cycle. So at the moment 40 is producing the best results. I suppose over the winter months, I'll be building an ATU and I might try and do something about a decent aerial.

All in all its all looking very promising.

73's (for now)!

[Sideband]

Originally Posted by wd40addict

On the 840C I used to own the connections to the volume pot (wiper and top of track) were reversed ex factory! If volume adjustment seems a bit strange worth a look to check yours is OK.

Well another update: The set was working quite well but was plagued by a hum which wasn't smoothing....not too disturbing on speaker but a real show-stopper on headphones. Tonight I decided to investigate further. It didn't take long to discover the hum was coming in on the grid of the AF amplifier. Something wasn't right in the grid circuit. A few resistance checks revealed that the volume control wasn't wired as the circuit suggested it should be. It didn't appear to have been touched and I found that indeed it had been wired as in the quote above. I reversed the slider and top of track connections and tried the set again. Hum gone! As a result, I was able to spend a good hour on 40 metres listening on headphones which is much better for hearing some of the fainter signals, catching lots of amateurs in a contest. All European, the only slightly exotic one was a Bulgarian who came in with a thumping signal. I have a very old International Callsign list which now appears to be hopelessly out-of-date...some callsigns I logged were not included in my list (it still has Czechoslovakia listed)!

It's a pity there isn't a bandwidth control. It could do with better selectivity but I'm not complaining. It's been fun getting it to this stage and I think I can improve it still further. As a matter of interest, I used the signal genny to check the alignment of the radio around the amateur bands....pretty good, they are where they are supposed to be.

Call me old fashioned, but to me this is real radio, carefully tuning in and finely adjusting for best reception (using the RF gain and BFO) hoping to hear a VK or VE just for the fun of doing it.

[Jon_G4MDC]

Pardon me, but an 840C and selectivity... in my experience they are mutually exclusive.
Maybe fit a Q multiplier?

It is very rewarding hearing DX on such a set and I did that with mine in my younger days. In standard from you need to be lucky that the strong Euro QRM gives you a window to make the most of what you have.

[Bazz4CQJ]

Originally Posted by Sideband

Top band here is just as noisy as MW and 80 is only slightly better. !

If by noise, you mean electronic noise, make a magloop .

And with QRM, the loop is directional, so could help with the lack of selectivity

B

[Sideband]

Jon.....yes I'm not expecting great selectivity from this set...at least not for SSB work. Back in my younger days, I built an external Q multiplier which worked well so I might do a similar thing.

Bazz....yes a loop is also on the cards. It's certainly worth a try. It is mainly electronic noise....not helped by my next door neighbour who has all his security PSU's and routers up in his loft. I get on too well with him to complain......!!

[AdrianH]

My 870 has got to be worse for selectivity than your 840, it does not have the first preselector/V1 stage as it all goes into the mixer valve direct. I did consider making an external preselector and may still do so.

But it can still pick up stations well, there just may be two stations in there at times. Bandwidth is always going to be an issue as the sets are basically broadcast receivers so will have around 9 or 10kHz bandwidth. But they are fun to use and this little thing takes up less room then a typical tabletop radio, so all is good.

If you are having fun with it then I say a job well done.

Adrian

[Restoration73]

It's good you have got some enjoyment from this receiver. Most single conversion
superhets can benefit from an outboard preselector rather than an ATU and can increase
the gain by up to 20dB, easy to build new versions of the Codar PR30 (valve) or PR40
(FET) circuits are out there.
Same dial but slightly deeper and considerably heavier is the 940, which is a 13-valve
design with mains transformer. The last of the Eddystone valve sets c.1965.

[wd40addict]

I wonder if all 840C's have this volume control issue...

Glad you're enjoying the set, it also has a very distinctive smell when fitted with the dropper! Mine had uneven sensitivity across the band, this was cured by realignment. Once I got my RA17 it just sat on the shelf so I sold it on.

[Bazz4CQJ]

Originally Posted by Sideband

Jon.....yes I'm not expecting great selectivity from this set...at least not for SSB work. Back in my younger days, I built an external Q multiplier which worked well so I might do a similar thing.

It's a while since I heard anyone mention Q-multipliers, which seemed to feature quite often in mags and books in the past. I've no experience of them myself. I wonder if anyone has made any use of the ceramic filters (small three-legged devices) in valve receivers? They were certainly intended for use in solid state circuits where they offered a low-cost and very compact way of improving selectivity. It might be worth a few minutes on Google to find out if some clever person worked out a way to improve selectivity of a valve Rx by using them?

B

[turretslug]

There's certainly a good choice of 450kHz ceramic filters available cheaply, I believe that these typically expect in/out impedances of 2k, i.e. not chiming easily with traditional high-impedance valve + IFT topology. I suppose that capacitive impedance matching would be relatively straightforward to devise, but these filters expect wideband resistive behaviour from input and output feeds, otherwise out-of-band characteristics deteriorate. Perhaps a twin-triode arrangement with the filter between them, cathode-follower feed and grounded-grid on the output with appropriate resistive padding (on the assumption that one is sticking to valves).

I fitted an additional IFT and EAF42 to the mains transformer-converted 670A here (second IF stage now fed with 2/5 AGC tapping to keep strong-signal AM distortion low) for much improved skirt selectivity, but the 840-series sets would be more of a squeeze.

This morning, I wondered if the current weather had any bearing on HF propagation, so as well as using the 670A for a tune through 16m and 13m broadcast allocations, I checked the graveyard 11m broadcast allocation- just a weak, noisy Russian(?) catch, but the 26.5-27.5MHz region was absolutely awash with strong Italian CB stuff. Sometimes finding good reception with a basic radio can be more satisfying than using something sophisticated!

[G6Tanuki]

I vaguely remember something from one of Pat Hawker's column about replacing the IF stage cathode bypass capacitors with crystals, the idea being that the crystals present a high impedance [so maximum negative feedback ] at all frequencies except their series resonant frequency, at which they present a very low impedance so that the valve gives maximum gain. Perhaps this might work using modern ceramic filters?!

Q multiplier would to me seem problematic to retro fit safely in a live chassis radio like an 840.

[Bazz4CQJ]

Yes, use with valves was definitely not their intended purpose, but I am sure that somebody somewhere has had a go. I have an HRO and the crystal filter in that is intended purely for use on CW and I have no interest at all in that mode. Given the cavernous box that houses the filter circuitry in the HRO, I thought that I could remove the existing components and use the space for the ceramics to be used, perhaps with suitable transformers? I bought quite a few, but that project is still (lowish) on the "To Think About" list .

I've just uncovered this article discussing ceramics and the HRO in my file (forgot I had it). I think the author is of this parish??

B

[Jon_G4MDC]

That's a nice article and thanks for posting it.

Something like the Murata Ceramics to improve the IF selectivity is just what the 840 (and maybe others) need. It would mean shifting the IF from 450 to 455kHz but I think it should cope.

I always meant to try it on an HRO but I never have.

[G3PIJpeter]

Originally Posted by Bazz4CQJ

... Given the cavernous box that houses the filter circuitry in the HRO, I thought that I could remove the existing components and use the space for the ceramics to be used ... I've just uncovered this article [attached pdf] discussing ceramics and the HRO in my file (forgot I had it). I think the author is of this parish?? B

Yes - I am of this parish and the one of whom you speak! The article was based on my experience of adding a mechanical filter to a rebuilt HRO. There is no need to open up the filter unit in the HRO, which would require dismantling quite a lot else. The extra filter of whatever sort can be added under the chassis, immediately following the mixer IFT, with relay-controlled switching. Matching transformers could be added if needed (not in the case of the Kokusai) with a transistor amp if needed. No doubt the relay could be replaced by diode switching, but the relay circuit suggested earths the relay contact capacitance that could degrade the filter response when in circuit.

Good luck - but I woudn't add a 3 kHz filter if you mostly listen to BC stations.

Peter G3PIJ

[Bazz4CQJ]

Thank you for sharing your experience Peter. My HRO project has been "on-going" for a while and I need to refer back to the files kept on my PC to recall some of what I've looked at. I have to admit to you (and it may shock some) that I have already taken out the filter box on the HRO (I still have all the components) anticipating it's re-incarnation.

Your comments about the incompatibility of BC stations with 3kHz filters is noted.

@Jon, please keep us posted on anything you try with the ceramics. I have files on Toko, Murata and Morgan (?) components if you cannot find info you need.

I had another look on Google today, and it may be that Peter's article is just about the only one that touches on such improvements on vintage receivers.

B

[Bazz4CQJ]

I've spent the evening digging out my little stock of ceramic filters and looking at the datasheets. The insertion loss with those components looks like it's 6-7dB in a circuit which was designed to take them. You have to wonder what the loss is if you throw them in to a valve circuit. I wonder whether including some amplification would be a really good step? What such an amplifier might look like is something that I'm not at all sure of . Maybe two or three transistors?

B

[Jon_G4MDC]

My present thinking is something like an emitter or source follower between original crystal filter output and the new ceramic. That could be raised in impedance by a series resistor if need be. This could perhaps feed more than one filter so that BW could be selected only by switching the output. Is that a silly idea?

On the output side I was wondering about lifting the tuning capacitor and inserting a higher value fixed capacitor to make a capacitive tap on the IF1 input tuned circuit. So far I haven't even thought about the values that would need.

Another method I wondered about was to put the ceramic in one of the IF cans. As these use critical spacing between the two coils to determine coupling that would only work by introducing a screen. I have more or less discounted doing it that way. Peter's idea is better.

Of course all of this could pertain to an 840C as well. Just not to stray too far from topic!

[Bazz4CQJ]

Originally Posted by Jon_G4MDC

Of course all of this could pertain to an 840C as well. Just not to stray too far from topic!

Absolutely! I was thinking that if I can get you do this on the 840C, then I can just copy it on the HRO . PM to follow.

B

[Jon_G4MDC]

Ah - my 840C went a while ago at a Stockwood Park Rally. It had acquired a transformer and E series parallel heater valves but never any improved IF selectivity. I did all that long before I knew about this forum. All it's Plessey Red & Blacks probably wanted changing!

Now I'm in it for the HROs.

[Bazz4CQJ]

ooh...I've lost the plot Jon . I had started to think that this was your thread about the 840 but of course, it is Sidebands. Well, my apologies to him.

But going back to the issue of ceramic filters, could Sideband update us on his views.

I don't know the 840 very well, but I think the challenge of improving selectivity is probably much the same as with the HRO, and that the ceramics look interesting (esp after digging out Peter's article). Shall we start a new thread?

B