Eddystone 840A

[turretslug]

Oh... That's a shame, I'd expect more change over a few hours with viable electrolytics- some restorers show considerable patience here but I'm wary of trusting anything that hasn't gone to low leakage within about half a day. It's true that the 2BA mount aluminium can paper types are generally better than their cheaper moulded case contemporaries but there's always the chance of a duffer. If you're lucky, there might be enough connection flexibility to unbolt and lift clear of the chassis and temporarily isolate each one in turn with a slip of card/plastic sheet or somesuch. Another plus point here is that Eddystones frequently don't have "secure mechanical connection" applied to connections, (just placed and soldered) so you might be able to isolate each can electrolytic in turn quite easily and spot any rogues. Having downloaded the circuit, I see that the manual RF gain involves a 270k resistor to HT on one end of the control- again, this is only a slight leakage but as even old good electrolytics can settle to under a mA leakage, it's another thing to be aware of as things progress. Good luck.

[WaveyDipole]

I think it will be easy enough to isolate the four 2BA's and the big electolytics. Even if one of the electolytics is too leaky, then on the basis that they are all the same type and age and probably batch, I will probably re-stuff all 3 of them. On the other hand, what does one replace the 2BA types with? Is there a particular reason why this type was used rather than paper waxies? Voltage and capacitance rating does not appear to be a factor so is it down to the shielding perhaps?

[turretslug]

Supposedly, being in a shielded cylinder reduces the self-inductance- important for effective wide-band decoupling in something like a general-coverage front end- and it'll also guard against stray coupling that's possible if the outer foil is "hot" to AC currents. These capacitors abounded in military kit like the R1155 and CR100- I suppose that Eddystone might have aquired a few tons on the surplus market, they certainly featured liberally in their stuff. Modern polyester types (the ubiquitous "yellows") from various sources ought to be a good substitute, with the precautions of tucking them close to the chassis and keeping leads short and something like the original placement. I haven't tried restuffing the original stud-mounting types- I expect it would be fiddly but a neat end result. One hurdle here is a neat and effective chassis connection as regards the aluminium stud. At least modern 47uF 350V or thereabouts electrolytics should be diminutive compared to the original smoothers.

Regarding the original not-much-changing voltage drop across the limiting resistor, have any additions by a previous user affected anything- someone might have tucked a Zener in for oscillator feeds, say?

[WaveyDipole]

Thanks for explaining that. I have just arrived home and removed the 4 x 2BA mounted caps from the chassis and just left then dangling loose. Fortunately, there is enough clearance and the wiring stiff enough to place them clear of anything. Anyway, I applied power again and I'm still getting the same current. That rules these caps out which probably means its one of the big electrolytics. Will have to eliminate them one by one. I don't see any additions to the circuit, certainly not zenners, but I will have a good look over it in a while. I will leave it on again over dinner to see if anything changes.

[WaveyDipole]

Well it would have to be the last one!

I removed each electrolytic can in turn, first C42 (AF stage), no change. Next C62 (first one after the rectifier), a change of about 2v. Finally C63, a whopping change down to 6.15v which equates to around 1.3mA which is much more like it, although I would be hoping for less than a mA based on the cumulative effect of the potential dividers alone, so I guess this is not too bad?

So what to do next? It would seem that C63 is beyond recovery. The other two may be OK perhaps? As I said before, given their age, the fact that all 3 are the same type and possibly same batch, would it be safe to assume that it can't be long before the others fail?

I think my best option therefore is to replace them, by maybe restuffing the cans. Anyone think it is worth persevering with reforming the other two cans?

Oh, and I am now getting mains hum, which, without soothing caps or valves present, is undoubtedly to be expected.

[G6Tanuki]

C63 - the big 50uF electrolytic after the choke - my approach would be to subject it to line-voltage for a day or so and see what happens.

[To be honest I've never really got the whole "reforming" thing for any post-1940 electrolytics - I just shove full-expected HT voltage up them for a day or so which either frightens them back to life or makes them spill their guts catastrophically in which case I replace them]

My Eddy 840A has Plessey electrolytic 'cans' dated Dec-66. I've shoved 240V into the input socket with the selector appropriately set. So far nothing has exploded.

[WaveyDipole]

Ok, so this is where things get a bit weird. Having disconnected the big electrolytics, I temporarily soldered in some new modern caps and checked the readings again. I was surprised to get exactly the same readings of around 46v / 4.7k which gives about 10mA. I left it for a while and no change. The only thing I can think of is that its something to do with that choke?

Given that I got the same readings from old and new caps, I removed the new ones and soldered the old ones back into the circuit. I then re-installed the valves and tested. Dead. A quick check revealed that I had missed a wifer connecting the HT to the other side of the radio. I soldered this on and tried again. This time, after a minute or so, the radio came to life!

I was able to use a signal generator and known stations on MW to set the pointer to the correct position on the scale, but I have been playing with it for about 40 mins or so using a random wire antenna slung out through the window. The sound is clear and without any hum although the sound, without the cover on is more in keeping with a communications receiver than a general broadcast receiver. All bands appear to be working. There are no crackles while tuning or on the tone or volume pots. I am pleased with the result.

[David G4EBT]

Originally Posted by WaveyDipole

Well it would have to be the last one!

I think my best option therefore is to replace them, by maybe restuffing the cans. Anyone think it is worth persevering with reforming the other two cans?

It might be worth my mentioning a thread that I started back in January, where I mentioned a new range of Rubycon miniature electrolytic capacitors. When it comes to considering re-stuffing vintage multi-section electrolytic cans, these capacitors tick all the boxes. They're 105 degrees C, high ripple current rating (often not stated on other makes so we haven't a clue how they'll stand up to ripple), 450VW, in a range of values to suit our needs, and best of all, not expensive. Three will fit nicely inside a typical can, and it makes a much neater job than say leaving the can in place for appearances sake, but disconnected, and putting new caps under the chassis, if space can be found.

https://www.vintage-radio.net/forum/...s+for+stuffing

Since writing that post, I've used these caps several times to good effect.

Hope that's of interest.

[WaveyDipole]

PS, can anyone help me with the BFO control? I'm not sure whether it works.

On my DX390 when I scan the ham bands with SSB on and when I find a signal, then with a bit of patience I can use the BFO control to clarify it. I expected the control on the Eddystone to work in a similar fashion. So I tune to the appropriate band (80m or 40m) and turn BFO on. I didn't find much activity, but the couple of signals I did find were distorted and the BFO control wouldn't clarify them. In fact, it seemed to have little or no effect. I'm not sure what to expect on a valve radio though. Some Eddystone models have crystal based oscillators for better stability. This one does not so I expect some drift but I was hoping to be able to get a clear signal.

[G6Tanuki]

When using the BFO you need to wind the RF-gain back quite a bit or the received signal overwhelms the BFO injection-level and you get distortion.

[I generally set the AF gain to 'max' and control received volume with the RF gain control].


A simple test that the BFO is working is to tune to a normal AM broadcast-station then turn the BFO on - you should get a loud whistle, whose frequency varies with the setting of the BFO control.

[WaveyDipole]

By that test, the BFO does appear to work as I do get the heterodynes either side of the received broadcast signal and the frequency does vary with the setting of the BFO control. Dead centre seems to kill the whistle altogether which I presume is as it should be?

Its not a particularly loud whistle but it is there. It was much louder on the other (transistor) radio. Unfortunately I didn't pick up any signals on the 40m or 80m band last night so I didn't get the chance to experiment with the RF vs AF gain suggestion you made.

[G6Tanuki]

If you want a reliable source of a SSB signal to try to resolve, listen out for the "VOLMET" transmissions.

http://www.dxinfocentre.com/volmet.htm


I'd suggest Shannon [5.505MHz] and the UK Military one [5.450MHz] would be good tries, as they broadcast continually and should be audible throughout the UK irrespective of time-of-day.

Shannon on 13.264MHz is another good one during the day.

[WaveyDipole]

Thanks. I did try Shanon on 5.505Mhz but couldn't pick it up at all. It was very late evening (10.00pm) and I struggled to pick it up on the Satellit as well so I resorted to the 80m and 40m bands. I generally can't pick up the Military one, except very occasionally and then only very faintly. I will try Shannon including the other frequency again though.

[Station X]

Just tried the three frequencies mentioned in post #52 on USB and I had no difficulty in receiving any of them.

[WaveyDipole]

It might be the combination of my geographical location and my antenna (which came with the radio and which serves for testing purposes) which consists of about 10m of a single insulated core of mains wire. About 3m runs across the room and the reminder I throw out of the window. Thanks for confirming though that all 3 can be received. Where is the Military one transmitted from?

[G6Tanuki]

The military one is usually radiated from RAF St. Eval

http://www.alancordwell.co.uk/hfradio/steval.html

but as is the case with most MoD stuff they have several other transmitter sites and may switch to using another without warning.

[Note: the site is sometimes satirically described as "RAF St. Ivel" after a once-popular manufacturer of dairy products]

[WaveyDipole]

Well Leicester is about the same distance from Newquay as Ipswich just in a different direction, so it is curious that I struggle to pick it up even on the south side of the house.

PS, why does the AGC need to be disabled (by shunting to earth) when applying BFO?

[WaveyDipole]

Now that I am back home from work, I gave it another go. I did pick up both Volmets, Military on 5.450Mhz and Shannon on 5.505Mhz. Its probably for the one on 13.264Mhz as I can't pick it up on either radio. The suggested technique of upping the AF gain volume and using the RF gain to control volume works nicely. I found that setting the RF gain to more than mid-point does indeed cause it to distort. I had it up to almost full previously which is why the signals were distorted. I found that setting it about 40% of the way up seems to work the best. The BFO then works quite nicely although it begins to drifts after a few seconds and needs adjusting every couple of minutes or so, but this I guess this is to be expected. Anyway, thanks for helping me sort it out.

[G6Tanuki]

Originally Posted by WaveyDipole

PS, why does the AGC need to be disabled (by shunting to earth) when applying BFO?

1] Because the BFO would appear as a constant 'very strong carrier' and cause the AGC to throttle-back the gain of the receiver even with no signal at the antenna.

2] Because even if it didn't do [1] the effective signal for the AGC to sample is inconsistent when resolving CW or SSB. Normal "AM" AGC works by sampling and averaging the essentially always-present carrier-level of the AM signal; this carrier's not there in either SSB or CW.


(OK, it *is* sort-of there with CW but its average-level is a composite of signal-level and the ratio of dots-and-dashes to spaces... and the time-constants needed to handle this sensibly are wildly different to the time-constant needed for old-style AM audio).

[turretslug]

Originally Posted by WaveyDipole

The BFO then works quite nicely although it begins to drifts after a few seconds and needs adjusting every couple of minutes or so...

The BFO may drift a little, but drift in the HF LO is more likely to be significant as regards SSB intelligibility in the first hour or so- the sturdy die-cast coil-box and rigid attachment to the front-panel tuning assembly that Eddystones are famed for makes for very good mechanical stability but it does take a while to all warm through. Once it's got over the warm-up drift, it should be pretty good. On my 750, I found that replacing the EF91 LO's plain aluminium screening can with an identical but matt-black finish type made a surprisingly large improvement to initial warm-up drift- I'd have been very sceptical of someone who told me that it would make a difference but it did!