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Old 25th May 2024, 9:24 pm   #1
WaveyDipole
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Default Eddystone 840C receiver

I have started work on an Eddystone 840C that I acquired recently at Retrotech. I powered the radio up gradually on a variac and it seems to work but the audio is rather quiet. I have the AF Gain control set to almost max but getting barely an adequate level of sound. This might be down to alignment which has yet to be determined.

This later version has the addition of a magic eye, but although the radio is picking up stations, another problem is that the magic eye indicator is not showing any signal. One of the 2.2meg resistors is out of tolerance at 2.6meg which may be a factor, but does not seem so far out as to be causing the indicator to show nothing?

This set does not have any waxies present which would normally have to be replaced on sight. However, I have replaced "that cap" because it is one of the Hunts plastic cased Mouldseal types which are known to be unreliable.

The set does have quite a few of another type of Hunts capacitor, marked 'G50' which I am not familiar with and which does not appear on Paul Stenning's capacitor guide here:

https://www.vintage-radio.com/repair...apacitors.html

I do not necessarily want to carry out a bulk replacement of capacitors unless it is necessary to do so as a handful of them are present in very difficult to get places in the tuning assembly. Therefore I wanted to ask what type of capacitor are these and whether they are reliable and can be left in?

Am I right in thinking that the small grey .01 capacitor is also of a plastic coated paper type that is unreliable?
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Old 26th May 2024, 8:50 am   #2
cathoderay57
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Default Re: Eddystone 840C receiver

I'd be inclined to agree that the small grey Hunts caps aren't particularly reliable. How do the valve electrode voltages compare to the service data? Jerry
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Old 26th May 2024, 10:04 am   #3
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Default Re: Eddystone 840C receiver

The magic-eye on these was always considered a bit of a joke, "an expensive power-on neon" as I have heard it described.

Check all the valve voltages - and home in on any where the cathode voltage is low [which could point to problems in the screen supply, either the feed resistor gone high or the decoupling cap gone leaky].

Also the high value resistors in the first AF amp [anode and screen feed] would be worth some close scrutiny; on my 840A the 1st audio stage's anode feed resistor had gone way-high so the anode voltage was lower than the screen. Not good for gain!

Eddystone's QA procedures were not always that hot either - on my 840C there was a wiring mis-step in the coil box which meant it could *never* have worked on the highest frequency range. Yet somehow it was aligned at the factory, went through QA checks and was sold to some unsuspecting customer. Your receiver could have some similar factory-fitted performance-reducing feature!
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Old 26th May 2024, 1:36 pm   #4
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Default Re: Eddystone 840C receiver

Well I found the reason for the low volume this morning. I hadn't got around to checking the valve voltages yesterday, but found the table in the manual this morning, printed off the necessary information including circuit diagrams and set about some troubleshooting. There was nothing on the screen of V5 due to an open 680k feed resistor. I didn't have this value to hand, but I figured a 470k + 220k in series should be close enough. That solved the volume problem, which there is now plenty of now. It didn't solve the magic eye problem though, which still shows nothing.

The voltages around V3 and V5 are still not correct though:

V3: A=125, S=111, C=7.15
V5: A = 18.4, S=31, C=1.5

The screen voltage on V3 varies from 83V to 111V depending on the setting of the RF gain control, but even so seems much higher than indicated in the table. I checked the 330 ohm on the cathode and that was fine. Since there is a difference in the volume level of the signal when the A.G.C switch is operated, I am presumaing that the A.G.C is working? I checked the 330 ohm resistor on the cathode and that is fine.

V6 and V7 look a bit on the high side but in about the right ball park:

V6: A=69.36, S=69.22, C=0.01
V7: A=114.9, 122.59, 7.2

I was using a DMM but also compared with an AVO but there was next to no difference in the readings.

The undersides of V1 and V2 are practically impossible to get to so I have not been able to check these.

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Old 26th May 2024, 5:02 pm   #5
David G4EBT
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Default Re: Eddystone 840C receiver

Regarding the DM70 'exclamation mark' magic eye, it's a very simple device with just two connections for the filament (cathode), plus the grid, and the anode. Its rather fragile electrically in that the filament is 1.4V @ 25mA. With 60 - 90V on the anode, and zero volts on the grid (no signal tuned in), the exclamation mark should be fully illuminated.

As the AGC voltage goes negative on the grid when a signal is tuned in the flow of electrons from the cathode to the anode is impeded and the exclamation mark reduces in size. When I restored a Unitra 'Figario Special' which had a DM70, I found that with -4V on the grid the DM70 was half extinguished, and at -9V, fully extinguished.

The first pic shows the pinouts of the DM70, the second is of a DM70 Of tune, fully lit.

Fortunately, as magic eyes go, the DM70 is fairly plentiful and typically can be found at £10.00.

(They never seem to loose their luminescence, but like any 1.4V filament valve, they won't stand much higher filament voltage).

Hope that might help a bit.
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Old 26th May 2024, 6:02 pm   #6
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Default Re: Eddystone 840C receiver

I've encountered a few of the red Hunts G50 capacitors. Most have been leaky, so I suspect they are paper dielectric.

Paula
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Old 26th May 2024, 7:18 pm   #7
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Default Re: Eddystone 840C receiver

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Originally Posted by frsimen View Post
I've encountered a few of the red Hunts G50 capacitors. Most have been leaky, so I suspect they are paper dielectric.

Paula
Yes, those red Hunts are always a change on sight capacitor for me, ‘Orrible things!


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Old 26th May 2024, 7:32 pm   #8
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Default Re: Eddystone 840C receiver

Quote:
Originally Posted by David G4EBT View Post
Regarding the DM70 'exclamation mark' magic eye, it's a very simple device with just two connections for the filament (cathode), plus the grid, and the anode. Its rather fragile electrically in that the filament is 1.4V @ 25mA. With 60 - 90V on the anode, and zero volts on the grid (no signal tuned in), the exclamation mark should be fully illuminated.

As the AGC voltage goes negative on the grid when a signal is tuned in the flow of electrons from the cathode to the anode is impeded and the exclamation mark reduces in size. When I restored a Unitra 'Figario Special' which had a DM70, I found that with -4V on the grid the DM70 was half extinguished, and at -9V, fully extinguished.

The first pic shows the pinouts of the DM70, the second is of a DM70 Of tune, fully lit.

Fortunately, as magic eyes go, the DM70 is fairly plentiful and typically can be found at £10.00.

(They never seem to loose their luminescence, but like any 1.4V filament valve, they won't stand much higher filament voltage).

Hope that might help a bit.
Thank you for this explanation of the DM70 magic eye. Now I understand what "exclamation mark" refers to! Once I pushed the DM70 up a bit further into its shield, I could see a dot on the far left and the thin faint line at the other, but it isn't quite an exclamation mark yet. I now understand that this works kind of in reverse and counter-intuitive way to the bar indicator magic eye that I am familiar with, where the glowing bar increases in length as the strength of the signal increases. The current situation with this one is that nothing changes during tuning. I will now check how the voltage on the grid behaves in response to tuning. Much appreciated.

Quote:
Originally Posted by frsimen View Post
I've encountered a few of the red Hunts G50 capacitors. Most have been leaky, so I suspect they are paper dielectric.

Paula
Interesting to note. I had wondered, since they are of a different colour to the usual brown ones, whether they were paper or foil. If they are paper dielectric then it sounds like they might be worth replacing for the long term. For now though, I am going to see where the diagnostics take me.
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Old 26th May 2024, 7:39 pm   #9
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Default Re: Eddystone 840C receiver

Quote:
Originally Posted by WaveyDipole View Post
It didn't solve the magic eye problem though, which still shows nothing.
When you say 'shows nothing' do you mean it's not illuminated or that it doesn't move when tuned in?

If it's not illuminated, check the anode load resistor R25 470k. The one in my 840 was open....well it was very high....above 20Meg. If it's OK then it might just be an O/C filament in which case a new DM70 will be required. The heater chain in this set is a series/parallel arrangement. The DM70 has an 8 ohm resistor in parallel with the filament. As stated above, it's not very effective as a tuning indicator but it does work....sort of!

Edit, Crossed with your reply above! OK check the high-value resistors (and the anode load) but be aware that it could be leaky caps (like C56). Those Hunts red caps are paper so are likely to be leaky. Check the cap on the AGC rail (C48)....even minor leakage here will upset things...and therefore the way the DM70 works.

I spent two weeks recapping mine....it had the notorious black Hunts that were cracked and split. Yes some are an absolute pig to get to involving the removal of switch linkages and screening covers. Not a job for the faint-hearted! take LOTS of photos!
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Old 26th May 2024, 9:21 pm   #10
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Default Re: Eddystone 840C receiver

I am seeing a dot on the far left and a thin line on the right. Not illuminated or "fat" enough to make an exclamation mark. The intensity of the line or dot does not change with the tuning.

I took some measurements around V4. The anode voltage is 50.96V, which is about right. I dare say the heater must be OK otherwise it would not illuminate at all? R22 was high at 2.75Mohms so I put a 10meg across it which reduced the overall resistance to 2.16Mohm but made no difference.

Voltage at the screen (pin 1) varies from -0.440V when tuned to a strong station, to -0.104 when tuned to static. From David's comment above I can already see that we should be getting as low a -9V, possibly lower. I disconnected C56 to rule it out, but that made no difference, which leaves C48 which I need to locate.

Nobody came back to me about those TSE capacitors, so I ordered some JB capacitors instead from a reputable electronics vendor.
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Old 26th May 2024, 9:40 pm   #11
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Default Re: Eddystone 840C receiver

I can remember reading here I think a few years ago about a mod to make the magic eye more sensitive on these. I've had a few 840's over the years and the magic eye didn't do much on any of them.
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Old 27th May 2024, 9:18 am   #12
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Default Re: Eddystone 840C receiver

It sounds like you have very little AGC voltage being developed, this will explain the lack of magic eye action.

I would be replacing all the capacitors on the AGC line, yes some of them are hard to get at!! But it only takes one of them to be masquerading as a 5Megohm resistor to really upset things.

There are a couple of horrible little brown encapsulated paper capacitors in there, they are about 3/4 inch long and 1/4 inch diameter, with rounded ends, hence the colloquial nickname of 'rat droppings'. Hunt these down and replace them!!
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Old 27th May 2024, 10:41 am   #13
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Default Re: Eddystone 840C receiver

Re the DM 70 Magic Eye, under no signal conditions it should look like the diagram below, with full brightness. At -9.0V on the grid, only the dot of the exclamation mark will remain illuminated. It does need a swing of several volts on the AGC line to vary the display. I've always felt that magic eyes of all kinds, (except perhaps on some test gear), are a bit of a pointless gimmick. It seems to me that our ears are sensitive enough to discern if a radio station is on or off tune.

That said, if a radio does have a magic eye, if it isn't functional it does detract a little from its overall appearance.

(With the demise of so many stations, the DM70 will spend most of its time fully lit!).
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Old 27th May 2024, 12:24 pm   #14
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Default Re: Eddystone 840C receiver

Even if you don't intend replacing all those Hunts paper capacitors, if you have to open any areas up and clear other components out of the way to get in to change any components at all, check to see if you've now also got access to any of the little devils and make the most of the opportunity.

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Old 27th May 2024, 3:42 pm   #15
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Default Re: Eddystone 840C receiver

Quote:
Originally Posted by linescan87 View Post
I can remember reading here I think a few years ago about a mod to make the magic eye more sensitive on these. I've had a few 840's over the years and the magic eye didn't do much on any of them.
I wonder whether it was this post by any chance?

https://www.vintage-radio.net/forum/...41&postcount=3

Its the only reference I could find to a mod. It mentiones that the 840C AGC develops only -4V at full signal, but up to -15V is required to fully extinguish the eye. The mod allows the AGC line to go down to -7V making the eye more sensitive.

Quote:
Originally Posted by G6Tanuki View Post
It sounds like you have very little AGC voltage being developed, this will explain the lack of magic eye action.
Quote:
Originally Posted by David G4EBT View Post
Re the DM 70 Magic Eye, under no signal conditions it should look like the diagram below, with full brightness. At -9.0V on the grid, only the dot of the exclamation mark will remain illuminated. It does need a swing of several volts on the AGC line to vary the display.
David, there is no thickness at all to the exclamation symbol, its just remains a thin line. I may be imagining that its intensity is varying very slightly, but just barely enough to discern. Since I get -4V max on a strong MW station, its starting to make sense that as per G6Tanuki, there is perhaps an issue with the AGC that needs investigating.

Quote:
Originally Posted by G6Tanuki View Post
I would be replacing all the capacitors on the AGC line, yes some of them are hard to get at!! But it only takes one of them to be masquerading as a 5Megohm resistor to really upset things.

There are a couple of horrible little brown encapsulated paper capacitors in there, they are about 3/4 inch long and 1/4 inch diameter, with rounded ends, hence the colloquial nickname of 'rat droppings'. Hunt these down and replace them!!
Quote:
Originally Posted by Radio Wrangler View Post
Even if you don't intend replacing all those Hunts paper capacitors, if you have to open any areas up and clear other components out of the way to get in to change any components at all, check to see if you've now also got access to any of the little devils and make the most of the opportunity.

David
Thank you for the heads up. That sounds like good advice. I have heard about those "rat dropping" types and will keep an eye out for them. I see at least 3 Hunts red types below the tuning coil assemblies, but don't see any of those brown ones. Could be hidden from view though. Might as well think long term and replace any in such difficult to reach places while I am in there. In fact, I do intend to replace all of the Hunts paper caps.

BTW, I located what I think is C48 on the AGC line and snipped it on the IF can end. The result was a somewhat quieter but distorted audio and no change to the magic eye. I have temporarily re-attached it.
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Old 27th May 2024, 11:02 pm   #16
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Default Re: Eddystone 840C receiver

A very quick and easy way of reducing the anode voltage of the DM70, increasing the change of deflection with AGC voltage, is to connect it to the junction of R9/R10, i.e. the mixer screen grid potential divider. The current through this divider is far greater than the maximum drawn by the DM70, so its connection makes negligible difference to the mixer screen voltage. This is what I did with my 670A, essentially the same basic circuit as the 840 series without the BFO, to get more liveliness from the DM70.

I suspect that the red Hunts paper capacitors were essentially the same innards as the infamous murky-brown encapsulated ones, perhaps slightly better environmental protection but still faring poorly after 60+ years. Some of the later Eddystones were blessed with Mullard mustards but likely only the very last 840Cs.
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Old 29th May 2024, 7:42 pm   #17
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Default Re: Eddystone 840C receiver

That’s an interesting alternative approach to the 10k resistor divider suggested by the author of the linked article. I might give that a try when I am up and about again. Have unfortunately come down with something in the last 24hrs. Hopefully replacement caps and resistors will have arrived by then.

Unfortunately this example was not blessed with Mullard mustards. Would have been great if it had.
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Old 31st May 2024, 12:28 pm   #18
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Default Re: Eddystone 840C receiver

It struck me as a simple and pragmatic approach that avoided excess modification and additional circuitry and taking additional HT current- this type of "AC/DC" PSU arrangement doesn't give much leeway. After all, it's only a small and simple tuning aid rather than a precision measuring device. Additionally, the voltage at the junction of these two resistors rises slightly as AGC action with strong signals reduces mixer screen current- this adds extra visual discrimination with strong signals.
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Old 31st May 2024, 9:15 pm   #19
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Default Re: Eddystone 840C receiver

Some progress has been made. All of the accessible red Hunts capacitors have been replaced, including the one on the AGC line. I have also replaced one of the 25uF Electrolytics as well as 470k resistors R19 and R20 which were out of tolerance. I have ordered and am waiting on 2M2 resistors to replace R21, R22 and an open R35 between the anode and the input to the coupling cap on V7.

Quote:
Originally Posted by Sideband View Post
Edit, Crossed with your reply above! OK check the high-value resistors (and the anode load) but be aware that it could be leaky caps (like C56). Those Hunts red caps are paper so are likely to be leaky. Check the cap on the AGC rail (C48)....even minor leakage here will upset things...and therefore the way the DM70 works.
Replacing C48 on the AGC as Sideband suggested has made the magic eye functional. The line is brighter and almost full length on static and it fades and reduces to about half length when a strong MW station is tuned. The the original Hunts cap must have been leaky.

I haven't attempted the DM71 circuit modification yet but might have a go at that once replacement of parts has been completed. I still have a couple of small Electrolytics and those 2M2 resistors to replace in the accessible section, after which the next step will be the tricky one - replacing the Hunts in the tuning coil section.

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Old 31st May 2024, 10:17 pm   #20
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Default Re: Eddystone 840C receiver

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Originally Posted by WaveyDipole View Post
Replacing C48 on the AGC as Sideband suggested has made the magic eye functional. The line is brighter and almost full length on static and it fades and reduces to about half length when a strong MW station is tuned. The original Hunts cap must have been leaky.
Even a 3 megohm leak will affect the way the AGC works as the impedance of the circuitry here is very high.

That sounds about right for the DM70. You won't get much more movement out of it in this radio. Very similar to the way mine is working.

R21 and R22 might affect the DM70 if they are way out of tolerance. R35 provides a small amount of negative feedback in the output stage.
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