EM 80 in Wega 201 radio.

[David G4EBT]

The EM80 requires several Volts negative to have and appreciable effect on the display. Depending on which data is referred to the spec states that the negative swing can be up to 16V (with 200V HT) or up to 20V with 250V HT.

I only discovered this after much head-scratching when - some years ago - I'd restored a Pye Fenman 1 which worked fine in all respects except that the magic eye barely moved whether on or off tune. In my ignorance, I naively assumed that the eye was faulty. Fortunately, NOS Russian EM80 equivalents are cheaply available. The only slight difference is a physical one the Russian ones are slightly larger in diameter - no more than 1mm as I recall, which in most circumstances won't matter but in my case, as the eye is housed in a 'shroud' made of hard rubber, I had to make a new one from beech.

The new eye duly arrived and it didn't make a scrap of difference. I therefore made up a little test circuit to enable me to vary the negative voltage on the grid using a few batteries - 1.5V, 3V, 4.5V 9V. So for example, a PP3 on the grid of the valve with the negative battery connection to the grid and the positive to ground, showed the effect. The little tester exonerated both valves.

That still didn't solve the problem of the unresponsive display in the Fenman 1. Then it dawned on me that all I'd been using for an antenna was the piece of twin flex woven in and out of the slots on the back panel. In terms of the set's performance, that's all that was needed as an aerial to receive signals at good strength. However, on plugging in a external aerial the display was very lively.

Basically, without an external aerial there was very little AGC voltage swing, which the eye relies upon to vary its display. The same phenomenon applied to a little Unitra Figaro Special I restored a year or so ago. It uses an 'exclamation mark' DM70 Magic eye. The set works fine on it's internal ferrite rod, but whether on or off tune, the display hardly varies. As soon as I attached a short wire external aerial, on a strong station the exclamation mark all but disappears.

So, my guess is that there's nothing much wrong with the set or with the eye, beyond a lack of AGC Voltage swing.

The attached sketch and notes might help a bit, and this too:

http://www.bunkerofdoom.com/tube/man/eyet.pdf

[ms660]

The antenna has already been covered in Post#2 in this thread David, parallel threads on this radio.

https://www.vintage-radio.net/forum/....php?p=1048698

Lawrence.

[David G4EBT]

Thanks Lawrence - I hadn't been following that thread (or this one, for that matter).

In now looking at both threads, it's been re-stated several times that for the eye to operate correctly, it requires a negative voltage on the grid - the display varying in proportion to the negative volts. Secondly, that the negative voltage is derived from the AGC line when the AGC comes into play on strong signals. In post #7 of the parallel thread, the OP stated:

"I have monitored the voltage on the grid (pin 1) of EM 80 (6E1N or P) and it does not vary whilst tuning station".

So until a swing of several volt negative on the AGC line can be attained, the problem will prevail, but that's re-stating the obvious and of little help.

[ms660]

Out of interest here's an article (No: 24) by the Mullard Technical Service Department on the EM80, it's in the 3rd link of Post#1 in the thread below:

https://www.vintage-radio.net/forum/...ad.php?t=62844

Lawrence.

[David G4EBT]

Thanks Lawrence - an interesting read.

I've often heard the display referred to as 'angel wings' which open wide on strong signals, but in fact Mullard use the term 'petals'.

What the EM80 has going for it is that unlike EM34s, they seem not to loose their emission over time, and they're cheap and plentiful.

(EG: two NOS for £19.45 inc post from Ukraine).

[paulsherwin]

It's not that they lose emission, it's just that the metal targets stop fluorescing in the electron beam. All the metal target magic eyes fail in this way after prolonged use. The EM84 with the phosphor on the glass seems to last much longer.

EM80s remain relatively cheap because the USSR used them in military equipment that remained in service into the 90s, so there are warehouses full of the things even now.

[longneck90]

Hello to all, and many thanks to Lawrence, David G4EBT, and everyone else who has provided a mass of very interesting information, which I am reading and re-reading, so as to have a better understanding of the workings of valves like the EM 80. Sorry for delay in replying. First, the good news; I appropriated a digital multimeter that registers - minus voltage. Sadly, no progress with finding out why 6N1P isn't working. Well... it is working, if the proper minus voltage was reaching it. The voltage at Pin 2, EABC80 is - 0.56 volts. The voltage at Pin 1, 6N1P is varying between 0.7 volts to - 1 volt. When I disconnect R22 from circuit, the voltage is - 0.45 volts at Pin 2, EABC80. I connected a 12 volt battery, + to Pin 2, EABC80, and - to Pin 1, 6N1P. The envelope opened fully, as seen in photo. I connected - 3 volts in the same way, and the envelope also fully opened. What I intend to do now, (tomorrow) is, get new, correct value resistors and capacitors, implicated in the AGC circuit, and install them in place of original components. If that does not result in the 6N1P working properly, I'll leave well enough alone All the best. John.

[ms660]

Originally Posted by longneck90

The voltage at Pin 2, EABC80 is - 0.56 volts.

Was that with respect to chassis and with an FM station tuned in?

Lawrence.

[kalee20]

It's a good and easy test, longneck90, and you've seen your eye do its stuff into the bargain!

The EABC80 should be throwing out several volts of signal-dependant DC. If it isn't, then it could be because you have a very weak signal - but in a hat case reception would be weak and hissy. Or there could be a fault in the circuit - but in that case, reception would likely be distorted.

Is there a small electrolytic capacitor in this part of the circuit? Probably between 2uF and 10uF. It's an essential part of the ratio detector used for FM. Try measuring the voltage across it. It should be a few volts as a station is tuned in. Possibly this capacitor has gone leaky. It may be worth replacing, as a matter of course.

[longneck90]

Hello to all, and thanks for all your help and information. Yes Lawrence, the voltage - (minus) 0.56 volts was in respect to the chassis, and there was no difference between MW and FM. To kalee20; there is good reception here, with 40 foot ariel across the garden. The radio has great sound and no distortion. There is no variation in - voltage when tuning in a station. Capacitor C38 (2uf) is bothering me, as it is shown + side to earth. I have replaced it, but with + side to earth, as shown on circuit diagram. The voltage across it is 0.56 volts. Thanks to all, and I'll update if there is any positive result. All the best. John.

[kalee20]

Yes, it really will be +ve side to chassis. It generates a negative voltage.

0.56V across it is just wrong, though - there should be several volts as a station is tuned in! Do you have a spare EABC80 you can swap in? It's unlikely to be the problem (though possible), but it is such an easy thing to do, that it's worth it just to discount it.

[longneck90]

Hello to all. Just to report on the ongoing saga of the Wega 'Magic Eye'. I have tried a different EABC80; no change. I have traced and re-traced the wiring countless times. I can't see anything wrong there. There seems to be a resistance between 11 and 12 on wavechange switch. Ie. Between Pin 1, 6N1P, and R22. Anyway, what about this? While writing this, I had to go and check something on the radio (Wega), and I decided to try some resistors between Pin 2, V2 and Earth. Lo and behold, when I tried a 20K resistor between Pin 2, V2 and Earth, the sound increased dramatically, and the 6N1P opened completely. It also operated in synconisation with the broadcast. Now that has got me thinking. Thanks to everyone for their help and information. I need some 'thinking time' now, so all the best. John.

[longneck90]

Hello to all. I am afraid I made a mistake in my last post. Excitement... I inserted the 20K resistor at junction R10/C23. While the 'Magic Eye' works on MW, it does not work on FM. All the best. John.

[julie_m]

It looks to me as though the fault is low gain somewhere inside the AGC negative feedback loop. If there is less gain somewhere than there should be but still just enough to give a listenable signal, then it will develop a lower "turning-down" voltage. And this voltage is what the magic eye is measuring: the gain is already low, so it needs less AGC voltage to reduce it.

Valves can go low-gain as they age, but the first suspects should be the passive components within the AGC loop. Resistors can go out of tolerance, and capacitors can become electrically leaky. AGC circuits in particular often have high-value resistors and demand low-leakage capacitors. It's very unlikely to be an alignment problem (there ought to be some form of audible distortion, not just low gain, if that were the case), so don't go there until all else has failed.

[longneck90]

Hello to all. Wega 201. I have just discovered that, by connecting 20K resistor between Pin 2, V2, and Earth, the 6N1P is working perfectly on MW and FM. Now I just have to find out why. All the best. John.

[ms660]

Check the AGC line resistor(s)

Lawrence.

[kalee20]

Originally Posted by longneck90

Hello to all. Wega 201. I have just discovered that, by connecting 20K resistor between Pin 2, V2, and Earth, the 6N1P is working perfectly on MW and FM. Now I just have to find out why.

I'm at a disadvantage - which valve is V2? I don't have a circuit diagram of this set! But I'm intrigued!

[longneck90]

Hello to all. To Lawrence; I'll check the AGC line resistors again. I am a bit pre-occupied with football just now. To kalee20; Valve 2 is EF89. I hope the photo shows the circuit clearly. All the best. John.

[longneck90]

Hello to everyone, and thanks to all who advised me and provided useful information on the above. This is an update on 'The Story so Far'. Today, I found the culprit, and the 'Magic Eye' is working perfectly now. The offending component was C 15. This is a 5n capacitor, situated between Pin 1, V1, and earth. All the best. John. I forgot to say; the capacitor was short circuit.

[julie_m]

Result!