How do magic eyes fail (EM80 tuning indicator)

[HoverJohn]

Ok so the radio is up and running in the Dynatron picking up stuff from a long way off(all the London FM stations half way to Bristol on the internal Ariel ). But the tuning indicator is not indicating much just a green line it’s not fanning out. Is that dead or have I affected it with the recap of the main radio? (See pic)
Also a school boy error I should have tested it on the bench as the 2 inaccessible bulbs are dead so I have pull it out of the set again!!

Ta JB

[jjl]

It looks as though the high value resistor in the anode circuit of your EM80's triode amplifier has increased in resistance or gone open circuit. This prevents the eye responding to the AGC or discriminator voltage applied to it. This is a very common fault.

The other common fault with magic eyes is that they lose emission and the green illumination becomes very dull or extinguishes altogether.

John

[kalee20]

Agree with John - it sounds very like an open-circuit resistor on the DC amplifier triode anode. So the anode voltage is very low, and as it is connected to the deflecting pins, the electron beam is focussed into a very narrow beam.

Originally Posted by jjl

The other common fault with magic eyes is that they lose emission and the green illumination becomes very dull or extinguishes altogether.

Yes... 'losing emission' is usually considered a cathode problem with respect to electron emission. But a milliammeter on the target connection generally shows the current is OK.

What the degredation seems to be is failing phosphor activity - it ceases to emit light when bombarded with electrons. So it could be considered to be 'losing emission,' just a completely different form of emission from a different electrode!

[HoverJohn]

Ok I will have a look at the resistors and check the valve is seated properly
I’ve attached the schematic and guess it’s either R37 or R36 causing the issues R36 was removed and put back in swapping C49 as access was tight. It’s the one bottom center on, looks the same in the after pic.

I also have a cheap soviet spare tube to try but it’s not a mesh type.

John & John your help us appreciated

John.

[jjl]

It's most likely to be R37, but there's no harm in checking R36 as well while you're at it.

John

[HoverJohn]

Ok well when the bulbs arrive I will pull the set and have a good look.

[David G4EBT]

EM80s are generally long-lived, unlike say EM34s, and NOS ones are plentiful and cheap too. (They also go under the designation 6E1P and 6BR5). Three years ago, typically £10 inc post, but now, a bit more expensive (largely because eBay now adds VAT on imported goods, which it collects on behalf of HMRC).

EG. Google '6E1P / 6BR5 / EM80 Magic Eye NOS Tube' will typically bring them up on eBay at £12.60 post free, from the former Eastern Bloc.

From my limited experience, if the HT Voltage is adequate and the display is dim, the emission is low and the eye is past its best. If the emission is good but the eye varies little when on tune, it's most likely due to the AGC voltage varying little when the radio is tuned in to a station.

Back in 2017 I was restoring two radios which had EM80s, (a Pye 'Fenman1' and a Ferranti '045'), neither of which altered when a station was tuned in at seemingly good strength. I ordered NOS EM80 replacements which were no better, then I decided to make a little EM80 test unit on which I could vary the AVC Voltage to make it more negative. I tested each one with the AVC at -1.5V and -3.0V The old eyes and the new ones all worked fine. When I checked the AVC voltage on the radios, though the audio level was at good listening volume, the negative AVC voltage varied little.

To check whether an eye is working or not, a quick test is to connect say an AA or PP3 battery with the negative to the grid of the eye, and the positive to ground. If the display responds, the eye itself is working. If an eye is dim, it can be due to lack of HT if resistors have gone high, but as a rule it's because the phosphor has gone to magic eye heaven.

Pic I below is the circuit of the little EM80 tester I made on which I could apply 1.5V or 3V negative to simulate variation in the AGC voltage.

If an AM set has an internal frame aerial or ferrite rod, it’s worth connecting an external aerial, if only a few feet of wire, to compare the eye display. If it’s on FM and has an internal aerial – often just a simple dipole on the back panel of the radio - try plugging in an external ‘ribbon’ type aerial. The second and third pictures below are of a Pye Fenman 1. Pic 2 is with R4 of VHF tuned in on the internal dipole on the back panel of the radio; pic 3 is on a ribbon VHF dipole hung up in the workshop. The difference is quite marked.

Pic 4 is a Ferranti’ 045’ tuned to RTE on 252M Long Wave with a 20 Metre end fed wire aerial.

(A 20 Metre wire aerial is just 1/12th of a wavelength on 252 Metres, so it isn't a 'long wire' aerial, it's a very short one, but the best I can do).

Pic 5 is with the radio connected to my homebrew Gary Tempest designed ‘Wellgood’ Amplified loop antenna, (which featured in the BVWS Bulletin and other threads on the forum). It shows the difference in signal strength, compared the the wire aerial, and proves that both the eye and the loop antenna are doing what they're supposed to do.

Some links which might be of interest:

General magic eye site:

http://magiceyetubes.com/

Two EM80s in a stereo amp:

https://www.youtube.com/watch?v=jXsG...M&spfreload=10

I hope these musings are of interest.

[HoverJohn]

Ok thanks for this I know it used to work before the set sat idle so I will check the grid voltages and resistors.
Does anyone know where I can source the type of em80 with a perforated glowing bit to match mine it’s like the fine mesh’s screen in a gec rf86

[Walterh]

Hello John

If you have an EM80 like in the attached picture, you have to search for an NOS Siemens EM80. But note - in later years, Siemens also sold EM80 from other makers under their name, after stopping producing them themselves.

Also note, that the russian substitute for EM80 wear out much faster than e.g. NOS Philips. Generally, EM80 are not long lived tubes compared to the later EM84.

Kind regards, Walter

[HoverJohn]

Yes that’s the beauty right there. I’m hoping it has life in it still as it’s not been used much since my grandfather passed in 1978.