Testing magic eye tubes

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EM85 under test. A perfect 100% working specimen



So, I bought a large amount of tubes from a gentleman, who had been restoring radios for more than 30 years.

Many of the boxes was of little interest for me. Tv tubes and pure "bc radio" tubes, but there was a large lot of magic eye tubes, you know, the green glowing things, that was made to aid in tuning in to stations back in the analog VFO era.


Function

Magic eyes, or indicators, work by directing a stream of electrons onto a phosphorus coated surface. 

The phosphor starts to glow green when hit by the electrons, and we can control the way they hit, by inserting a negatively charged element in the way of the electrons or beside the electron stream. By varying the amount of negative voltage, the shape of the beam varies.


Different types

I mostly had EM84´s, which is a "closing" bar. Used in fm receivers and tape decks among other things. The bar is projected vertical on the side of the tube.

I had a few EM80´s and one EM85 (rare) that has a leaf shaped projection area.

Also, there were a lot of EM34's and variants of that. The EM34´s projects a circle, with a two range "pie" cuts, that vary in size depending on the negative voltage applied to the grid.


Testing

I used my Utracer as testbed, but I only used the tube sockets and the filament psu. I had a smalll DC-DC nixie tube power supply which I used in conjuction with a variable PSU to generate the negative voltage, which worked fine.


A bit dull EM80






A very bright UM80, an EM80 with 19V heater


7j
A very nice looking EM84


EM34, here with both "pie cuts" open





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