Quote:
Originally Posted by Devon60Ben
NOS magic eye arrived and looks great but doesn't open & close very much - staying around 3/4 open so need to look into that.
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The magic eye display depends on the AGC voltage swing on its grid. (Pin 5 of the 6U5G base and the plug to which it is connected). Unless there is a swing of several volts on the grid (up to 20 Volts AGC swing for the 6U5G), the eye display won't vary much, if at all. Easiest way to check the AGC voltage swing is to check the voltage with the set off tune, then again when tuned into a strong station. The thing that has most effect on signal strength (and hence AGC voltage), is of course the aerial.
This article, from the National Valve Museum site, originally from Radio Constructor March/April 1969, cites the 6U5G to explain how magic eyes work, so you might find it of interest:
http://www.r-type.org/articles/art-137.htm
As an aside, I restored a Pye 'Fenman 1' which sounded well enough at good volume on the internal wire aerial, but the EM80 magic eye didn't vary whether on or off tune. As EM80s are mercifully quite cheap, I took the easy option of fitting a new one, which made no difference. I made a little EM80 tester to vary the voltage on the grid to see how much it needed to go negative to vary the display. That proved that both the new and old eyes were working fine. It then dawned on me to plug in an external aerial, which gave a swing of several volts on the AGC line, and hence on the grid of the EM80. An interesting learning experience which cost me several hours of my life which I'll never get back!
Likewise, I restored a Unitra 'Figaro Special' which has a DM70 magic eye. (The 'exclamation mark' display). The set has an internal ferrite rod aerial on which it performs very well at good volume. However, the AGC varies little when off tune or tuned into a strong station. The set has an external aerial socket too, and when I plug an aerial into it, the AGC voltage swings by -9V and the display shrinks to just a dot.
Hope that might help a bit.