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    Re: Improving the Link A-12
    From: Geoffrey Kolbe
    Date: 2012 Aug 24, 17:33 +0100

    >What I found, was that the unit has some serious 
    >limitations when used at night. For one, the 
    >lighting system on the index vernier scale 
    >seriously sucks, at least as far as my 55 year 
    >old eyes are concerned. So I replaced the old 
    >filament bulb with a red LED drop in 
    >replacement*, and ended up with something I 
    >could actually read in the dark. But by far my 
    >biggest gripe was that the backlight for the 
    >bubble cell really doesn’t work at all. What 
    >you want to see is a uniform luminous 
    >background, preferably red, where the bubble 
    >stands out as a nice black circle and the star 
    >is clearly visible as a small white dot. What 
    >Link provides is a far cry from that.
    >Because I’m an engineer by trade, I am 
    >genetically disinclined to leave well enough 
    >alone � especially when “well enough” 
    >doesn’tt really work in the first place. 
    >Luckily, the lighting system in the A-12 is a 
    >small flashlight-like affair that can easily be 
    >removed. So I designed a small cap that fits 
    >over the light bulb which does the trick. (See 
    >the attached shot.) It backlights the bubble 
    >cell with a uniform red glow. I’m having a 
    >really hard time figuring out why the good folks 
    >at Link didn’t think of this 70 years ago. Works like a charm.
    Nicely done Paul, but there are a couple of 
    points. First, there should a diffuser on the 
    bubble unit which diffuses the light. Another 
    light diffuser should not be necessary to achieve 
    a uniform background light for the bubble - at 
    least, this has never struck me as a problem. 
    Second, the correct bulb for the unmodified 
    bubble illuminator is the round type with a 
    single uncoiled filament wire. That is important 
    as you get a much more linear variation in light 
    intensity as you vary the rheostat with a 
    straight filament lamp than you will with a 
    coiled filament lamp. That lensed bulb you have 
    in your modified bubble illuminator unit is the 
    bulb for the index scale illuminator. The round 
    one with the straight filament may not have come 
    with your octant, which may have contributed to 
    your problems. They are probably impossible to 
    find now, but that is the type of bulb that should be used.
    Converting the scale and bubble illumination to 
    red LED is a good idea in principle, though 
    personally I have not found that the bubble 
    illumination ruins my night sight and I don't use 
    the scale illuminator. I use a red LED head lamp 
    to read the scale, read my watch and write down 
    the time and altitude. I made a variable red LED 
    background illumination device for my theodolite 
    and to make sure the illumination varied in a 
    linear fashion with potentiometer rotation, I 
    powered the LED from a two transistor 
    multivibrator so that the time the LED was on for 
    varied in a linear way with potentiometer 
    rotation. A similar unit could be made to fit 
    into the battery holder of the bubble illuminator on the A-12.

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