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    Re: The development of bubble sextants
    From: Jean-Philippe Planas
    Date: 2009 Aug 25, 05:17 -0700
    I had the same issue on an A12 I purchased 3 years ago.
    Following is the story on how I managed to "fix" the problem.

    [NavList 418] Re: A-12 Bubble sextant
    From: jeanphilippeplanas---com
    Date: 26 Jun 2006 12:47
    Previous messageNext messageReply to this message
    Hi all
    Further to Ken Gebhart and Geoffrey Kolbe suggestions I succeeded in extracting the Bubble Illumination Assembly that was stuck in the instrument body.
    I first proceeded in taking off the front glass window which gave access to the bulb. I then relatively easily extracted the bulb with a small pair of pliers from this opening. This is rather delicate because one has to squeeze the bulb to induce a rotation movement in its tapping without braking it.
    I thereafter had a partial access to the front end of the light assembly.
    I then proceeded to take the black rheostat cap off which enabled me to confirm that fortunately there was no old battery present in the tube.
    The next step was to find a way to push the tube out of its housing. I used an adjustable clamp actuated by a screw actuator and resting on the front end of the light assembly on the zinc casting and resting on the bottom side on a fixed part of the sextant body. This way I imposed a pressure on the front end of the light assembly and more precisely on the zinc casing. The tube almost instantly unstuck and was pushed 15mm (0.6 inches). Unfortunately the tube would not slide down any further and seemed to have hit a stop in its travel and to have got locked in this new intermediate position.
    I then had to add an extension arm between the clamp top side jaw and the zinc casting in order to be able to go on developing a downward pressure. Sadly at this moment the top of the zinc casting fractured and the tube was still stuck in its bore.
    The next step was to pour a lot of penetrating oil in this area and try to move the now protruding bottom side of the tube by hand. I also used a pliers at one time on the protruding bottom side of the tube but quickly stopped because the thin walled tube could not withstand the smashing force induced by the tool and got locally deformed. I resumed pouring penetrating oil and working the light assembly back and forth in its bore to induce shocks on the stop and it suddenly came out.
    The consequence is that I saved the rheostat and the tube part of the light assembly (had to put it back locally in a cylindrical shape to even out the pliers prints). Both components are fully usable now. Sadly the original zinc casting is destroyed beyond repair, but it can easily be duplicated on a lathe in any conductive material. I am currently thinking of brass or aluminium if I cannot find zinc.
    The origin of the problem does not seem to have been a question of corrosion or galvanic couple, but as Ken Gebhart stated, a question of geometrical expansion of the zinc cast diameter.
    My special thanks to Ken Gebhart and Geoffrey Kolbe for their precious advices and valuable help.
    JPP


    Ken Gebhart <gebhart---com> wrote:
    On 5/30/06 9:18 AM, "jean-philippe planas" <jeanphilippeplanas---com> wrote:

    Hi all,  
     
      
    I just purchased an A-12 bubble sextant that seems in fine condition but features nevertheless a serious flaw.
      
    The Bubble Illumination Assembly (with a bulb on, and also named the Night Illumination Assembly in the manual) is stuck in the instrument thus preventing the removal of the bubble assembly in place (and also preventing any night shots as I cannot put a battery inside or change the bulb). I am also unable to clean this part of the instrument.
      
    Could anybody familiar with these instruments suggest a procedure to extract this Bubble Illumination Assembly because I don't want to do anything foolish that could damage the sextant or the assembly.
      
    May be Mr Ken Gebhart could provide useful advice to solve this probleme if he is still monitoring this list.
      
     
      
    Looking forward for any advice
      
    JPP

    JPP,

    The problem is that the light holder has a zinc part that holds the bulb inside.  They say that zinc expands with age, and accounts for it being stuck inside after many years.  Our procedure is to take the black rheostat cap off, and using a long screwdriver as a pry-bar, try to pry it out using a wooden block as a fulcrum.  Using Liquid Wrench or a penetrating oil may help.  If no luck, you must take the sextant completely apart in order to have access to the front end of the light assembly.  First, break the light bulb (if installed), then using a wooden dowel, try to knock the assembly out with a hammer.  This may well fracture the zinc casting, but at least you will have the unit out.  You could then jury-rig a penlight, or something to stick into the cavity in order to illuminate the bubble.

    Good luck,
    Ken




    --- On Tue, 8/25/09, engineer@clear.net.nz <engineer@clear.net.nz> wrote:

    From: engineer@clear.net.nz <engineer@clear.net.nz>
    Subject: [NavList 9619] Re: The development of bubble sextants
    To: NavList@fer3.com
    Date: Tuesday, August 25, 2009, 12:34 AM


    Peter
    You wrote of your A12 sextant battery/bulb holder

    "I can pull the tube out about an inch or so, but no further. There are slits in the walls of the tube and I can see light from the slit on the opposite side. This means that I'm fortunate that there is no battery in the tube with the resultant corrosion. Do any of you have any special tricks for removing the tube? Heat/cold, etc.?"

    The A12 is notorious for corrosion and the two battery holders should not be stored in the instrument. You could try pushing it back in and removing the bubble unit and popping out the tinted skylight. Then employ a small child to apply some grease with its finger around the bulb holder, which is an aluminium casting attached to the thin-walled battery holder. Also squidge some grease through the slits from the inside while pulling and rotating. Then try pulling harder while pushing with your own finger. Forcefully rotating the thin casing with a wrench is likely to crush the rheostat housing or buckle the casing.

    I once had to break up the bulb holder casting and remove it in bits - a bit like old-fashioned obstetrics really - but the battery casing then came out easily. It wasn't too difficult to make a new bulb holder.

    Bill Morris
    Pukenui
    New Zealand



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