NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A-12 Bubble sextant
From: Jean-Philippe Planas
Date: 2006 Jun 26, 12:47 -0500
Ken Gebhart <gebhart@celestaire.com> wrote:
From: Jean-Philippe Planas
Date: 2006 Jun 26, 12:47 -0500
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@celestaire.com> wrote:
On 5/30/06 9:18 AM, "jean-philippe planas" <jeanphilippeplanas@yahoo.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
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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
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