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Jean-Philippe Planas wrote


>2) The A12
>-The bubble assembly(ies) always need refilling. But it is a very easy
>task to be performed provided you've got the correct tools (Bristol
>wrench). Thereafter the refill must be redone every now and then to keep
>the bubble the correct size due to undetectable small leaks.
>-The shades tend to separate from their mounts due to the cement ageing.
>If the shades are not lost they are easysy glued again in their mount. If
>they are not present in the box, one has to find a substitute (not easy to
>find)  to be cut to dimension and glued in place.
>-The battery cannister allowing the night shots can be missing preventing
>any nignt observation, or it might be stuck in place. Then it is very
>difficult to extract without damaging eather the sextant or the battery
>cannister.
>-Sometimes the reflecting mirror located in the bottom of the mirror tube
>requires re-silvering.
>-The main issue with Ebay A12s is that they almost often feature damages
>on the edges of  the sector gear and on the anti-backlash mechanism due to
>mishandling or inappropriate use of the instrument. If to many teeth are
>missing the instrument is useless and nothing can be done.
>
>The A12 is a light rugged instrument very simple to operate and maintain.
>The altitude is measured via a vernier which require a magnifier to read
>to two minutes precision. The night lighting sytem definitly does not
>provide good performance.
>I had to buy four of them to get two in excellent shape and working order
>with the complete set of accessories along with a good calibration and the
>last two are working to some extend.

And

>The conclusion is that, depending on luck, you'll have to buy several
>units (on Ebay) to be able to eventually get one in good working order.
>But it is also part of the adventure as it is nothing more than a hobby.
>Finally, for the ease of use and overhaul simplicity I would recommend the
>A12.
>
>JPP

My experience coincides with that of Jean-Philippe. However, I would add
that if the bubble chamber on the A12 has dried out then there may be a
white precipitation on the inside of the clear window of the chamber.
Putting new Xylene in the bubble chamber does not dissolve this
precipitation, which then has two effects. The first is to make the bubble
picture a bit murky, which is a minor inconvenience. But the second effect
is that the bubble does not slide smoothly over the window as the sextant
tilts. This 'stick-slip' effect seriously affects the accuracy of the sextant.

With a clear bubble and good clean optics, the A12 is capable of returning
altitudes accurate to one minute (most of the time) or two minutes
(sometimes). If the bubble chamber is dry and the window is not clear....
well, I have not yet succeeded in removing the small ring nut which holds
the window in place.  If I could get it off and remove the window, I am
sure I could mechanically clean it with something like 'Brasso' which is a
metal polish.

Perhaps Jean-Philippe or someone else has managed to remove a window from
an A12 bubble chamber.....?

Geoffrey Kolbe


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