NavList:

Bruce

Issues with A10: Main bearing often frozen and difficult to un-freeze; silvering of prisms often deteriorated, especially the "horizon" prism which won't work properly without it.

Issues with A12: Corrosion everywhere but especially the scale and bubble lighting units, which sometimes have to be destroyed to extract them; teeth on toothed sector ofen stripped.

Issues with Mk IX: relatively complex but bubble unit ofen intact after 70 years; mirror usually in fair to good condition, but easily replaced.

Bubble unit refills:
A10 is moderately difficult. Suggest using Viton O-rings to re-seal. The vapour pressure unit (black control unit) is easier than the air-bubble unit (bulkier, chrome plated unit). Must refill vapour pressure unit with xylene (or naphtha).

A12 is easy though if the glasses are dirty it can suddenly get difficult. Can be refilled with alcohol, xylene or naphtha.

Mk IX: Fairly easy to refill though needs soldering iron and de-soldering wick. Must NOT be refilled with alcohol. Originally filled with hexane, but xylene or naphtha will do.

Though the MkIX looks clunky, it is very easy to use and lacks the "complification" of later US designs. For actual use in aircraft, the MkIX A, AM or B have a much better averager (60 shots versus 6), but are heavier and really need a hook from which to hang them.

See www.sextantbook.com about overhaul manuals

Bill Morris
Pukenui
New Zealand.
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