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What Bill says is true, but the backlash spring was designed to be
too weak.  The A10 and A10A were known as the "backlash sextant"  by
old air navigators.  Backlash would creep in after even a short
period of time with the lubricants then being used.  There is an
accessable screw under the averager plate that was often turned a
little CCW to loosen things up and restore backlash free operation as
a quick fix.

Ken Gebhart
On Nov 28, 2008, at 1:15 PM, engineer wrote:

>
> Bruce
>
> There shouldn't be any backlash with an A10, as it is provided with a
> backlash spring. The index bearing after 60-odd years is often in need
> of lubrication to ensure free movement (I once had one completely
> seized). The bearing can be difficult to pull apart and reassembling
> with the anti-backlash spring correctly tensioned can be a trying
> exercise. You can probably improve the index error by setting the
> instrument to a sea horizon, removing the read-out counter, setting it
> to zero (allowing for dip) and replacing.
>
> I will be covering these and other details in an e-book on A10
> restoration, due out at the end of February. It will be along the
> lines of my e-books on the reconditioning of the Mark IX series bubble
> sextant and my "Naked Nautical Sextant". See www.sextantbook.com for
> details. Meanwhile, you can contact me off line about your A10 if you
> wish.
>
> Bill Morris
>
> On Nov 28, 4:08 pm, bruce hamilton  wrote:
>> All times GMT 27 Nov 2008
>>
>> A-10 Sextant with 31.5' index error off the arc (horizon reading
>> is lees
>> than zero in case if am mixed up)
>> Sextant has no other calibration. I frequently get LOP's within 2
>> miles
>> of my AP so I am happy. It has backlash so I try not to change
>> directions.  I suspect some other error as stars with higher
>> altitudes
>> tend to give me bigger errors.
>> I am actually quite surprised that I am getting a fix that is close
>> enough to find the airport (or Bermuda) if I had do. I have an
>> A-12 and
>> Mark IX, but I only drag out one an evening.
>>
>> Thanks
>>
>> I am presently using CelestNav 3.0 for the palm pilot to work out the
>> sites. I would appreciate any comments on the program. I like it for
>> everything but the fix calculation, but I think 3.0 is only a Beta.
>>
>> AP 49d  16.1 N  123d 7.1 W
>>
>> Pollux   06:19:33   27d  59.3'
>> Vega    06:16:31   13d    0.3'
>> Betelg   06:10:53   28d  44.3'
>> Altair    03:56:11   22d  37.0'
>> Vega    03:54:45   32d  06.9
>> Capella 03:52:46   39d  35.7'
> >


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