Hello,
Over the past three years I've renovated five of these instruments, all purchased on ebay, using to begin with Bill Morris's excellent manual, which I can heartily recommend. I can also agree with those members who believe that these are the best bubble sextants available (although I know little about other models), because they are strongly built, straightforward to use and relatively easy to access and restore. (This includes refilling the bubble chambers, if necessary. Again, Bill's manual gives full details, and there used to be instructions on the internet too.)
The models I have done are Marks IX, IXA, IXB and IXBM. The prices I paid varied enormously, from roughly 80 GBP down to only 10 GBP -- but the latter was a 'freak' purchase, fetching a low price because the case and instrument looked in much worse condition than it was. Looking at recent Ebay prices, however, there does seem to have been a steep rise in the prices that some sellers ask. If I bought another now, I wouldn't want to pay much more than about 50 GBP.
When I bought my instruments, I always asked whether the mirrors are damaged or tarnished and if all dials and knobs move freely. (I don't ask about whether there is still a bubble, because there most often isn't and, if there isn't, I don't mind refilling the chamber. But this is a personal preference: there are models out there which
can still produce a bubble, though if the bubble is large, the chances are there's a slight leak, so you will need to refill anyway.) However, it helps to be aware that if the chamber has dried our completely, the insides of the bubble-chamber glasses may be coated with a residue that can't be removed without dismantling the chamber. This isn't an easy job, and there's no guarantee that reassembly can be achieved without a leak. So, to avoid this, you might also ask the seller to look and see if the view is cloudy when they look through the eyepiece in daylight, with one of the lower filter-numbers, say 2 or 3, in place. (A few sellers will have a go at this, though some will be a bit baffled by your request!)
It is true that mirrors can be removed and resilvered (though this is a job I've always tried and managed to avoid) and the inner workings lubricated. And I have always guarded against getting a model with damaged gears because (as Bill rightly points out in his manual) cutting/replacing damaged gears isn't a job that most users will be able to do. (One of my models has badly worn gears, and gives results more erratic than my other instruments.)
Finally, models sold by Ebay sellers as 'functional' will not necessarily mean that they are in calibration, so anyone buying a Mark IX must be prepared for their purchase to arrive out of calibration. Of the seven sextants I bought, only two were accurate, and with both (and with great care, and some luck!) I can get readings within one arcminute. The others, however, were all about 5-6' out when they arrived. It's a purely personal thing, but while there are more scientific methods, I've recalibrated my other instruments by taking lots of 1st mag star sights, reducing immediately on PC, and adjusting accordingly. This is a lengthy process, but with these instruments I get results within 2-3 minutes. (NB, these figures are for single shots, as I don't use the averaging mechanisms.)
I hope this helps...
Ian Payne
> Date: Sat, 15 May 2010 12:37:37 +0200
> From: scheele@telkomsa.net
> To: NavList@fer3.com
> Subject: [NavList] buying a Mark IX A bubble sextant
>
> I would much appreciate it if list members could advise me on what I should be looking out for when buying a Mark IX A bubble sextant and on how much I can expect to pay for a "fully functional" model. What would an overhaul entail? I understand that this is a subject which has been discussed before, but I would like to approach it again to get some piece of mind. A couple of things scare me away from buying one, the extreme difficulty of finding replacement lightbulbs being just one example.
>
> Christian Scheele
>
>
>
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