NavList:

Though it has taken two years and considerable help and guidance from 'PB' of this parish, I've managed to dismantle the innards of a Mark IX A, (save for the clockwork) and have successfully got it all back togather (thrice) and up and running.  By posting this news here, I hope to give heart to anyone who has long contemplated a renovation of such an instrument.  Herewith  follows a few topics which may be of help in being prepared for such undertaking.

You may already have a wide array of screwdrivers, I thought so too, yet had to resort to grinding down the blade tips to make an exact match with the various sizes of screw slot.  Even so, it is amost inevitable that a few od the slot flanges will be sheered off in the removal....and you will find that even the good ones, if original, will incur damage if re-used.  On the neccessary screwdrivers, I would make special mention of the need of one particularly for a retaining collar to whose access is almost impossible. What is needed, is a minimum length of 4.5 inches of blade, with a fine flat tip slightly less than 1/8th an inch and having a handle that can be re-shaped to the thickness of a little finger.  In the end, I found  an old fashioned spoke from a bike had the best steel  for the task...just bend the other end in a suitable offset for a twist handle.

Of replacement screws,  a couple of dozen each of.... 9BA by 1/4 inch slotted c/s;   6BA by 1/4 inch raised c/s; 8BA by1/4 inch, there may be others,( for which you'll need a vernier gauge to measure up) depending on heaviness of hand, but the real stinker ( for the specially described screwdriver) is 8BA by 1/8th inch, interior slotted, grubscrew of which only 1 is actually needed, though I advise a few more will be prudent. Better have a good stock of all screws as spare, cos at this stage,you can have no idea of just how many times you will need to open her up again, for minor checks and adjustments.  and for finding disappeared screws.

Not technically minded, I often need to resort to a plan of dis-assembly, organising, on paper,  the correct sequence and flow. A dozen or so of styrofoam cups, are easily labelled with with content details and, more importantly, the order in which the unit content is used.

Washers are crucial parts of those internal lever-pivot posts and they look almost all the same . It is of vital importance that these are put back exactly where they came from, otherwise, having used a wrong washer,on operating the instrument, you will need to strip down yet again and shift the washers around and try again.

It would serve well,  to make tiny witness marks to indicate the alignment of all screw heads and  be diligent at every re-insertion,  in checking the correct length of screw being used.  And , do talk yourself out of giving screws the extra half-turn (for luck) as you will most surely sheer the slot flanges and, if you're really unlucky and re-use an original screw, you may well sheer off the entire screw...meaning a drilling out and a re-tapping of the thread, always assuming there is space enought to work.

To remove the index bar, it is pre-requisite to extract the gearbox assembly, once free'd from its connectors. In the hand, it is a truly fascinating bit of engineering genius and unless there are crashingly obvious and compelling signs of internal damage, is best left alone.  

   Of the general layout of the internals, one of the most obvious pivot points is permenantly under tensioning so as to  reduce the effect of sloppy operation, ..quite understandable really , as accuracy is paramount.  This means that when being tampered with, it will tend to spring , deceptively, easily and swiftly ,apart.  You will doubtless find that its proper re-assembly  needs great deal of demanding patience and tactile skill.  I found that doing this was the most difficult and wearing of all of the work...made even more so, when providence conspired that several attempts were to be needed, one reason being the wrong washer was initially used.

If needed, a replacement index mirror  is easily fitted in approximate position, with just some minor re-shaping of the hold-down tabs, but its proper alignment will include either the use of a collimator device or a graduated level mark at some distance away.In any event, the actual alignment is set by miniscule tension applied by the adjustment screws and, the back tension from the static hold-down tabs.

When it comes to setting the correct mirror alignment, it can be done  by both adjustment of the index screws, and/or by the lower of the twin capstan screws that are abreast the long lever to the lower mirror.

To calibrate zero, I used a horizontal mark some 6335 feet distant and established by theodolite. At that sort of range an angle of 0.5 arcmin is a shade less than 12 inches and , even  additional optics and clumsy hands  together with tiny movements of the relevant screws, I would think it  a job well done to get the index as close to true as perhaps 1.5 arcmin.

I hope the above has gone some way to encourage those contemplating a furtherance, to at least, the getting apart of the chassis.

Many thanks, PB,

Loz 

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