I was setting up my scopeless No4(T) (The one where the crate went to US via ebay some months ago) for the weekend when I discovered that the elevation reticle sticks and needs approx. 10 clicks before it moves again either way.
Can anyone advise how to fix this please?.
The scope looks to be in excellent condition so I do not wish to ruin it by tinkering.
Thanks.
you need to ask peter
Contact the helpful people at Fultons of Bisley. They may be able to put your scope in front of the right person for it to be properly repaired.
What you describe is a trait of dirty Mk3 and L1A1 telescopes and not Mk1's and 2's. In the meantime, here's a little test. Go up to 10 (or down 0) and slowly rotate the drum upwards or downwards but look at the inner/central brass part of the drum. This reverse slot spindle is the lead screw. It should turn at the same rate as the main outer thumbscrew.
What you are indicating to me is that as you turn the outer thumbscrew, the inner leadscrew is not turning at the same time/rate. This leadscrew is/should be locked to the outer drum via the lead screw lock nut and in turn, is threaded into the diaphragm nut. Lead screw moves, then so does the diaphragm and grat.
Do that test first
There you have it.
Got the adjusting tool?
Thanks very much for the advice Peter. I have made the tools, and and tried again. The lead screw seems to be moving with the drum correctly . A clunk at top and bottom of travel (64 clicks)followed by 14 clicks before the reticle starts to move and then only slightly for a few clicks then on to 64 clicks with 1MOA movement per click. With the locking ring loose, and the lead screw at stop(bottom or top)the lead screw needs a just over a quarter turn before the reticle starts to move. As if the threads are worn at either end or the scale or perhaps play between the diaphragm nut and the erector cell?.
Is there an exploded diagram for the Mk2 showing this mechanism or is it easier just to remove the turrets and have a look?.
If you feel confident you could remove the turret head assembly (4x 8BA cheesehead screws) and carefully lift it clear. Treat the graticle block as delicately as you can and even then, only hold it at the left and right edges. But to be really honest, if you have suddenly found yourself with the equivalent of 14 clicks of backlash there's something more than backlash at large there. But see what you find.
The lead screw nuts have a mechanism built in whereby you can tighten the nut up on the lead screw but I'm not going to go into detail here.
There is an exploded diagram from the original 1942 parts list but not out in the public domain
I took the elevation drum apart last night, with the help of lots of magnification. Fascinating miniature engineering and all looks OK. So how can the graticle not move when the threads are (almost) directly engaged. Either the base of the lead screw is finding movement at the base of the turret, which seems to have been disturbed previously as the screws have been undone, or the lead screw nut threads are worn. The lead scew nut is split into two halves and presumably can be adjusted by shaving off a small amount from each half to tighten it around the lead screw threads.
It is almost worth what I paid for it just to see the internals. No, maybe not!.
Detent grooves picked clean of hardened 70 year old grease at 2am this morning, greased and re-assembled. Nice clicks. I will try it again today and see if I have inadvertently changed anything.
Is there a coil backlash spring in the bottom of the reticule block, or the twin wire springs on either side of the steel reticule slide?
Yes. I gave up for a while having dropped a blob of grease onto a lens at 2am!. I think the spring may not be sitting correctly on top of the grat block. I will have another look when I am feeling up to it as I am now determined to understand how it works. There seems to be quite a lot of opportunity for wear and backlash in this design, between the brass to steel threads, the right angle connection and the pin to grat block link, so the springs would seem critical for it to work properly?.
And of course I now have to get the grease off the lens!.
Hats off to Peter. What a fiddle!.