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 Post subject: Wild M420 focus mount problem
PostPosted: Wed Jun 13, 2018 2:31 pm 
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Well, I got a little caught up in bidding on eBay and ended up buying a Wild M420 (at what I think was a good price). I really should get the standalone makrozoom lens working first but I was hoping this would be a shortcut to the same end result (not to mention it's more fun to have eyepieces).
Unfortunately it arrived with a couple problems. Two minor ones on the base (which has a precision x-y stage), a broken-off thumbscrew and a few loose rollers, neither of which are likely to cause problems for me. The real issue is the focus mount. Focusing works smoothly as I turn the knob until I hit a certain point in the rotation where it suddenly loses grip and the head falls to the bottom of the rail. I'm hoping this is possible to fix because the image I see through the eyepieces looks really darn crisp! Has anyone run into this problem before?
I will get some pictures of it in a few hours, but it's the original Wild-only version with a cylindrical stand, not the later Leica one.
edit: here's a picture:
Image


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 Post subject: Re: Wild M420 focus mount problem
PostPosted: Mon Jun 18, 2018 8:08 pm 
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OK, I had a look through the service manual and started to get it apart.
Step 1: remove the makrozoom objective. Very easy to do, it's a dovetail connection held on with a single screw.
Image
Step 2: unscrew this hex screw (then two others) to get the focus drive housing off of the head
Image
That's as far as I got, because this screw is STUCK on. Is there a product that would help loosen it that would be safe to use?

On the plus side, removing the objective did give me a chance to get eyes on the problem:
Image
Unsurprisingly it is a broken gear. The head uses a rack and pinion style gear mechanism and one of the teeth is just straight up gone. The whole thing is also fairly worn, it must have seen a lot of hard use.

So at this point I am not 100% sure how to proceed. While it appears to only be a single part that's broken I doubt I'll be able to find a replacement just for the pinion? This may mean I'll need to replace the entire focus drive by cannibalizing one off a (more) broken scope. It looks like the M5 and M8 stereoscopes might have compatible focus drives in addition to other 400 series makroscopes. And of course I'll have to get this screw unscrewed somehow to access it.


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 Post subject: Re: Wild M420 focus mount problem
PostPosted: Thu Jun 21, 2018 9:52 pm 
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OK, I did try putting a little bit of liquid wrench in the thread and leaving it overnight. Unfortunately as events unfolded I couldn't go easy on this screw:
Image
Turns out you can strip a hex screw even with a perfectly sized wrench (3mm). We finally got the screw extractor seated, and after the application of some tremendous force it suddenly popped.
Image
Yup, whatever son of a fish last maintained this did use loctite or some other epoxy.
Once I knew that, though, I was able to get the other two out by exerting a lot of force and pressure. Both popped free and came quietly thereafter.
Image
This allowed me to move the rack down and get a look at the collar that holds the focus mechanisms together:
Image
thankfully these screws were much easier to get out, since if I stripped one of them I'd definitely be up a creek.
Image
With the collar off, the knobs on the right side unscrewed:
Image
Allowing me to get two separate parts, the right knob with a small threaded section that goes into the collar, and the left knob which has the pinion gear attached:
Image
Image
I think the next thing I'll have to do is get one of those two-toothed key things to get this unscrewed. This part is officially past where the service manual goes, so I am getting into uncharted territory here.

Image
Not only is the pinion badly worn, you can also see it appears to have been hand-cut, and the teeth are diagonal relative to the long axis of the pinion. I am not sure why.
Image
The rack also shows some wear, but nothing catastrophic. Hopefully replacing the pinion will be an adequate long-term solution (especially since I don't plan to abuse it; I don't know that that's what happened here, this may be normal wear and tear; but based on the state of that gear I think there is an outside chance that the lab it was in was staffed by overexcited howler monkies.)

So, I guess once I figure out how to get it loose I'll see if I can have a replacement made. I have had Al Meekins at gearsmade.com recommended, so I will probably try there first.


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 Post subject: Re: Wild M420 focus mount problem
PostPosted: Fri Jun 22, 2018 10:43 am 
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wow!
The epoxy was likely put on at the factory. Also the two screws you picture have those very interesting lock washers from switzerland. You are VERY lucky that you didn't run into a big headache with those. I ended up having to drill out three of them because I snapped Wiha (Willi Hahn german made) allen keys in them. They would not budge. I drilled carefully and got three barrel shaped rings. The two parts separated. There were three threaded studs with conical drill starts in them. These unscrewed with two fingers. No thread locker at all. Just impossible to break loose lock washers. It was one of my more remarkable repair experiences.

If the screw you drilled out bears weight you should replace it with one made by UnBrako or Holokrome.

You need a pin wrench to take the knob off. Photograph the disassembly because there may be a clutch or focusing friction adjustment in there. Sometimes those are difficult to get re adjusted after. There was probably one guy at the factory that did that and with a particular lubricant too. You can get a snap ring plier , right angled and use it too though you may break it.

the angulation of the pinion teeth is to improve the smoothness of the focusing knob. They are not hand cut. They may be rolled with a knurling type tool but it doesn't look it.


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