Moskva shutter won't stay cocked - any suggestions?

[Coldkennels]

This Moskva shutter has been in pieces on my desk for months. I just cannot get the damn thing to stay cocked - the black catch lever isn't able to keep the mechanism in place once the shutter is cocked.

Here's the shutter at rest:



Everything runs perfectly and the timing is good, it just won't stay cocked. Here's how it looks when the shutter is cocked:



That black catch lever sits into a notch to hold the shutter primed until the shutter release pushes it out of the way - but as you can see, it fits very loosely right now.

I have absolutely no idea how to solve this. The spring wire on the catch isn't replaceable, and I'm not even sure that's the issue anyway. It's driving me nuts trying to figure out a fix; I'm half-tempted to use a Dremel to reshape the catch lever slightly so it can sit in there tighter, but that seems drastic (and, of course, is irreversible).

Any suggestions?

 

[Muggins]

The only thing I can think of is that something isnt quite seated as it should and is standing proud so that something else doesn't quite engage, but that probably isn't a huge help. The shutter doesn't look as much like a Compur-Rapid as I'd have expected, or I'd suggest trying Chris Sherlock's videos. Good luck!

 

[JeffS7444]

Wondering if the mating surfaces of the pawls have become a bit rounded due to wear. If so, a light touch of a jeweler's file might be more appropriate than a Dremel tool.

 

[Coldkennels]

Wondering if the mating surfaces of the pawls have become a bit rounded due to wear.

I've considered that. They do seem to slip apart a bit too easily. My thought with the Dremel was to take off some of the "meat" opposite the pawl on one of the components so there's more space for the two parts to mesh together - if you look at the second photo, it seems almost like there's not enough room for the catch lever to lock in properly.

 

[caila77]

Had the Same issue many years ago. Unable to solve it

 

[joe bosak]

May not be relevant (and I know nothing about shutter mechanisms...) but zooming in, the end of the black catch lever has the same toothed shape as the next notch along on the shutter cocking ring, which also has more of a lip there.

 

[Coldkennels]

Yeah, I noticed that. I'm not actually sure what that notch is for - there's nothing for it to latch onto in normal operation, and there's no way to reposition the shutter cocking ring in such a way that the catch lever can hook into there. Very odd.

 

[GMOG]

It kinda looks like the entire tip was rotated counter-clockwise a bit. The shape is not ideal, for sure.

If I were to start somewhere I'd file off material from the back side of the catch pawl/lever's tip. That would allow the tip to get into the notch a little better.

 

[Coldkennels]

That's the point I'm thinking about - however upon sleeping on it, that curve of the catch allows it to slide into place. I'm more inclined to file away the corresponding point it makes contact with on the shutter cocking ring so that it has more space to sit in there.

 

[GMOG]

That's the point I'm thinking about - however upon sleeping on it, that curve of the catch allows it to slide into place. I'm more inclined to file away the corresponding point it makes contact with on the shutter cocking ring so that it has more space to sit in there.

That's good thinking.

Are you also thinking of staying away from the locking surfaces on the pawl and cocking ring? Changes there would impact the cocking ring's cocked position but that's probably a non-issue because the change would be small.

 

[Coldkennels]

Again, I suspect "sharpening up" the locking surface on the cocking ring should be the first port of call. I don't really want to tinker too much with the catch itself if I can help it!

Amusingly, when I first looked through Maizenburg's book on FSU camera repair, I found this recommendation:



...not exactly an easy feat! That's a surprisingly complex little part.

 

[GMOG]

Is "problem 3" mended by "sharpening up" the locking surface on the cocking ring? If the book recommends it then go for it.

Agreed on doing as little as possible!

 

[Coldkennels]

No, "problem 3" is to do with the shutter release getting stuck and holding "pawl 6" (i.e. our little problematic catch lever) in a position where it cannot hook the shutter mechanism in place. I wish the problem was that simple!

 

[msarkki]

I had the very similar issue after dismantling the cocking ring and release lever and cleaning the shutter. But unfortunately I cannot 100% surely remember in detail which was the dislocated spring causing the issue. Here’s however some tips on what I do remember. Look closely the black spring, that is close to the sync contact at the end of the long lever driven by the shutter release lever. It was either that or then there was a spring underneath those levers that spins around and jumps easily to a wrong position and then the claw does not go deep enough to the groove. Thus the shutter trips way too easily and in B position it will not keep cocked at all. If you get hold of Maizenbergs book there are diagrams how shutter should work and if you study how the shutter works in B position you can pretty easily see which spring was spun into wrong position. There’s only one or 2 that push that B-mechanism levers and the levers that keep the shutter properly cocked. You can cock the shutter covers dismantled while supporting the cocking ring time selection ring in place with fingers. Do not file anything, that’s probably not the issue.

 

[msarkki]

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[Coldkennels]

Thanks for the input, @msarkki; I finally got some time to have another look at the shutter this afternoon, and after dismantling the shutter release section and fiddling with the springs shown here -



- I managed to get the shutter to stay latched between 1sec and 1/100.

Now B will not engage at all, but for a different reason - either the delay mechanism is sitting slightly low or the speed selector ring is sitting fractionally too high, and the lever 3 in this diagram is slipping under the speed selector ring:



As for 1/250: that still won't stay latched. The heavy spring that comes into play when 1/250 is selected is far too powerful, and the black catch pawl still isn't making a positive enough lock to withstand the spring. I suspect fixing whatever's causing the speed selector ring and the lever on the delay mechanism to not mate properly may help as tightly squeezing the whole unit where the catch pawl is located while priming the shutter seems to help, but I still can't shake the feeling that the actual mating surfaces of the catch mechanism might need refinishing slightly.

 

[msarkki]

On the last picture you’ve posted it looks like that at least the small hinged catch/hook on the cocking ring is in wrong position (just barely visible on the lower left hand corner). But maybe you already put that in place. B not engaging, do you mean it just runs through, the catch does not catch the ring as it should. So the required levers do not move close enough towards the lens? I hate that I cannot any longer remember clearly how I managed to fix it, but I do remember that following that mechanism I found the issue. Somehow one of the levers had gone into wrong position so that it prevented the others moving freely far enough - all the way up to the black catch keeping the shutter cocked.