Depth-of-field preview lever broken on Nikon FM

[nikonfmuser]

I was recently testing out the depth-of-field preview lever on my Nikon FM and it snapped off. I was obviously disappointed but I didn't think too much of it as the camera seemed to be fine other than this. Then, when I was out taking pictures, I took a picture at one point and the aperture blades closed but did not reopen again for about a minute even though I had the speed set at 125. This happened again another time so I decided not to use the camera for the day. I tested the camera out the next day by taking a picture. For the first two everything seemed perfect but when I took another picture the same thing happened again where the aperture blades did not reopen for about a minute. I took the lens off and checked the aperture stop down lever and it seems to work fine, I am able to push it down fully. About midway through pushing it down though it does sound like it is lightly brushing/scraping against something but this cannot be felt, only heard, and it is very faint. I'm quite new to analog photography so I am just wondering if someone with a bit more expertise would be able to give me some insight into what the problem might be. I assume the depth-of-field preview lever has something to do with the issue but I am just a bit confused as it seems to work fine sometimes but then other times the aperture blades get stuck for a while. Thank you.

 

[reddesert]

Take off the lens and look into the lens mount. Watch the aperture stop down lever at the left (subject's left, photographer's right) as you fire the shutter. This lever should move down and up quickly - when it moves down, it allows the the lens to stop down, and when it returns, it holds the lens aperture open. If it's not returning quickly that's a problem.

Also look at the lens to make sure that the lens aperture opens and closes freely when you push the linkage on the lens by hand. Sometimes if lenses get oil on the blades the aperture gets sluggish (IME this happens less often with Nikon brand manual focus lenses than some others, but it can happen).

My guess is that something was bent and the bent linkage is causing the hangup. It may be the combination of the parts and hard to see when you have them apart. There may already have been friction due to a bent linkage and that's why you pushed hard enough on the DOF preview lever to break it. It is possible to bend these linkages if one forgets that Nikon lenses are removed righty-loosey rather than lefty-loosey (I did this to my first Nikon SLR over 30 years ago and had it fixed by a shop).

Incidentally, this isn't totally specific to analog photography, as many Nikon DSLRs have the same mechanical linkage to stop down lenses that have a manual (non-G) aperture.

 

[shutterfinger]

Nikon uses a clear thread locker on the screws that hold the mount ring onto the lens rear and the body. It can be dissolved with Acetone.
With the lens mount ring removed from the lens you should be able to access the aperture operating lever and return spring. Carefully reform it if it is bent, (90° to the lens body is normal).
The Stop Down lever is part of the mirror box assembly and may require a major disassembly of the camera front to repair.
These factory service manuals are free: https://learncamerarepair.com/productlist.php?category=2&secondary=8
The mirror box exploded view is on pdf page 133 (fig 9) of https://learncamerarepair.com/downloads/pdf/Nikon-FM-Revised-Service-Manual.pdf
The body lens mount ring starts on pdf page 126 (fig 7)
The mirror box assembly instructions begin on pdf page 177.
I do not recommend attempting to disassemble the body beyond removing the lens mount ring to gain better access to inspect the stop down linkage unless you are an experienced camera repair person. The manual will aid a tech not familiar with Nikon.