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07-24-2013, 06:44 PM   #1
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Pentax K1000, aperture doesnt want to stay open

Hi everyone, new to the forums here.

I pulled my old K1000 out of storage, and I want to start shooting with it again.

It has a Pentax 50mm F2 lens

The only thing I can see wrong with it, is that the aperture doesnt want to stay open. I set it to F2, and it springs back to F8ish each time.

When I take the lens off the camera, it stays at F2 just fine, it doesnt spring back.

This has me suspicious that the "stop-down actuator" (i think thats the name of the little metal tab on the mount-face of the camera, that opens the aperture to give more light for focusing) is maybe at fault.

I'm going to take it apart this weekend. Anyone have any pointers/ideas? Is this a common issue?

07-25-2013, 01:27 AM   #2
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I think we should try, as the lens will work on a different camera
07-25-2013, 12:33 PM   #3
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Off the camera, set the aperture ring to f22. The blades should be almost fully closed. The lever on the mount should move the blades until they disappear, and releasing it should allow the blades to snap back to f22 in the blink of an eye. If you put the lens on the camera with the ring still set on f22, the blades should open fully once the lens is properly mounted. A bent lever might not mate up with the camera's aperture controller and cause your symptoms.

Still want to take it apart? The five screws on the mount are JIS screws and fit the JIS screwdrivers the best. A Phillips screwdriver might work if you are lucky and careful, but it won't actually fit the screw. In the US, JIS screwdrivers are labeled with Phillips sizes to confuse everything. You want the #00 size.

There are at least three versions of this lens. The oldest Pentax-M 50mm f2 lenses say JAPAN on the front. At some point the construction was changed, then later there was a Pentax-A version that was cosmetically different, similar build internally and has an A setting on the aperture ring. The lens on the left is the older style, the one on the right is the newer A style. (Actually both are the f1.7 versions but construction is the same.) The newer M50/2 is almost identical to the A version on the right.

Here are some shots of the newer M or A version. On the left is a lens with the mount removed. Note that the aperture ring is now loose and can slide right off. You don't want to let it come off because the bearing will fly off into space and never be seen again. Om the A version, three or four more parts will also disappear. There's a stamped plate with a notch that you can lift right off. Use this photo when you can't remember how it goes back on. The assembly that moves the aperture blades is held on by three screws and is below the plate. You can see it more clearly on the right hand lens.

On the left is another view of a lens with the plate removed. In the center is the spring-loaded aperture control mechanism, removed from the lens. The right-hand lens has a copper tab removed; don't do that. It guides the focusing.

If you have the older version of the M50/2, there's a different stamped ring that acts as a spacer. This shot shows it in place; it just lifts off. You have to take off the aperture ring to see anything. There's a small bearing trapped between the ring and lens body. It is just below the orange diamond mark, and spring loaded. You can put the lens in a plastic bag before sliding the ring off, or put your thumb over the orange diamond to catch the bearing. The bearing is in place in this shot.

There are three screws hidden by the aperture ring that hold on the aperture control mechanism. You can slide it off, it's usually tight.

Inside that there's another ring that's loose. Here it is removed and flipped over.

That's the mechanism for both types. If you fiddle with it you can usually figure out how it's supposed to work.
07-25-2013, 01:16 PM   #4
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thanks Dave, thats some really good info.

Do you think the lens is at fault, if I take the lens off the camera, and the Aperture behaves normally (aperture snaps closed perfectly when I play with the little aperture actuator tab)?

07-25-2013, 08:42 PM   #5
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My gut feeling is that the lever is not working together with the camera's arm that should move it. The lever should stick out perpendicular to the lens mount, roughly in the center of its slot side to side. Turning the aperture ring will move it up and down through its whole range, from the top of the slot to the bottom. Do you have any other lenses to compare to it or put on the camera?
07-26-2013, 02:13 PM   #6
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nope, no other lenses to try.

i'm going to do some troubleshooting with it tonight and see what I can figure out.

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