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Metaphorce
Tinkerer
Username: Metaphorce
Post Number: 1
Registered: 03-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, March 27, 2007 - 05:03 pm: | 
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I have a Pentax M 28mm lens (k-mount) with sticky aperture blades. they move, but very slowly.
I have removed the rear element and the blades do not appear to be oily, but I don't know how to get any further into the lens assembly safely.
Any guidance would be greatly appreciated. |
Alex
Tinkerer
Username: Alex
Post Number: 25
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, March 27, 2007 - 05:31 pm: |
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I don't know this particular lens, but I had a similar problem with a 28mm Hoya lens which came with an old M42 camera I acquired. I removed the rear lens assembly, as you have done, to get access to the diaphragm blades from behind, and using a squirrel-hair brush, painted the blades with lighter fuel and exercised them repeatedly (opening and closing). Eventually, when it dried out, it seemed to free it up, so I reassembled. It worked for a few days, then I had to open it up again. This time, I repeated the 'paint' procedure several times, using a lint-free pad taped to the other end of the brush to dab up the wet surfaces of the blades. This took a couple of days (not solid, of course). I then painted the blades a final time but this time with a smattering of powdered graphite in the lighter fuel, to act as a dry lubricant. It doesn't take a lot (you don't want bits of graphite migrating on to the lens surfaces). When the lighter fuel evaporates, there's a microscopic amount of graphite left on the blades. This did the trick, and I reassembled the lens, which has been working perfectly for over a month now, snapping down instantly and returning just as fast. Others with more experience than me may have more sophisticated procedures, but this worked for me, and saved a lens which was otherwise destined for the bin.
Alex |
Metaphorce
Tinkerer
Username: Metaphorce
Post Number: 2
Registered: 03-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, March 27, 2007 - 05:40 pm: |
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Thanks, Alex.
My first problem is getting the diaphragm assembly out, because it sits right on top of a coated lens element, and I don' dare work with any quantity of solvents in that position.
I tried dabbing minute amounts of highly pure Isopropyl Alcohol on the blades, but that didn't help. In fact, I think it made it worse. And I don't want to use any larger quantity because of the lens element.
I suspect there's some other part down beneath the diaphragm blades, where the actuator-lever links up, that's all gunked up.
I'm hoping somebody will know about the assembly of the lens and give me some tipes on getting it apart (and back together).
SteveM |
Metaphorce
Tinkerer
Username: Metaphorce
Post Number: 3
Registered: 03-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, March 27, 2007 - 07:09 pm: |
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Hey, I did it!
I decided to have a go at it. I found a description of taking apart a Pentax-A lens, which is only somewhat similar. But it had some useful cautions in it.
In brief, I removed the 5 screws on th back place. The back assembly lifted out.
I marked the position of the rear lens element and unscrewed it, noting the number of turns and the position of the index mark when it came free of the threads.
At this point (as things later turned out) I could have fixed it without further disassembly. However, I was trying to get the diaphragm assembly out, so I went a bit deeper. I will describe, for the benefit of those who might have other things to fix.
Next the aperture-adjusting ring lifted off. There's a little ball-bearing stock in the side of the housing that becomes free when you remove the outer aperture-adjusting ring. Be careful not to lose it. Mine was bedded in enough grase that when I test-fitted it into its hole, it stayed there and wouldn't be removed even with a magnet.
Then there's a black metal flat ring inside which has to be scooched out because it has a tab that projects out through a slot in the housing. (This tab engages in a slot on the inside of the aperture-adjustment ring)
Next I removed three screws around the outside of the next large assembly, and it lifted off. When it lifts off, a spring-loaded chrome tab with a cam-post on it rotates outwards. This will have t be rotated back inwards to re-assemble.
At this point, I could clearly see that I would have to disassemble the entire damned helicoid to get at the other side of the diaphragm. I really didn't want to. I felt the chances of irremediable failure would gte much higher from here on.
I decided to try Ronsonoling the blades. I took about half the cotton off of a Q-tip to minimize the amount of lighter fluid it would hold, put a drop of fluid on it and touched the blades. It spread, it evaporated quickly, nothing changed when I tried moving the blades: still very sluggish.
I put a couple of drops on, touched the blades. Suddenyl, they were all wet. I worked the action back and forth. On the fourth work, it suddenly freed up completely.
I then took dry q-tips and removed lighter fluid from the blades until it was all gone (trying to remove as much dissolved gunk as possible.
Reassembly was fairly easy. the chrome pivoting thing had to be moved out of the way to put on the big housing, and then moved and held again to replace the flat black ring. Placing that ring in the position where the pivot is furthest outwards puts the aperture blades at f22. This makes it easy to align the aperture-adjusting ring when replacing it. Have to be careful not to knock the ball-bearing out opf the side (it provides the detents for aperture adjustment.)
Voila. It works.
SteveM |
Alex
Tinkerer
Username: Alex
Post Number: 26
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, March 28, 2007 - 02:31 am: |
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Excellent. It's always good to see a report on how someone solved a problem, and this level of detail is bound to be useful to someone else faced with a similar recalcitrant lens.
Alex |
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