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04-20-2013, 04:40 PM   #1
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M 28mm f/2.8 slow aperture

Hi, I have just received a copy of the aforementioned lens. I bought it for a good price and it is in unused condition, but unfortunately it suffers from lazy aperture blades. It looks like a common problem with these M28s, as my previous one exhibited this phenomenon, although not as badly as this one.
I have seen the previous threads on this topic, but unfortunately they are about 3 years old, pictures have been removed, etc, so they are hard to follow. I possess the necessary tools for the job, and have repaired some Takumars before, but this would be my first K-mount job.
The problem I have encountered, is that after removing the set of 5 screws on the mount and 3 screws on the front (under the name ring), still absolutely nothing moves. I cannot remove the mount, therefore I cannot clean the aperture lever. I have also tried pouring lighter fluid into the lever cavity, which turned out to be the worst idea ever, because it seeped into the helicoid and the focus ring had a very weird, loose feeling for about an hour, until it dried. Then the focus returned to normal, but the aperture went even slower.

04-20-2013, 06:01 PM   #2
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The mount should come off without any other hidden screws. Sometimes when you get it off, there is oil right under the mount, sticking it on with surface tension. I guess that might have migrated from the aperture ring which is typically lubricated. You could use a lens cap and try twisting it off. Or use the lighter fluid on the mount screw holes or bottom edge of aperture ring.

I don't have any photos of this lens but I did have a really stubborn one with slow blades. I cleaned the blades many times in place but they were still slow. I decided to remove them and clean them individually. When I got that far, I noticed that this bar was loose:



That's an M50/1.7 but the bar looks the same. I can't remember exactly whether you can avoid removing and replacing the blades to access the two tiny screws that hold the bar on. Probably the blades have to be taken out but there's only 5. Those are my only ideas at this point.
04-20-2013, 06:26 PM   #3
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QuoteOriginally posted by kcobain1992 Quote
after removing the set of 5 screws on the mount and 3 screws
This is entering the scary zone as far as I'm concerned.

I tried this with a wrist watch when I was young... had an envelope of bits that never told the time again.
04-20-2013, 06:38 PM   #4
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QuoteOriginally posted by kcobain1992 Quote
pouring lighter fluid into the lever cavity, which turned out to be the worst idea ever
No kidding, you are supposed to have disassemble the lens and apply the lighter fluid sparingly to the affected parts with a brush. In order to maintain a lens like this you have to be able to disassemble the lens completely.

04-21-2013, 01:24 AM   #5
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Slow aperture on the first type M-28/2.8 and even more the M 35/2.8, is a common problem with these lenses.
And cleaning does not always solve the problem!
I have cleaned multiple M 35/2.8 lenses, and after assembly, the aperture was still sometimes slow.
This sometimes slow is dependent on the orientation of the lens, point it to the sky, then it is slow, point it to the ground, and it is fast again.
I have an untested theory, that some sliding surfaces in the aperture mechanism become too flat, and not enough air is between them to act as lubricant.
Pointing the lens down solves that.
Next time I feel the need to open up a slow 28/ or 35mm pentax-M, I will try to roughen up some surfaces a little bit with fine sandpaper, to test this theory.
04-21-2013, 02:27 AM   #6
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QuoteOriginally posted by glasbak Quote
I have an untested theory, that some sliding surfaces in the aperture mechanism become too flat, and not enough air is between them to act as lubricant. Pointing the lens down solves that.
If movement speed is still too slow I find using graphite from a 7B pencil does the trick...you apply it along the edges of the aperture blades.
04-21-2013, 06:56 AM   #7
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Thanks for the replies so far. I hope the aperture blades won't need cleaning, as I hate opening up the optical assembly. They look pretty clean too. As I said, the lens was totally unused. Absolutely as new. I once heard that new-old-stock camera equipment is more likely to be susceptible of faults than heavily used ones. Which brings me to my dilemma: what feedback should I give to the seller? Because it was so unused, I doubt he even bothered to test it. But although he didn't mention this issue, I wasn't quite surprised when I discovered it. Also, he didn't send a lens hood that was pictured, but sent an unmentioned leather case
04-22-2013, 12:25 PM   #8
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The lens on the left shows the oil I mentioned which might be sticking the mount on a little.



04-22-2013, 01:17 PM   #9
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Thank you for the follow-up, I have managed to open the mount, but my fingers are now sore
I am a total stranger to what is inside these lenses, so I have absolutely no idea what to remove next, how to put it back, what does what and what's to be cleaned, but I would benefit from some guidance
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