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07-30-2018, 02:04 PM   #1
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Lens repair for M28/2.8

I purchased an M28/2.8 from Ebay a while back. Eventually I discovered that the aperture blade retracts slowly enough to affect exposures past f5.6. I took the base plate off enough to find that it was not oil on the blades, but some sort of bind in the mechanism. I could get the blades to move freely by hand, but did not know how to get in where the problem was to fix it.

Since I tinkered with it, it will not stop at infinity, which is not a big deal, but I was wondering if anyone had any experience with repairing this lens?

07-30-2018, 02:08 PM - 1 Like   #2
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Check this: Pentax Lens Disassembly Database - PentaxForums.com
07-30-2018, 02:23 PM - 1 Like   #3
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QuoteOriginally posted by DrawsACircle Quote
Check this: Pentax Lens Disassembly Database - PentaxForums.com
Specifically, this url: SMC PENTAX-M 1:2.8 28mm version 1 disassembly.

I used this to fix my M28 3.5. The problem on mine was the little lever in picture 3 that controls the aperture opening - it was loose. If you've never done this before, set some time aside (I spent a whole afternoon from start to finsh, but I'm not great at tinkering), and proceed with calm (easy to say).
07-30-2018, 02:29 PM - 1 Like   #4
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Oh, and, if you can, you should use a JIS screwdriver. I used a 00 philips succesfully, but I won't work on this same lens again without the proper driver. JIS drivers are unobtainable here in France, and international online ordering was expensive because of postage costs... If I ever need to open a lens again though, I'll spend the money.

07-30-2018, 02:47 PM - 2 Likes   #5
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Thanks

Thanks all of you!
07-30-2018, 03:48 PM - 2 Likes   #6
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Be sure to take your own photographs as you disassemble to help guide you through reassembly. Label parts, especially orientations.
Pause every now and again to practice reassembly of any particular step that seems complicated before moving further into
disassembly. Take your time. This can be a lot of fun but is also fiddly.
07-30-2018, 05:44 PM   #7
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QuoteOriginally posted by sergeremy Quote
Oh, and, if you can, you should use a JIS screwdriver. I used a 00 philips succesfully, but I won't work on this same lens again without the proper driver. JIS drivers are unobtainable here in France, and international online ordering was expensive because of postage costs... If I ever need to open a lens again though, I'll spend the money.
What I need right now is a spanner wrench. I have been able to get by in the past, but the rings on this one are very tight.
07-30-2018, 11:50 PM - 1 Like   #8
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QuoteOriginally posted by DSLRnovice Quote
What I need right now is a spanner wrench. I have been able to get by in the past, but the rings on this one are very tight.
I cobbled one up with two nails through a length of wood. Don't forget to blunt the nails before use (and proceed carefully, you do not want to slip).

07-31-2018, 02:03 AM - 1 Like   #9
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Look at the pictures of the disassembly, in the 8th picture and again further on you will see a small spring, this spring runs the length of the lens and is attached to pivot that has the shutdown cam (lever) attached to it, the reason your lens is slow to close down, is because this spring has become weak over time and use, obtain a new spring from Pentax or with a pair of sharp cutters, clip if the old upright attachment ring on the end of the spring and bend up a new ring from the coils of the spring, this in effect shortens the spring and will recover is action, when the lens is reassembled, test by holding down the closing/shutdown cam (lever) and release it, the action should be so quick as not to been seen.
Regards Tim
07-31-2018, 07:47 AM   #10
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QuoteOriginally posted by DSLRnovice Quote
Since I tinkered with it, it will not stop at infinity, which is not a big deal, but I was wondering if anyone had any experience with repairing this lens?
Probably best if you sent the lens to a trained technician and let him do a CLA?

Here is Eric's website:

Pentax Repairs

Phil.
07-31-2018, 11:31 AM   #11
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QuoteOriginally posted by Snappertim Quote
Look at the pictures of the disassembly, in the 8th picture and again further on you will see a small spring, this spring runs the length of the lens and is attached to pivot that has the shutdown cam (lever) attached to it, the reason your lens is slow to close down, is because this spring has become weak over time and use, obtain a new spring from Pentax or with a pair of sharp cutters, clip if the old upright attachment ring on the end of the spring and bend up a new ring from the coils of the spring, this in effect shortens the spring and will recover is action, when the lens is reassembled, test by holding down the closing/shutdown cam (lever) and release it, the action should be so quick as not to been seen.
Regards Tim
Thanks Tim. I really hoped that the full disassemble would not be required. What would be helpful but I know would take some effort, would to have a cutaway diagram from the sides. If it is a weak spring, I might only have to remove the front to get to it to shorten it. The back has definitely more complicated small parts to deal with.

However, some time back when I first took the back apart, it seemed like the aperture closed freely as if some other part was slowing it down which I could not see. It might require more dissassembly to know what is going on.

Dan

---------- Post added 07-31-2018 at 11:44 AM ----------

QuoteOriginally posted by gofour3 Quote
Probably best if you sent the lens to a trained technician and let him do a CLA?

Here is Eric's website:

Pentax Repairs

Phil.
I doubt the value of the lens would justify the cost of the repair. Besides, I like tinkering with such things.
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