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01-30-2017, 09:25 PM - 4 Likes   #1
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SMC Pentax-FA 28 - 200, f3.8 to f5.6 repair.

This is an older lens that I used for some time on my MZ5 film camera. Later it was used on my ist DS, and more recently on my K30. Up until a few weeks ago, it had been operating perfectly, then suddenly all shots were over exposed, almost to the point of white-out. A check on the lens itself showed that the small lever near the rear element that controls the aperture, was locked up solid.

I removed the four screws that hold the metal mount in place, then carefully removed the mount. This has to be jiggled a bit to disengage it from the aperture mechanism. The aperture ring must be held down in place, otherwise it is easy to lose the tiny indent ball, spring and operating button.

The offending part was the small lever and its pivot ( see red arrow ) that had seized up due to some corrosion. A tiny drop of penetrating fluid was applied to the pivot with a small screw driver, and then the little arm worked back and forward until it loosened up. This small arm is normally operated by a cam effect on the plastic aperture ring, but had seized in the wide open position, hence the over exposure.

A couple of things to keep an eye on are the tiny compression spring indicated by the yellow arrow, and the aperture ring release button indicated by the green arrow.

The screws holding the mount in place actually screw into plastic, so should be done up firmly without overtightening, which would strip the plastic female thread.

All good now, so I hope that I get a few more years out of this lens.

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01-31-2017, 12:46 AM   #2
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Thanks for sharing this. I find it quite interesting to get a look into the rear of this Tamron-made lens. It looks quite different to Pentax lenses from the same age.
01-31-2017, 03:57 AM   #3
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Although very differently constructed, I had the same problem with an old Jupiter-21M, through a combination of congealed lubricant and minor corrosion that seized the aperture control mechanism. A quick squirt of Servisol lubricant and some gentle back-and-forth movement was enough to get it going temporarily, followed by a thorough clean and re-lubrication later.

I suspect your fix will last a long time - good job!
01-31-2017, 01:22 PM   #4
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It's posts like this which make this forum such a useful resource, thanks for posting Terry.

01-31-2017, 03:06 PM   #5
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QuoteOriginally posted by dcshooter Quote
... Always a pain to get all 5 pins, their springs, and the ground spring all lined up when reassembling an AF mount. ...
Not really. Somewhere on the internet somebody posted a trick that really works: Put adhesive tape over the contacts before you remove the bayonet. Usually the contacts and their springs remain on the bayonet then.
01-07-2018, 02:02 PM   #6
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QuoteOriginally posted by Terry C Quote
This is an older lens that I used for some time on my MZ5 film camera. Later it was used on my ist DS, and more recently on my K30. Up until a few weeks ago, it had been operating perfectly, then suddenly all shots were over exposed, almost to the point of white-out. A check on the lens itself showed that the small lever near the rear element that controls the aperture, was locked up solid.............
I had the exact same problem with the same lens about 10 years back (about 10 years after I bought it new) and fixed it the same way. It was almost like they clamped the swivel too tightly when they put the pin in, but the cure was the same as you described. The iris closer has been working great ever since..........

More recently (yesterday, in fact) that lens developed a problem with the zoom which put a stop at about 80mm instead of allowing the helical to go on out to 200mm. I haven't opened it up yet, but found a used replacement for $45US so should be happy for a while.

ADDENDA: I opened my lens up again and had to go a bit deeper than before, taking the printed circuit board, electronics block, and underlying layer off. That revealed the zoom ring which had a metal tab which bridges a plastic track to the inner helical barrel. The plastic track serves as a stop for the zoom range with steps at both ends to engage the tab. The tab is secured to the metal helical barrel with two screws, both of which had untightened and were protruding into the plastic overlay above, presumably catching on it. Putting those back in place with light duty thread locker did the job and now I have my original lens with a good spare.

Last edited by Bob 256; 01-11-2018 at 09:40 AM.
11-03-2020, 11:42 AM   #7
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I need to thank you Terry for posting that picture. I had an issue with a Tamron 28-200 where the aperture ring inside the lens would not move when rotating the aperture control dial. And there was some clunky sound when shaking the lens... not good.

I decided to open the lens and see it for myself. Turns out that those two screws holding the vertical bar, which ends up controlling the aperture rign, got loose and fell off inside the lens.



After adding a bit of LockTite and some adjustments I was able to put it back together. Just be careful with the side screw (yellow arrow) as it holds the spring tight. If you remove this screw, you will need to reset this spring, and maybe add a bit of LockTite there as well to make sure to keep the spring in place.




And pay attention when opening the lense. I ended up opening it and noticing on my table a very tiny spring, and I had no idea where it came from. Turns out that was the spring from the picture at the top of this thread.

I thought I share my experiece in case it helps someone else.
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28 - 200 pentax lens, aperture, arm, arrow, button, k-mount, lens, lever, mount, pentax lens, plastic, repair, screws, slr lens, smc pentax-fa
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