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09-18-2011, 06:10 PM   #1
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SMC-A 35-70mm not always being seen as an "A" lens

I've got a SMC-A 35-70mm f3.5~4.5 that isn't consistently recognized as an "A"-type lens. I've tried it on two Super Programs and a K2000. I cleaned the contacts on the lens and all 3 bodies with isopropyl alcohol.

If I mount it and give it a little extra "twist" (in the direction you would rotate it to lock it into place), it is usually read as an A lens and I can take a few shots with it before it reverts. Since it behaves the same across 3 bodies, it's definitely the lens; and since the contacts have been well-cleaned, it seems to be something internal. I'm not sure how these actually work (in terms of the auto-aperture), but is there something inside that could need cleaning or adjusting? I'm pretty good with electronics and mechanical stuff, but I've never taken a lens apart, so a bit nervous about that prospect.

09-18-2011, 06:54 PM   #2
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Your biggest hurdle is that tiny little plate that holds, I think, the grounding contact... that was the issue in the two A series lenses I was trying to put right from my "as-is" purchases. Obnoxious little thing to set in place later and still set the aperture ring and ball bearing all the same time. So opening is easy enough and watch out for flying ball bearings and contact springs. Work in a box if possible and no open hardwood floors. If it is a second-hand lens, I would check for corrosion on the contacts under the pins and springs and the potentiometer or "Aperture-Volume" to see if it is loose. Look for signs of..."Has someone been in the lens before you?" "Did they forget something?"... oddness... Smears of dried grease or my favorite is gunk from WD40 or Appliance green grease or tons of white lithium grease.
09-18-2011, 07:09 PM   #3
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Maybe someone more familiar with that lens could comment but in the mean time, I have torn apart a handful of A lenses a there has not been anything that scary (at least not as far as you need to go). Note the alignment of the mount to the lens (make an index mark if you like). Unscrew the mount. with the mount facing up and the lens sitting on a large table (so if anything pops out you don't loose it) carefully lift the mount off being careful to keep the aperture ring down on the lens. Only lift it slightly at first and see if any of the contacts start to pull out of the mount. If any of the contacts do start to pull out, continue but be careful not to loose the little contact pin. The only ones I have seen where the contacts are not attached to the mount, it was just a pin that sits in place between the mount and the lens. If the aperture ring comes up there is a tiny detent ball that can pop out and get lost. On the ones I have taken apart, the mount will come off with all parts self contained (except for a contact one or two times). I would just leave the lens sitting there mount side up in case there are any loose levers. You should now be able to see the parts for the A pin on the mount. On many if not all lenses the A pin retracts when the aperture ring is not in the A position so look for something on the lens that would push it up. That should give you a view of everything involved with the A pin (mostly on the mount).
09-19-2011, 09:55 AM   #4
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So, when the aperture ring is in the "A" position, the pin should be out (ie, making contact with the body), correct? And when the aperture ring is in any other position, that pin should retract, correct again? If so, then this is definitely strange because the pin does not seem to retract when the ring is moved out of the "A" position, so, logically, I would think that my problem would be the opposite. That is, if the pin is stuck out, it would always be in contact with the body and the lens would always be seen/detected as an A lens regardless of the aperture ring position.

09-19-2011, 10:12 AM   #5
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QuoteOriginally posted by wedge Quote
So, when the aperture ring is in the "A" position, the pin should be out (ie, making contact with the body), correct? And when the aperture ring is in any other position, that pin should retract, correct again? If so, then this is definitely strange because the pin does not seem to retract when the ring is moved out of the "A" position, so, logically, I would think that my problem would be the opposite. That is, if the pin is stuck out, it would always be in contact with the body and the lens would always be seen/detected as an A lens regardless of the aperture ring position.
I don't know that they have to be that way. I just know looking at several A lenses, that is how mine work (And I was thinking it might not be coming far enough out due to an obstruction or something). Looking at a few auto focus lenses, they have contacts that do not retract so there has to be some way for it to work that way. Thinking about it, I have ricoh lenses where the ricoh pin retracts so I guess they all can't retract (or ricoh lenses wouldn't sometimes get stuck).
09-19-2011, 10:51 AM   #6
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QuoteOriginally posted by ripit Quote
I don't know that they have to be that way. I just know looking at several A lenses, that is how mine work (And I was thinking it might not be coming far enough out due to an obstruction or something). Looking at a few auto focus lenses, they have contacts that do not retract so there has to be some way for it to work that way. Thinking about it, I have ricoh lenses where the ricoh pin retracts so I guess they all can't retract (or ricoh lenses wouldn't sometimes get stuck).
Hrm...perhaps it's the other side of the pin that's not making good contact. I'll have to (carefully) disassemble it and check things out, I guess. I just wish I had another, non-functioni lens to practice with.
09-19-2011, 11:39 AM   #7
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QuoteOriginally posted by wedge Quote
Hrm...perhaps it's the other side of the pin that's not making good contact. I'll have to (carefully) disassemble it and check things out, I guess. I just wish I had another, non-functioni lens to practice with.
If you are mechanically inclined, its not that scary. I'm willing to bet after the fact you will be wondering what you are worried about. The only risk is loosing small parts so do it somewhere you will be able to find them (not over plush carpet etc). If you keep the aperture ring in place and down, there is not much to loose. On the ones I have worked on most didn't have loose parts. For instance 1 had a floating contact pin but the rest had contact pins stuck in the mount that didn't come out. Just be patient and go slow. Of course tearing into it further can get substantially more complicated like tearing into the focusing helix but you don't need to go near that far. If you decide to do it let us know how it works out.
09-19-2011, 01:49 PM   #8
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There are photos in this article about the contact on the A50/1.7.

https://www.pentaxforums.com/forums/maintenance-repair-articles/88051-pentax-...ng-repair.html

I can't remember if your lens works like that or uses different parts. I often think they could have invented something simpler.

For tiny disappearing parts that I really need to find, I use a rare earth magnet to search the carpet. This is after being completely confident that I won't lose anything. Fortunately, a lot of stuff is magnetic.

09-19-2011, 03:08 PM   #9
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QuoteOriginally posted by Just1MoreDave Quote
I often think they could have invented something simpler
It is amazing what kind of complex designs they came up with back then. I tore apart a soligor cd lens for parts (canon lens with fungus so I was just gutting it), and not only was it a pretty complex design, but it must have had well over 100 tiny screws holding everything together. If memory serves the mount alone must have had maybe 40 or more parts in it. It's an old manual focus lens too. Luckily most lenses don't seem to be built like that. I have repaired a few and changed mounts and gutted some for parts, but even after having fully disasembled the soligor, I don't think I would ever try to repair one (I would never get it back together).
09-20-2011, 06:27 PM   #10
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OK, disassembly wasn't too bad. I managed not to lose any parts, but during reassembly, that tiny brass spring got good & mangled. Anyone know if or where that spring can be purchased? Or would I be better off locating a broken but intact A lens and scavenging it for parts?
09-20-2011, 07:57 PM   #11
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QuoteOriginally posted by wedge Quote
OK, disassembly wasn't too bad. I managed not to lose any parts, but during reassembly, that tiny brass spring got good & mangled. Anyone know if or where that spring can be purchased? Or would I be better off locating a broken but intact A lens and scavenging it for parts?
What does the spring go to?
09-20-2011, 08:10 PM   #12
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QuoteOriginally posted by ripit Quote
What does the spring go to?
It goes from a small brass block into the back of the pin. This being the only lens I've ever taken apart, I don't know if all A lenses have that block/spring/pin arrangement. I guess I should have taken pictures of it.
09-20-2011, 08:28 PM   #13
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I assume you mean the pin is for one of the contacts. I have mostly taken apart aftermarket lenses so I'm not sure about pentax lenses but no it is not the way they are all made. Maybe someone that has worked on more pentax lenses could shine a little more light on it. For the A pin, there has typically been a flat spring. For other contacts, most have been just pressed into the mount. Is the pin you are talking about for the A pin or for another contact?
09-20-2011, 09:05 PM   #14
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Besides complexity, lens designers also love to make each lens with unique parts. I haven't had great luck with acquiring one lens to fix another. One cheap source of a lot of springs is a film camera, especially with a KA or newer mount. Film cameras are easier to find for $5 than an A50/2 for $5. The springs still might not work, but there will be maybe a dozen springs in a camera - better odds.
09-20-2011, 09:21 PM   #15
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QuoteOriginally posted by Just1MoreDave Quote
Besides complexity, lens designers also love to make each lens with unique parts. I haven't had great luck with acquiring one lens to fix another. One cheap source of a lot of springs is a film camera, especially with a KA or newer mount. Film cameras are easier to find for $5 than an A50/2 for $5. The springs still might not work, but there will be maybe a dozen springs in a camera - better odds.
That'skind of what I was thinking. If it is just a contact spring then finding a generic spring somewhere close enough is size might be the best bet. Something else to consider is that if it is not the a pin (if it is another one of the contacts), all the contact has to do is ground against the mount. It most likely doesn't have to be spring loaded (be careful trying it as I don't have the lens to see how it is designed). The non spring loaded ones are flush with the mount on the side that goes to the camera. If it is flush, as long as its held in place and grounding against the mount, I would think it would be fine even without a spring. Maybe if there is a gap something as simple as a piece of tin foil could serve.
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