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Pentax-A 50mm f1.7 Aperture ring repair
Posted By: Just1MoreDave, 01-24-2010, 07:52 PM

The Pentax-A 50mm f1.7 is a very good lens except for one problem. A tiny part can break and jam the aperture ring at some place other than the A position. It often won't return to the A position, making the lens far less useful and valuable. The fix is not that hard and doesn't require special tools. It is probably better to read through this once before starting, then follow the instructions.

A clean workspace with good light is helpful. You may want a clear plastic bag to control a spring-loaded bearing. More on that later. A good quality Phillips #0 or #00 screwdriver is required to take off the mount. Tweezers, a small dab of lithium grease and a small flat-bladed screwdriver are optional.

I have heard that the mount screws have slots for a slightly different screwdriver head, but Phillips will work OK with proper technique - make sure you have good contact with the slot and apply even pressure. With the lens face-down on the work surface, remove all five screws. Grab onto the bayonet lugs and lift off the mount. You'll see this:

At this point, a lot of things are loose - the mount was holding everything together. The first thing you can do is lift off the stamped steel ring around the rear element. It links the aperture to the aperture ring. This photo will help you put it back in the right place.

Next, there's a thing on the left side of the photo marked with a green arrow. Pentax calls it the "A-M Selecting Plate", part number 38305.X50801. It has two more associated parts, a tiny spring and a metal plate. This part is critical in telling your camera you've mounted a KA lens. You can slide it out now with the tweezers, or leave it for later. The main thing is, these three parts are important for maintaining the A position functions. A closer photo:

I think it's easiest to take all these off and reinstall them in the final stages, instead of worrying about them staying in place. Here is the part, its spring and contact:

OK, now we'll talk about the second part that you don't want to lose. There's a 1.5mm ball bearing that makes the clicks between aperture settings. A spring is pushing it outward against the aperture ring, so when you remove the aperture ring, this bearing will launch into space. The bearing is just to the right of the off-white bump. In this photo, it is perched on its little spring:

If you have the lens in a plastic bag, when you slide the aperture ring off, the bearing will end up in the bag. I can usually just orient the lens so the bump is in my left palm, slide the ring off with my right, and the bearing is in my hand. This probably only works because I have spare bearings, so losing one is no big deal for me. The spring can also get lost when you're not looking at it, but it usually stays hidden in its hole. The aperture ring has the small black button in it too; keep track of that.

With the aperture ring off, you now have access to the problem. You probably see something like this:

And you have another tiny part falling out, one of those flat steel springs. Pentax put these springs on with a plastic weld, which doesn't last forever. (In their defense, they may not have expected people to be using these lenses 25 years later.) You can fix the spring in several ways. Epoxy glue is one solution. Pentax uses screws on almost all other lenses, if you have some very small screws. Screw heads or epoxy should not stick out at all, because the aperture ring has to slide by them. Here is a successful screw repair:

It's a good time to clean the aperture ring of any dirt, and remove other grit or dirt you see. One more photo to show you the little v-grooves that mark each click-stop for the aperture:

Now all you have to do is reassembly. This can be challenging because of all the tiny unsecured parts moving around. I use a small amount of lithium grease to relube the aperture ring. I also use a bit to stick the A button in place, and stick the ball bearing to its spring on the lens. I hold the lens sideways and orient the ball bearing so it's at the top and not falling off. I hold the aperture ring so the numbers are going the right way and the v-grooves are lined up with the ball bearing.

Then I slide the opposite side of the aperture ring onto the lens first. I keep sliding it on until it runs into the first (lower) flat spring, then use a small flat-blade screwdriver to tuck that flat spring under the ring. Then I slide the ring on a bit more until it's up against the bearing. I use the flat-blade screwdriver to shove the bearing down and slide the ring over it. Then the upper flat spring gets tucked under the ring and the hard part is done. You should be able to move the aperture ring and feel click-stops, and the button to select the A position should work correctly. Set the ring to the A position for the next step.

Next, reinstall the parts for the A contact. The contact plate goes in first. It has a hook that goes over the outer rim. I use a bit of grease to stick this in place too. It should be in this position, except of course your aperture ring is installed now:

Then just slide the "A-M Selecting Plate" into its slot. The stamped steel plate goes on next, flat side up. Use the first photo to install it; the tab in the aperture ring fits into the slot (red arrow). Line the mount up so the aperture lever will fit through its slot and the A contact will fit into its hole, then put that into place. Reinstall the five screws.

Check your work by mounting the lens on your DSLR. Set the aperture ring to A and see if the camera will recognize the aperture setting. Use the DOF preview to see if the lens stops down to the selected aperture, then change the aperture with the wheel and stop down again. (Exact controls to do this will vary with different camera models.) Then go use your repaired lens.
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03-23-2012, 03:55 PM   #16
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Awesome instructions on a much-needed topic! Thanks for this article.
I have one of these lenses, it isn't broken yet, but it feels like it might break anytime. Now I know how to fix it if it does.

03-27-2012, 01:20 PM   #17
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QuoteOriginally posted by Weber Quote
Awesome instructions on a much-needed topic! Thanks for this article.
I have one of these lenses, it isn't broken yet, but it feels like it might break anytime. Now I know how to fix it if it does.

I have this lens, too, but it's in great shape ... still good to know how to fix if ever needed!

Bravo! Salut, John le Frog
06-03-2012, 12:20 PM   #18
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ok.... is anybody listening, and willing to help me w/ the 1.5 mm bearing for the A lens repair? I've been working on fixing this 50mm, and each time I got it back together it was still extremely hard to turn the aperture ring, even though I had epoxied that plate/spring that had fallen off. And finally I shot the bearing off into space on my last attempt to reassemble. If someone has an extra bearing I'd be happy to buy it from them... I don't want to order 50 of them. Thanks folks. Adam
06-04-2012, 04:27 AM   #19

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Great fix. I found this while hunting around because a local guy has a couple M-series lenses with aperture rings that don't lock into the place anymore. They work, but don't lock. Would a repair on those be similar?

06-04-2012, 05:25 AM   #20
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Dave, thanks for the tips. It gave me the confidence to place a bid on a lens the owner thought was useless, he described the symptoms of a stuck aperture ring.
So I won a Pentax-A 50mm f1.7 for $36, with some very minor dust on the lens and as your picture (but only one) broken locking plate.
All good fixed it to night, used the screws off an old 3 1/2" floppy drive (I know if would be usefully one day).

I also have and A 50mm f2, so i pulled the ring off of that one to get a comparison and it looks like that fixed the problem when they designed this version, because it already has a screw in the top licking plate.

Edit: What the lens looked like before fixing.
Attached Images

Last edited by bdparker; 06-05-2012 at 07:43 PM.
11-03-2012, 04:51 AM   #21
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thank you so much for this tutorial! I couldn't figure out for the life of me where the ball bearing was supposed to go on my 50 f/2, and luckily it's the exact same place as the lens you took apart.
11-04-2012, 07:41 AM   #22

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I have one of these lenses, but have been lucky in that so far it hasn't broken. I very much appreciate the tutorial, although I hope I never have to use it.

02-14-2013, 08:58 AM   #23
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thanks for the tutorial. I bought a Pentax ME Super with this lens. The focus was stiff and i fixed it. But wouthout your tutorial I wouldnt have had the right informaiton to put it in working condition together.

the hardest part was setting the focus so, it will meet infinity and not go past it.

07-28-2013, 08:44 PM   #24
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Thank you for the tutorial! Just finished up fixing my A50-1.7 lens and the aperture ring is now smooth as butter. Thank god i had a donor lens (A50-2.0). I managed to accomplish all the cautions and errors mentioned in this thread the first time around. Lost the spring, lost the ball bearing, lost the aperture button, A clip and aperture button fell into the abyss, and the list goes on. Fix one problem, another one popped up like a sick joke. I laughed in horror many, many times. Now if i can find the lost parts somewhere on the carpet, i can repair the 2.0 lens. I shouldve vacuumed first had i known of the potential hazards. Would make the easter egg hunt much easier w/o all the crumbs and dog hairs --not to mention my carpet is dark green...(laugh in horror, again).
07-29-2013, 09:01 AM - 1 Like   #25
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I have some powerful rare earth magnets that I sweep the carpet with. They will pull up the tiny screws, springs and bearings. Non-magnetic stuff has to wait for vacuuming.
07-29-2013, 12:09 PM - 2 Likes   #26
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Another trick is to get an old or cheap panty hose stocking and put an section without runs over the vacuum cleaner tube end then vacuum the carpet/floor. Anything larger than a grain of sand gets caught in the material. As long as you aren't using fishnet stockings that is.
10-02-2013, 07:58 PM   #27
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Ok, I finally got around to taking off the stiff aperture ring on my f/1.7. Sure enough the same spring had broken loose as pictured above. It took me some time to scrounge up some screws of a suitable size. I cannibalized a couple of short screws from the top and bottom plate of the K1000 body the lens came on. I was concerned longer screws would cut too much into screw post adjacent to where the screw would go. I recommend you buy an appropriate sized drill bit to drill the holes. Typical sizes for these screws are 1.5mm and 1.7mm. I drilled a pilot hole with a 0.7mm drill (only drill I had under 1/16 inch). I then used a dental burr (my father was a dentist) that was slightly smaller than 1.5mm. This worked perfectly for the first spring so I decided to do the second spring even though it was intact. Unfortunately I did not drill the hole deep enough and the threads stripped out when the screw hit bottom. I tried patching the hole with some dental acrylic but this wasn't curing properly so I replaced it with some JB Weld epoxy.

To aid in reassembly I took a small bit of sticky tape (Scotch Brand Magic Tape). On the barrel of the lens where the spring and detent ball bearing goes is a flat area. The piece of tape was slightly wider than this area and long enough to fold over and reach the metal part that the diaphragm control lever is on. I stuck the bearing to the spring with a small amount of grease and pressed the tape straight down over the ball with my thumb. While holding the tape down I folded the loose end and tacked it down on the metal piece. Now I could replace the aperture ring without the plastic "A" button in place. Positioning it so the indicator was pointing to f/4 seemed to work best. You can slide you thumb along with the ring keeping pressure on the tape and the ball depressed. Okay, once the ring hits the first spring you can release the bit of tape and use your thumb to depress the first spring. Check to make sure the ball is in position. The advantage with using tape is the ball will stick to the tape 9% of the time and not go flying off into space although I recommend you still do the assembly inside a plastic bag. Once you get past the first spring you still have room to install the plastic "A" button part. At this point you can carefully pull the tape out. If it rips you can usually grab the other end with some tweezers. Compare the two pieces of tape to make sure none is left under the ring. If there is any left you have to start over again. Position the lens so the hole for the button is on the bottom and gravity will hold it in place. Use a toothpick, probe or fine tweezers to move the button into place. Now you can depress the second spring and push the ring on completely. Test to see if the ball is still in position and the ring moves freely and the "A" button locks and unlocks properly. Note the first spring keeps the ring from moving into the "A" position unless the button is depressed. The second spring keeps the ring locked in the "A" position. Continue assembly as outlined above.

Note that it appears the iris does not open completely when set to f/1.7 until the lens is completely reassembled. I made the mistake of thinking I did something wrong and started over again. Arggh!

On my lens the A-M plate differs from the one pictured. It consists of four parts. The plate contact that fits on the lens body first, the spring, the pin and clip. The last three parts fall apart easily and are so tiny it is an absolute bear to put them back together. Trying to put this back together was the most time consuming step other than fussing with the ball bearing. There is a groove at the base of the pin and it is tricky getting it back into the clip. And the only thing holding it together is the pressure from the spring. Which does not exist until the assembly is in place on the lens. It is also easier to slide the spring into the pin before fitting it into the clip as the pin has a habit of falling out more readily without the spring or when you try to insert the spring. Since these are electrical parts I did not want to apply grease to the pin or spring.

Fortunately all these parts are attracted to magnets including the A-M plate spring which appears to be copper plated. A magnet can be used to keep the parts from wandering around.

Whew! I wouldn't want to do this for a living. Maybe when I was younger and didn't need a magnifier for close-up work.

Last edited by Not a Number; 10-02-2013 at 10:02 PM.
11-25-2013, 08:40 AM   #28
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Is there a way to make this lens be always on A mode? some sort of short circuit... I have a problem of constantly loosing a contact between A and M.
05-22-2014, 10:59 AM   #29
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Thank you for the in-depth help!
05-22-2014, 11:32 AM   #30
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QuoteOriginally posted by zvon Quote
Is there a way to make this lens be always on A mode? some sort of short circuit... I have a problem of constantly loosing a contact between A and M.
If you remove one of the spring clips (I don't remember which one - the one that doesn't usually break) you can't take it out of "A" mode once it you set it to "A".

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