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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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07-14-2014, 02:09 PM   #31
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Success !!!!

Dave,

I hope and pray that I NEVER have to do that again, but it looks like your tutorial saved the day! I picked this lens up a couple of weeks ago from Goodwill.com, and of course, it had the broken spring problem. I have a couple of M50 f1.7's, but didn't have an "A" fast 50 for the K20D. It will now reside in my digital bag, perpetually on the "A" setting!

Thanks very much for the great pics and description. It was an afternoon I'll not soon want to repeat, but well worth the effort!

Griz

08-19-2014, 10:04 PM   #32
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Here is a PDF from a long gone website (Justin Pelling) that I saved to keep for reference. It gives a slightly different images of the process.
https://www.pentaxforums.com/forums/attachments/120-general-technical-trouble...m-lens-fix.pdf
08-20-2014, 04:29 AM   #33
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The author of that PDF makes it sound so simple. Maybe it is if you are under 45 or 40 years old and your eyes can still close focus. In my case fiddling with the A pin assembly consumed a good portion of my time. Without the pressure of the spring it has a tendency to simply fall apart. At least on my lens it isn't a simple matter of just pressing the "bullet" into the clip.There is a groove in the bullet and you have to slip it into the clip at just the right angle and pivot it in.

The pin assembly pictured above is different then the one I have - it looks like the pin or bullet is not separate from the L shaped part. And instead of a button or bump on the long leg of the "L" is just a stamped out collar. Simplification of manufacturing surely. It would be of interest to compare serial numbers. The one I have is similar to the one pictured in the PDF.

If you use a dab of grease on these parts to help keep them together and to lubricate them make sure it is not non-conducting grease. Otherwise you may find the A pin does not work even when it is extended properly. Then you have to take it apart, clean off the grease and start fiddling with it again, and again.

And if I had to do it again I wouldn't bother with the screws either and just glue the spring clip down with some JB Weld epoxy - the regular cure stuff. Finding the right sized screws is a chore - I took a couple off the top and bottom plates beat up K-1000 that came with the lens. Also where you drill for one of the spring clips goes into mounting post for one of the screws holding the rear bayonet - something I wasn't comfortable about weakening - JB Weld to the rescue.

Last edited by Not a Number; 08-20-2014 at 08:19 AM.
01-08-2015, 10:00 PM   #34
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I am not a technician in any sense of the word. I am about as mechanically inclined as a spoiled clam. I wonder if it would be possible to put some lubricant such as 3 in 1 oil on the sides of the aperture ring. Would I be creating a worse situation than I already have? It really is unfortunate that this has happened, since this lens is of genuine high quality glass. I depend on it greatly. Thanks again for your assistance.
The camera sales/service center I do business with stated they can effect repairs, but the cost would be from $50.00 to $89.00. I hate that.

Tonytee


Last edited by Tonytee; 01-08-2015 at 11:41 PM.
01-08-2015, 10:44 PM   #35
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The main problem is not really a lack of lubrication, it's the broken spring. The oil might migrate to other areas too. If your lens works in the A position, leave it there. If you really have to move the aperture ring, it's cheaper buy an M series 50/1.7 for those times than to pay for a repair.
01-08-2015, 11:42 PM   #36
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Thanks very much for all of the helpful replies.

Tonytee
01-12-2015, 02:58 AM   #37
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Okay, so someone suggested that I leave the lens on A permanently. I tried that with my Pentax SF-10. How can you tell what aperture setting was used? No indication of this on the viewfinder. Thanks for any help.

TT
01-13-2015, 01:16 AM   #38
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not every Pentax camera has the option to control the aperture manually. The sf-10 might not be able to do that. The cam will choose the "best matching" aperture. You will find if it is possible to control the aperture in the camera manual.

01-13-2015, 03:12 PM   #39
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I understand that taking the lens off the A position will allow me to manually control the aperture. On the SF-10's viewfinder, it will show the shutter speed necessary to take the photo, however there is no information as to what aperture setting is being used when the lens is in the A position. You are correct, the instruction manual should offer the information needed. I will check it and keep you posted. Thanks very much for your input.

tt

Last edited by Tonytee; 01-13-2015 at 03:30 PM.
01-13-2015, 10:45 PM   #40
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QuoteOriginally posted by Tonytee Quote
the instruction manual should offer the information needed
I looked at the manuals online and it looks like the only way to set the aperture is to turn the ring.
01-13-2015, 11:42 PM   #41
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that's the development. Lower range Pentax bodies were not able to manipulate the aperture with the camera body only, it is and was only possible with the higher range bodies like z1p. If you can't manipulate in the body and want to set the aperture as you wish, you will need the manual settings on the lens. However, some lenses do not offer this option anymore. The most people coming here have trouble with their lens. The mechanism which allows you to switch between manual an automatic modus has a weak spot. If the spring of the mechanism is broken and you use the lens on a digital body, it is recommended to leave it on a-mode, because you can manipulate the aperture there in the body too, or repair the mechanism. For switching between bodies it is better to have all options anyway. With a broken mechanism you might not be able to switch from manual back to the a-mode. The bodies will not be able to use any mode where it is important to change the aperture too. This leads to stop down metering and shutter priority won't work, at least not properly.

I hope this helped
02-23-2015, 02:23 PM   #42
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Thank you so much for the guide.
It has helped me resolve an issue I've had with this lens for 18 years! It was always very intermittant whether it would work in A., on any of my cameras.

It turns out that I was missing the angled plate that sat underneath the sprung pin that formed the A-M Selecting plate assembly.

Through trial and error I made a replacement out of one leg of a paper binder as it was about the right width and thickness of metal. Took me a couple of hours to stop it interfering with the aperture workings and get it just so. I think it contacts the inside of the lens mount, so it completes the circuit.
Plus I lot the spring and pin a couple of times, so spent a while trying to locate it on the carpet!

Now I can use it on my new K50 with confidence!

Thanks again!
02-23-2015, 02:35 PM   #43
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QuoteOriginally posted by bigyinn Quote
Plus I lot the spring and pin a couple of times, so spent a while trying to locate it on the carpet!
It's a good thing you were able to find them. One tip I read somewhere is to take a nylon stocking/pantyhose and put it over the tube of a vacuum cleaner and vacuum around where the items dropped. They will get caught in the nylon mesh and it works with non-ferrous parts unlike a magnet.
02-24-2015, 05:07 AM   #44
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QuoteOriginally posted by Not a Number Quote
It's a good thing you were able to find them. One tip I read somewhere is to take a nylon stocking/pantyhose and put it over the tube of a vacuum cleaner and vacuum around where the items dropped. They will get caught in the nylon mesh and it works with non-ferrous parts unlike a magnet.
I think I spent as long looking for the spring as I did carrying out the repair!
Luckily I have good eyes!
Very very happy to have fixed this.
05-25-2015, 02:06 PM   #45
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I have just joined the forums because I had this exact problem - and here is the solution, in exquisite detail.

Many thanks!
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