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07-16-2009, 11:18 AM   #1
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Problem with stuck aperture ring on Pentax KA 50mm f/1.7

I recently bought a Pentax KA 50mm f/1.7. The aperture ring wouldn't go any smaller than f/8. So I opened it up and on removal of the aperture ring, out popped the bearing, a spring and a couple of metal piece. One of them was the stop for the A setting and I stuck it back on with epoxy.

I assembled the lens without the bearing, spring (this is not the spring for the bearing - that spring is still on the lens) and the other metal piece to check whether the ring would move - it does steplessly, as expected - without the bearing. But the metering on the A setting is incorrect - I get the reading for the smallest aperture f/22 instead. I think this may have to do with the metal piece shown below (faintly circled). Any idea where this piece goes?



Also, when I tried to pick up the spring, it jumped and is missing. Does a spring go under the A button? The button works fine even without the spring - only problem is that it stays depressed on manual aperture settings once it clears the metal stop underneath.

Also, how do I get the bearing to stay while reassembling the ring back on?

Lens -






TIA!


Last edited by jhaji; 07-16-2009 at 11:46 AM. Reason: Replace with smaller image
07-16-2009, 12:36 PM   #2
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My friend, you need to find that spring. Turn the house upside down if you have to.

I can't remember exactly where the spring goes, but it has something to do with the recessed contact that tells the body that this is a PKA lens at A setting.

(The recessed contact is at 5 o'clock position if you look from the rear of the lens, with the red aperture/distance mark at 12 o'clock)

Remove the lens, move the aperture ring from the A setting (any of the number will do), feel the recessed contact. It's recessed, right? Now move the aperture ring to the A setting, feel the contact. It should not be recessed, but I bet you it still is on your lens.

Now that we have verified the caused of the problem, let go find that spring.

Edit to add: As I recall, the spring just sits at the egde near the pin. I had to use a bit of grease to temporarily hold the spring in place while assembling the lens. I also used a bit of grease to hold the little bearing in place.
07-16-2009, 03:49 PM   #3
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Figured out some of the stuff...

with a little help from SOldBear.

Unfortunately, I still haven't found the spring.

The small plate in the 1st pic goes on the inside of the aperture ring and makes contact with that A contact in the A position like SOldBear said. There is a coil spring that is missing... did you say that goes under the A contact?

The aperture take some wiggling to get it into place... how do you hold everything together... the bearing, the A switch, the A contact and the plate? What kind of grease did you use? Will petroleum jelly work?

Here's a disassembly manual I found for the KA 50mm f/1.7. Use "Pentax" for password if it asks you for one.

http://www.pentax-manuals.com/manuals/service/smca5017.pdf

The coil spring that I lost kind of looks like X509 / X506 in the picture... not sure where it goes... looking for a magnet to run over the carpet.
07-16-2009, 09:19 PM   #4
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QuoteOriginally posted by jhaji Quote
with a little help from SOldBear.

Unfortunately, I still haven't found the spring.

The small plate in the 1st pic goes on the inside of the aperture ring and makes contact with that A contact in the A position like SOldBear said. There is a coil spring that is missing... did you say that goes under the A contact?
Here is a page that should help you figure it out:

Pentax 50mm lens fix

QuoteOriginally posted by jhaji Quote
The aperture take some wiggling to get it into place... how do you hold everything together... the bearing, the A switch, the A contact and the plate? What kind of grease did you use? Will petroleum jelly work?
I use black lithium grease, a very small amount. You can also put some on the inside of the aperture ring. I think you can put the plate in afterwards. I use a small flat-bladed screwdriver and hold the lens so the bearing is at the very top and the focus ring is vertical (the lens is like it would be if you were mounting it). Rotate the aperture ring so the set of grooves is aligned with the ball bearing location. I begin to slide the ring on. It will run into one of those flat metal springs. I use the screwdriver to tuck that under the ring. I tilt the ring so I can tuck the second flat spring under it next. Then I use the screwdriver again to depress the bearing and slide the ring on.

This operation can go wrong. The bearing will be under spring tension and if your screwdriver slips, it will be launched into space. You can do this in a box or plastic bag to keep the bearing from disappearing. I would have liked to see how these lenses were assembled in the factory. At least they had extra parts on hand.

Here's where to buy bearings:

Ball Bearings, 1.5mm Package of 5 - Part #: BB-1-5M

I don't know where to get springs except from broken cameras.

07-17-2009, 09:59 PM   #5
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That's exactly what was wrong with mine too :)

Thanks for the link and the tips. I think mine went to around f/11 too.

Just a question of where a coiled spring could have popped out of left... when I pulled out the aperture ring It's not the one behind the ball - that one is still on the lens. This one was much thinner - the wire was almost the width of hair. Could it be from inside the A button... there is a small hole behind it and I noticed that once the A button clears the stop strip it doesn't pop back up when pressed down... once you get it back to the stop strip it comes back up.

They've probably got some specialized tool to hold it all together while assembling it at the factory. Either that or they've got some 4-handed folks.

Between the 2 of you, you've solved most of my problems... now I need to get the mother of all clear bags before I attempt to put it back together... and also some black grease.

Last edited by jhaji; 07-17-2009 at 10:09 PM.
08-15-2009, 05:22 AM   #6
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I too had the same query about the little brass item shown in your lens cap. I eventually realised it goes under the chrome bullet and spring assembly.

But I must have bent the spring as it now is L shaped! It no longer pushes up with enough force. I tried again but no joy, the contact gets pushed in and stays in. At the moment the lens works like an M series, but Iwould like the electrical contact to work properly again.

Just wondering if anyone has had success with replacing the spring with a bit of rubber or similar? Any Ideas. I would be very grateful, as would those who have lost the spring!
08-15-2009, 03:52 PM   #7
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The thin spring might be from the aperture lever.
It should spring close the aperture when you trip the lever to the biggest opening.
08-15-2009, 04:09 PM   #8
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Eveyone that takes lenses apart needs a LARGE magnet and the wisdom to NOT vacuum the floor til you find the missing pieces.

It's either that or you've accumulated a LOT of "fixer-uppers" and a shoe box full of parts you didn't think you'd ever use.

Ya pay your dues either way.

H2

08-15-2009, 04:10 PM   #9
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Would it be worth contacting Pentax about where to get the spring? Just a thought.

I was having problems with my A 50mm f1.7 as well (only with the auto aperture info kicking in and out) but luckily I found that repair link before attempting the fix. Ya, getting that bullet/clip/spring assembly back on is real troublesome.
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