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K100D doesn't detect when aperture ring is set to A position

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James Lin

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I could have sworn that I just saw a thread about this, but I can't seem to find it now, so sorry about the repost.

I just got a Pentax-A 28mm lens, but I can only get it to work like a -M lens. When I set the aperture ring to "A", the camera still shows "--" for Av.

I tried wiping off the contacts using a pec pad, but everything looked pretty clean to begin with. My DA lenses still work just fine.

The lens looks to be in really good condition, although I found that it would sometimes lock at the f/22 position when I'm trying to get it out of the "A" position. I'm not sure if that's normal or not.

Any ideas?
 
I've got the same situation with my FA 80-320mm.

--
Edvinas
 
On the lens check if the electrical contact pin for the A possition is moving out when you put the lens on A. If doesn't move, then I suppose it can be fixed if you open the K monture of the lens. Do you have problems with other A lenses?
 
Thanks for the reply. I don't notice any change in the contacts when I put the aperture ring in the "A" position. This is my only -A lens so I don't have anything to compare with.

Maybe I'll take it to a local camera shop when I have some free time to see if they can fix it for me. I'm not really up for taking apart lenses yet. :)
 
Actually, it does look like something is moving. There's a small hole where the row of contacts is and it looks like a small piece of metal shifts very slightly when I move the aperture ring into the "A" position. It barely moves though, so it seems like it wouldn't really make a difference in helping make electrical contact with anything.
 
James Lin said:
Actually, it does look like something is moving. There's a small
hole where the row of contacts is and it looks like a small piece
of metal shifts very slightly when I move the aperture ring into
the "A" position. It barely moves though, so it seems like it
wouldn't really make a difference in helping make electrical
contact with anything.
Click to expand...
The pin should move so that it protrudes above the contact surface. The movement is very small, less that half a mm I would say, but it is enough to ensure contact, and is quite noticable. Sounds like yours is stuck.

Alan
 
I've got an A-50/f:2 that did the same when I first got it.

It could be caused by either the cam on the aperture ring is worn so it doesn't push the contact up high enough, or it could be the contact pin bent a bit.

Warning...minor surgery involved...

I removed the mount flange and bent the contact piece a bit so it extends through the mounting flange further. Hasn't missed a beat since.

Be careful not to lose or misplace any parts if you try this though. My 50 was an old beater and I wasn't out much if I screwed up...plus it would still work as an M.

I don't think I'm brave enough to try this on a newer lens with multiple contacts, but a single-contact A is fairly simple under the mount flange.

I didn't think to take pictures of the surgery...
--
http://www.flickr.com/photos/moparmike90/
 
That last paragraph should say a single-contact A-50 is pretty simple under the flange. I can't speak for any other As, I don't know how the extra contact pins are set up inside.
--
http://www.flickr.com/photos/moparmike90/
 
Just be very careful when you remove the flange - I tried to fix my A50/1.7 like this and the spring-loaded 'A' pin went flying out across the (carpeted) room and I never saw it again. I now effectively have an M50/1.7 with a very loose aperture ring.

(which I don't use since I also have an F50/1.4)

--
Andy
http://www.caerphoto.com/
 
Ah. Thanks for the warning. I was afraid that there'd be some spring loaded part that would explode the moment I removed the flange. I will definitely keep that in mind when I take it apart tonight.
 
I removed the mount flange to take a look and I didn't really see anything that would extend out when the aperture ring is locked at "A". In fact, I didn't see anything that would really made contact with the camera

I drew a white arrow pointing to the only thing that I saw moving, aside from the aperture ring and it's clicking/locking mechanism (I'm sure you can tell that I'm an expert on the internal workings of lenses).

That little metal ring tilts up slightly when I rotate the aperture ring towards f/2.8.



 
Here's a picture of my a-50/f:2 with the flange removed.



It looks like you're missing the very upper part of the contact piece. On my 50, it's an L-shaped piece of metal with a round-tipped pin riveted onto the short flat of the "L". Yours has a hole in the piece where the pin should be.

Unless you can find a parts lens to rob the whole contact piece, I fear it's stuck as an "M" lens.

Or you could do like I've heard others do and stuff a little ball of aluminum foil into the hole where the pin goes and maybe it'll protrude far enough to touch the body contact. Not a very good fix...

The contact is spring loaded and held down by the aperture ring. It shouldn't move up until the ring is in the A position.

BTW, if you dig any further into the back of the lens, be careful not to lose the detent ball and spring for the aperature ring or the spring & button for the A position lock button.

A trick I learned when I ran my electronics repair shop...When I disassemble small stuff like this, I put the whole thing inside a clear plastic bag (zip-lock baggie or whatever ya got handy) to best contain any flying parts.
--
http://www.flickr.com/photos/moparmike90/
 
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