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12-20-2011, 08:14 AM   #1
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Lens mount for A* 300mm f2.8

Hello Everyone,

I need to replace the lens mount on the back of my A* 300mm f2.8 lens. This is because the small hole on the mount that engages the pin on the camera side is worn out. So, the lens will not lock in place when it is twisted on the camera body. It stays loose and comes off easily.

Does anyone have experience replacing the mount? Can I use the mount of an A lens (say the A 50/1.7) as a replacement? Or do I need to send it to Pentax for the repair?

Please advice on how I should proceed.

Thank you very much.


Sai

12-20-2011, 09:48 AM   #2
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I think since the mount communicate with the camera its aperture value, you need to find an equivalent mount (i.e. A lens f2.8). That should not be too expensive I don't think.
12-20-2011, 10:07 AM   #3
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Hello Doanh,

Thanks for the comment.

So I need to match the f-stop? The electronics is attached to the mount then? Can the electronic not be reattached to the new mount?

Thanks,
12-20-2011, 10:26 AM   #4
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There are no electronics really, the pads are either shorted to the mount or not. On most lenses you can tell just by looking at them, if the pattern of plastic/metal matches it should be good. And if it doesn't you can probably move the plastic part between the mounts.

12-20-2011, 11:33 AM   #5
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So, I can take any A lens and take the mount as the replacement?
12-20-2011, 12:15 PM - 1 Like   #6
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QuoteOriginally posted by swip Quote
So, I can take any A lens and take the mount as the replacement?
The short answer is no. The contacts on an A lens define the maximum and minimum F-stop for the lens. There are a total of 5 contacts that generate a binary 1 or 0 pattern that tells the body the maximum (2.8 in your case) and the minimum (32) F-stops for that lens. There is also another movable pin that engages the "A" functionality. If the lens is in the "A" setting the pin protrudes, if it is in any other f-stop it is retracted. In some lenses the "1" is just the metal part of the lens while the "0" is a plastic insert to prevent electrical contact. Some others have pins for "1" and plastic insulators for "0".

You can find a detailed description at Dojidar Dimitrov's site Features and Operation of the Ka Mount

Besides the correct pattern, the screws that attach the mount must match to be able to swap the mounts. The two A lenses that have F2.8-F32 are the A 135mm F2.8 and the A 400mm F2.8. It is unlikely that third party lenses will have the same spacing (or even the same number) for the screws. In addition, if pins are used to indicate "1", they are spring-loaded so if you remove the mount, they will come apart. Unless you have experience in putting these things together you may end up with a K or M version of the lens.

My recommendation will be to find a reputable repair facility and have it fixed there. This is a very nice and expensive lens to mess it up.

Good luck,

Demetri
12-20-2011, 12:34 PM   #7
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Hi Demetri,

Thank you very much for the great insight.

So it looks like I need to find an A 135mm f2.8 or A 400mm f2.8 lens. I guess the A 135mm f2.8 will be the cheaper choice.

Regards,

Sai
12-20-2011, 12:40 PM - 2 Likes   #8
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A possible low risk solution used in many similar situations. This is sort'a like going to the dentist to have a filling made. I would also determine what was causing the excessive wear.

Obtain some J B Weld steel/epoxy at any store that sells hardware. Other brands of epoxy with metal filler will work too.

Scribe / mark the precise center of the latching hole on the ring.

Protect the base of the lens from dirt and damage.

Fill the damaged hole with the epoxy and let set for 24 hrs.

Carefully mark and re-drill the hole with a bit just slightly larger than the latch pin on the body. A drill press is recommended.

If you make a mistake, you can clean out the hole with the drill bit or Dremel tool burr and start over.

If you're not comfortable doing this yourself, find someone at a local model train or aircraft club and explain what you want done -- they'll have the necessary tools and skills to do it and you may find some interesting photo opportunities with them too.

H2

12-20-2011, 01:08 PM   #9
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Changing a mount is easy. Just needs a replacement A-lens with an equal amount of "Holes", et voilą... (or drill some extra holes in the embossed spaces in the mount).

Here a link to a recent DA*-repair of mine (Google-translate): Link to the BEST Dutch Pentax-forum
12-20-2011, 01:47 PM   #10
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Hi Pacerr,

That sounded like a very quick and easy solution. I may give it a try if I can find the J B Weld in my area.


Hi Frederik,

Your link is very instructive. This will definitely provide the permanent fix to my problem. But I am not sure I have the same courage as you did tinkering with my lens.

Thanks,

Sai
12-20-2011, 05:13 PM   #11
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Hey Sai,

The difficult part is removing the 6 small screws by which the chrome mount is fixed to the lens barrel. In some lenses the are 'glued' to the mount by accumulated dirt or screwed in to firmly to remove by regular consumer screwdrivers. Your local watch or jewelery seller probably has more professional material to unscrew them (regular cheap screwdrivers can mess up the screws easier than you would think).

A good tip is to take pictures of each step (just for yourself and for an interesting thread on this forum )!

The only tricky part in dissembling the mount of a A-lens is the spring that drives the diaphragm driving plate (circular shaped piece of metal). It's a good idea to remove the actual chrome mount very slowly, and try to look between the half remove mount and the rest of the lens, while keeping the diaphragm flange in its place and look at the position of the spring, the copper contacts and the little boomerang shaped piece of metal that works like a contact switch between the A-setting and the manual F-stops.

Maybe have a try with a cheap A50/2 or so?

You are always welcome to come over to Belgium and let me do the fiddling

Good luck!
Frederik

Last edited by frederik9111; 12-20-2011 at 05:22 PM.
12-20-2011, 06:39 PM   #12
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Here is a thread where I think they tried the same thing you need to do to your lens with some success.

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/163700-lens-mo...0mm-if-ed.html
12-21-2011, 12:07 PM   #13
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I tired pacerr's idea but the worn out area is too small. The material I applied did not stay in place. I may have to open the hole a bit more to make sure what I applied have enough surface to stick to.

If this does not work, I guess I will have to get a sacrificial lens and get the mount replaced.

Thanks everyone!

Sai
12-21-2011, 12:37 PM   #14
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Hmmm...home repair of a $3k lens vs. sending it to Eric for around $50-75. Hmmm.............errr.......good luck.
12-22-2011, 10:37 AM   #15
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What I ended up doing is using a Dremel with a dental bur (557, with a flat top and straight sides), I carefully cut out the worn out part (there was a step created by the wear thus allowing the pin to slide out of the hole) and created a slightly bigger hole for the locking pin. it wasn't pretty but now the pin locks properly. There is also some minimal rotational free play but I don't think it will affect picture quality.

Thank you everyone for the good suggestions. I may need to use some of them when the problem surfaces again.

Have a Merry Christmas!

Sai
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