Pentax 50mm lens fix

I acquired an SMCA 50mm f1.7 from eBay. Well, what do you expect.. The aperture ring was would only turn between f1.7 and f11. Meaning no A position action for my DS. So I did a bit of reading round the net and found that this seems to be a problem peculiar to this one specific Pentax lens. I need very little encouragement to take things to bits, so I went for it, and here are my observations.

First, unscrew the five screws securing the bayonet mount.

Take it off, and it looks like this

The aperture lever cam just lifts off, like this.

Next, the tricky bit. You have to take the aperture ring off, VERY CAREFULLY. It is worth doing this in a box or perhaps a plastic bag, because there are a couple of tiny bits that could fly off and get lost. If you happen to have one of those handy magnetised parts trays (available from all good automotive tool supply shops), so much the better. Once the ring is eased off, and all the bits are accounted for, relax. But not totally. You may have one or more minute ball bearings rolling around. They are teeny tiny. Keep good track of them and BEWARE, they are soo small that if you brush against one accidentally, you can pick one up on your skin without realising it - until it is too late and you've lost it. Best advice - pick them up with a bit of Sellotape until you are ready to put it all back together. The A position button will probably fall out of the aperture ring, but it's large by comparison and hard to lose.

Now it looks like this. Note that little silver bullet-on-a-spring thingy. That's the 'A' connector, as it were, that tells the camera when the lens is in A mode. Most likely it fell off when the aperture ring came off. The bullet thingy sits on a metal tab in that grove. The bullet thing disassembles into three bits; the bullet, the spring and the carrier. If yours drops off and falls to bits, just press the bullet bit back into the carrier and shove the spring back up its arse and, come reassembly time, put the metal tab back in the grove and sit the bullet thingy assembly back on it.

Now to the main event. There are two metal tangs controlling the A position clickstop. These are held in place with plastic welds. These welds are a bit fragile and commonly fail. It seems that Pentax only used this fixing on this one specific lens, but I haven't looked inside any others to check. Yet. So there's the problem - the weld has failed on one tang and it has become detached, jamming the ring.

Solution: I have read that some people have drilled tiny holes and reaffixed the tangs with tiny screws. Far too much effort for me. I just used a blob of Araldite adhesive, which seems to have done the job perfectly. Perhaps you could lash it down with duct tape. There there, all better. Original plastic weld on the right, Araldite blob on the left.

Now to stick it all back together. Remember the detent - here's the spring, pulled a bit out of its hole so you can see it.

And there you have it. It's not brain surgery, just a bit fiddly. I do hope you didn't lose any of those ball bearings now...