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12-07-2019, 06:32 AM   #1
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Unfortunate Samsung GX-10 aperture problem

Hi
I have just acquired some extension tubes with all 7 contacts for the KAF mount( marked as pentax on end cap ). I have used these on my GX-10 and K-70 without issue when using any of my DA kit zooms or the DA55-300mm and sigma 17-70mm. All work fine. Unfortunately I also have the so called plastic fantastic DA-50mm and this is where my problem has arisen.

The DA-50 when interfaced with a tube would have a very stiff aperture movement but seemingly only on the shorter tubes. When I looked closely this would show itself before any metal to metal contact of the aperture lever in the lens with that of the tube by slow mating of the tube and lens. It was as if the plastic body of the DA-50 was distorting the frontplate/aperture lever fixing of the tube and causing the tube lever to seize or bind. None of the metal bayonet lenses caused any issues at all.

Unfortunately when testing with the longest tube that had seemed to work ok the problem raised its head again to the extent that the aperture slider in the body of the GX-10 no longer works. The slider will move up and down against its spring by finger movement but does not actuate on shutter or DOF preview. The cog behind the slider can be seen to work rotating each time. I've looked at the k-70 and it works similarly and the cog does not seem to be attached to the slider. Does anyone know if the aperture slider is driven from that cog or by a solenoid that has possibly failed under the stalled/seized aperture. Has anyone had a similar experience with GX-10 or K-10d with a seized lens aperture mechanism, it must happen from time to time. Has anyone repaired such a fault themselves if they have had a problem.

I'm seriously annoyed with myself but more so the plastic bayonet mount which clearly is behaving in a manner that the stiffer( or better toleranced ) metal to metal mountings don't. I feel heart sorry for my GX-10 that was enjoying semi-retirement but still in use and running well turning against the DA-50( I should have just got an A-50 f1.7!) . I noticed a thread list john pye as a new repair centre but if it's fixable myself I would look into doing so.

Hope you can chime in with any similar experiences and the resolution or not.

12-07-2019, 09:47 AM   #2
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My K-10D succumbed to this, too. A sticky lens aperture mechanism can damage the camera body by making the camera mechanism work too hard trying to re-open the aperture after each shot. Old, worn, or damaged lenses can have sticky aperture mechanisms. Add-ons such as extension tubes and teleconvertors increase the workload. Honestly, I'm surprised that more people don't have problems like this.

I did not try to repair my K10D but, instead, dedicated it to use with M42 lenses and other preset lenses (e.g., the Pentax 28/3.5 Shift) that don't need aperture actuation.

Good luck!
12-07-2019, 11:13 AM   #3
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Hi Photoptimist

My worst case scenario was to do exactly as you did and consign it to M42 lenses( if I ever use such ). I'll see if it is repairable but theres little video wise on the net and like yourself I'm surprised that it doesn't happen more frequently. Though from an engineering perspective I'd never expect it to happen as you would design to protect against situations like this as they are likely to happen in real life. Quite depressing and it isn't really a fault of the tubes as they work perfectly in a metal to metal interface, it's the plastic mount that has caused the stuck lever. It's a nice camera and I don't want to give up on it if I can sort it out in a financially viable way.

I have downloaded the GX-10 service manual with exploded views and disassembly instructions but whether you can access/purchase the part is unknown. You'd think it would be an easy fix being a likely failure point. Not a happy chap!
12-07-2019, 01:31 PM   #4
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The diaphragm control block G100 used in the K-30/50, KS-1/2 looks similar to that used in the K-10D. You can buy it from USCamera probably with the notorious green solenoid. You may be able to rebuild the GX-10 G100 by replacing the broken parts. It is not a trivial task removing the G100 block.

https://uscamera.com/product/pentax-77860-g0100-diaphragm-control/

12-07-2019, 02:14 PM   #5
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hi Not a Number

That photograph of the module is useful as it lets me see how this works mechanically, I haven't found any good images of this area of the camera. The phrase "not a trivial task" wasn't what I was hoping to see .

I'll do more research then if it's a runner I'll see if it's available in the uk too.

Edit: I've now viewed the photos and unfortunately this unit doesn't use return springs on the slide system and the main gear has a different placement. I'm a little concerned that the k10d and GX-10 may have all of this control integrated as part of the mirror box and not a removable subsystem. I'll continue digging around, I also now have to k10d service manual to view too for further clues.

Edit 2: Well on looking at the K10D service manual it does indeed show the G100 unit and on playing with the slider on my camera, the return springs that I can see are actually underneath the slider in a manner that would be hidden on the web images so this may well be worth investigating. The pentax manual is a lot shorter than the samsung yet gives more photographs of each sub assembly the samsung is good for disassembly/assembly but its exploded views are graphic and not particularly detailed compared to the photos. The G100 seems to have a "reset" position where the slider is held down and a latch clicked while setting the drive gear at at fixed point. This may be worth looking into if I do disassemble it. The samsung manual seems to have no images nor reference to the G100 unit but clearly is identical to the k10d other than the external case shape.

Last edited by Richard_Fiddler; 12-07-2019 at 03:14 PM. Reason: Added an update to reply
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