Stuck focus ring on Pentax-A 50mm F2

ZombieArmy · 08-12-2015, 05:38 PM

Just picked this lens up and a AF160SA flash for 3 bucks at a local thrift store (good deal regardless

) but the focus ring is stuck and won't move.
When I try to use a bit of force I move the rubber instead.

I figure I could use this for a bit of experience to take apart this lens and fix the issue! But I don't want to go in blind and would like some advice.

**patrick9** · 08-12-2015, 05:58 PM

I tore apart a 55mm Nikon lens that had a mild case of fungus.and stuck focus ring. I used A few drops of lighter fluid to free up the focus ring so that I could get to move. I need to disassemble it again and apply a tad bit of Helicoid grease .It finally came. It is a little stiff one direction but free and smooth the other . If you disassemble the lens I would recommend marking the parts for alignment purposes on reassembly. The hardest part I has was removing the front outer ring.

Not a Number · 08-12-2015, 06:02 PM

As I understand it on "A" lenses you have to remove the rubber ring so you can press in the tabs that hold the outer trim ring in place. From there you can disassemble the helicoid.

dcshooter can advise you better.

ZombieArmy · 08-12-2015, 06:33 PM - **1 Like**

Originally posted by dcshooter

Here's the basic process for relubing the helical on the simple (i.e. non 1.2) 50s. I advise that you take pics at each step to aid in reassembly.

From the front:

Remove trim ring, front barrel (screws) and focus ring (screws). I don't have the f/2 in front of me, but it could be as Not a Number says and be tabbed in (common on the cheaper lenses), or it could be a regular screw in. I actually rarely work on the F/2s, since most people just elect to replace them since they are so inexpensive. It's a great lens to practice on, though, so keep reading if you want to proceed.

From the rear, remove the rear mount (be very careful not to lose the contact/spring/collar reassembly. Remove aperture lever guide cam. Remove helical synchronizing guides (2 screws each), marking which guide goes in which slot, and see if either side of the helical will rotate. If not, give it a squirt of lighter fluid to loosen, then remove both helical halves. Take note of exit point of helical when separating the two halves for later reassembly. Clean out the helical grooves with a toothbrush and lighter fluid.

Lightly grease and reassemble both helical halves and reassemble, rotating the lower helical half the same number of turns it took to get it off originally. If it seems like the threads are thrashed by sand/grit, you can sometimes recover them by applying Mother's Mag polish (available at auto parts stores) to the threads and giving the helical about 500 turns or so. The polish will wear down the rough spots and leave you with a smoother helical.

Reinstall helical guides (greased) and reassemble rear mount. The toughest part is getting the little aperture contact assembly in the right place when you put the ring back on. Tweezers help. So does a good baseline level of sanity. You will be able to tell the position to install it in by a notch in the ring that you can slide the little collar down into before turning the ring to lock it down and reinstalling the mount.

Mount the lens on your camera and adjust to infinity focus. Attach the outer focus ring with the 3 screws in infinity position. Reattach front barrel and trim ring.

If your rubber grip is spinning, then the adhesive underneath has given way. Reglue it with contact cement.

Enjoy your working lens.

This is great! I appreciate it

I will most certainly give it a try during the weekend, wish me luck

Not a Number · 08-12-2015, 06:52 PM

The "A" series lenses usually have two trim rings:

an outer one on the focusing collar with the focal length and speed engraved - usually held by tabs under the rubber ring
one inside the filter threads usually blank - screws out with the usual rubber stopper method

I believe, but not 100% certain, you can disassemble the helicoid without removing the inner ring.

Ontarian50 · 08-13-2015, 09:07 AM - **1 Like**

The A and M 50mm 2.0's are prone to getting really stuck - more often than not caused by a strong knock on the front.
The outer part of the helicod is aluminum, but the inner part (with the optics) is plastic.
All the instructions for taking the lens apart only apply if the helicod will move - and if it has been knocked enough to skew the plastic threads inside the aluminum, it won't.

The only solution to this I've found is to remove the trim ring around the front element. Unscrew the screws inside to remove the filter ring and then the focus barrel - this will expose the end of the helicoid and you should be able to see if there's a larger gap on one side than the other.

If so, you have to remove the whole helicoid from the rest of the lens barrel - by taking off the rear mount (A lenses are so much fun here, with the extra pins!) and then the focus guides.

In my experience, the quickest way to fix the helicoid is to press the remaining threads into alignment with the side that's already been whacked inward. Sometimes a gentle squeeze in a padded vice snaps the threads into place, and ta-da! it all works smoothly again. Other times a careful smack with a mallet and wooden dowel does the trick. The plastic is surprisingly resilient.

The hard part for the inexperience lens restorer, is to put all the parts back properly and get the infinity focus adjusted again.

If you've done this before, it usually takes less than an hour. If not, well, make sure you've made a lot of diagrams of where all the parts came from!

Not a Number · 08-13-2015, 09:43 AM

Excellent. He makes putting the ball bearing back look so easy! This should be added to the disassembly database. Too bad he doesn't take the helicoid apart. I had the same problem finding how to take apart the helicoid on a Minolta lens. Everybody seems to stop at the diaphragm.

https://www.pentaxforums.com/forums/114-maintenance-repair-articles/179884-pe...ml#post3297614

cantthink193 · 08-18-2015, 02:00 PM

I recently received a 50mm Pentax-A f1.7 which turned out to have a stuck aperture ring and stuck focus ring. I disassembled the back of the lens and sorted the aperture ring out, pins and all - the bearing had slipped and most of the grease was gone - but I cannot for the life of me get the focus ring sorted. My issue is with the copper-coloured helical guides (unhelpfully not dissected in the video above) - the focus ring now turns but doesn't connect to anything (and wasn't connected to anything). The helical guides prevent the helicoid from rotating when screwed in as I found them, and I can't for the life of me figure out any other way of attaching them (or indeed making the focus ring function). I've not changed anything in the lens (complete with photographic documentation at the time) beyond the aperture ring.

Anyone come across a similar problem in the past/is a A-lens maestro and could give a few hints?

08-12-2015, 05:38 PM	#1
ZombieArmy Pentaxian Join Date: Jun 2014 Location: Florida Posts: 3,210	Stuck focus ring on Pentax-A 50mm F2 Just picked this lens up and a AF160SA flash for 3 bucks at a local thrift store (good deal regardless ) but the focus ring is stuck and won't move. When I try to use a bit of force I move the rubber instead. I figure I could use this for a bit of experience to take apart this lens and fix the issue! But I don't want to go in blind and would like some advice.

08-12-2015, 06:52 PM	#5
Not a Number Moderator Join Date: Mar 2012 Location: Venice, CA Posts: 10,526	The "A" series lenses usually have two trim rings: an outer one on the focusing collar with the focal length and speed engraved - usually held by tabs under the rubber ring one inside the filter threads usually blank - screws out with the usual rubber stopper method I believe, but not 100% certain, you can disassemble the helicoid without removing the inner ring.

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08-12-2015, 05:58 PM	#2
patrick9 Site Supporter Join Date: Feb 2011 Location: Murfreesboro Tennessee Photos: Gallery \| Albums Posts: 3,458	I tore apart a 55mm Nikon lens that had a mild case of fungus.and stuck focus ring. I used A few drops of lighter fluid to free up the focus ring so that I could get to move. I need to disassemble it again and apply a tad bit of Helicoid grease .It finally came. It is a little stiff one direction but free and smooth the other . If you disassemble the lens I would recommend marking the parts for alignment purposes on reassembly. The hardest part I has was removing the front outer ring.

08-12-2015, 06:02 PM	#3
Not a Number Moderator Join Date: Mar 2012 Location: Venice, CA Posts: 10,526	As I understand it on "A" lenses you have to remove the rubber ring so you can press in the tabs that hold the outer trim ring in place. From there you can disassemble the helicoid. dcshooter can advise you better.

08-12-2015, 06:33 PM - 1 Like	#4
ZombieArmy Pentaxian Join Date: Jun 2014 Location: Florida Posts: 3,210 Original Poster	Originally posted by dcshooter Here's the basic process for relubing the helical on the simple (i.e. non 1.2) 50s. I advise that you take pics at each step to aid in reassembly. From the front: Remove trim ring, front barrel (screws) and focus ring (screws). I don't have the f/2 in front of me, but it could be as Not a Number says and be tabbed in (common on the cheaper lenses), or it could be a regular screw in. I actually rarely work on the F/2s, since most people just elect to replace them since they are so inexpensive. It's a great lens to practice on, though, so keep reading if you want to proceed. From the rear, remove the rear mount (be very careful not to lose the contact/spring/collar reassembly. Remove aperture lever guide cam. Remove helical synchronizing guides (2 screws each), marking which guide goes in which slot, and see if either side of the helical will rotate. If not, give it a squirt of lighter fluid to loosen, then remove both helical halves. Take note of exit point of helical when separating the two halves for later reassembly. Clean out the helical grooves with a toothbrush and lighter fluid. Lightly grease and reassemble both helical halves and reassemble, rotating the lower helical half the same number of turns it took to get it off originally. If it seems like the threads are thrashed by sand/grit, you can sometimes recover them by applying Mother's Mag polish (available at auto parts stores) to the threads and giving the helical about 500 turns or so. The polish will wear down the rough spots and leave you with a smoother helical. Reinstall helical guides (greased) and reassemble rear mount. The toughest part is getting the little aperture contact assembly in the right place when you put the ring back on. Tweezers help. So does a good baseline level of sanity. You will be able to tell the position to install it in by a notch in the ring that you can slide the little collar down into before turning the ring to lock it down and reinstalling the mount. Mount the lens on your camera and adjust to infinity focus. Attach the outer focus ring with the 3 screws in infinity position. Reattach front barrel and trim ring. If your rubber grip is spinning, then the adhesive underneath has given way. Reglue it with contact cement. Enjoy your working lens. This is great! I appreciate it I will most certainly give it a try during the weekend, wish me luck

08-13-2015, 09:07 AM - 1 Like	#6
Ontarian50 Veteran Member Join Date: Mar 2015 Posts: 530	The A and M 50mm 2.0's are prone to getting really stuck - more often than not caused by a strong knock on the front. The outer part of the helicod is aluminum, but the inner part (with the optics) is plastic. All the instructions for taking the lens apart only apply if the helicod will move - and if it has been knocked enough to skew the plastic threads inside the aluminum, it won't. The only solution to this I've found is to remove the trim ring around the front element. Unscrew the screws inside to remove the filter ring and then the focus barrel - this will expose the end of the helicoid and you should be able to see if there's a larger gap on one side than the other. If so, you have to remove the whole helicoid from the rest of the lens barrel - by taking off the rear mount (A lenses are so much fun here, with the extra pins!) and then the focus guides. In my experience, the quickest way to fix the helicoid is to press the remaining threads into alignment with the side that's already been whacked inward. Sometimes a gentle squeeze in a padded vice snaps the threads into place, and ta-da! it all works smoothly again. Other times a careful smack with a mallet and wooden dowel does the trick. The plastic is surprisingly resilient. The hard part for the inexperience lens restorer, is to put all the parts back properly and get the infinity focus adjusted again. If you've done this before, it usually takes less than an hour. If not, well, make sure you've made a lot of diagrams of where all the parts came from!

08-13-2015, 09:43 AM	#7
Not a Number Moderator Join Date: Mar 2012 Location: Venice, CA Posts: 10,526	Excellent. He makes putting the ball bearing back look so easy! This should be added to the disassembly database. Too bad he doesn't take the helicoid apart. I had the same problem finding how to take apart the helicoid on a Minolta lens. Everybody seems to stop at the diaphragm. https://www.pentaxforums.com/forums/114-maintenance-repair-articles/179884-pe...ml#post3297614

08-18-2015, 02:00 PM	#8
cantthink193 Forum Member Join Date: Aug 2013 Posts: 74	I recently received a 50mm Pentax-A f1.7 which turned out to have a stuck aperture ring and stuck focus ring. I disassembled the back of the lens and sorted the aperture ring out, pins and all - the bearing had slipped and most of the grease was gone - but I cannot for the life of me get the focus ring sorted. My issue is with the copper-coloured helical guides (unhelpfully not dissected in the video above) - the focus ring now turns but doesn't connect to anything (and wasn't connected to anything). The helical guides prevent the helicoid from rotating when screwed in as I found them, and I can't for the life of me figure out any other way of attaching them (or indeed making the focus ring function). I've not changed anything in the lens (complete with photographic documentation at the time) beyond the aperture ring. Anyone come across a similar problem in the past/is a A-lens maestro and could give a few hints?