Fixing the Stuck/Jammed Zoom on a Pentax/Tamron 18-250 Lens

I suspect this is a somewhat common problem, potentially affecting more lens models and brands in a similar way. I have found a few online posts reporting the same symptoms with this lens. Fortunately, the fix is quite easy, and requires very little mechanical skill to accomplish.

THE PROBLEM

The zoom mechanism on our SMC Pentax-DA 18-250mm F3.5-6.3 ED AL [IF] (apparently made by Japanese lens manufacturer Tamron, who sells it as the A18) lens recently stuck in its widest position. Experimentally working the zoom ring while orienting the lens in different directions would eventually result in the lens releasing. Sometimes, the freed zoom would work normally for many cycles. Sometimes, it would immediately jam after one trip back to full wide position. Sometimes, the freed zoom moved with some abnormally high drag. But the lens would eventually jam again at the full-wide end of the zoom range. Working the zoom lock switch at the circumference of the base of the lens made no difference in the symptom (and the lock still worked as a lock).

THE SOLUTION (the short version)

Tightening a single screw which had worked itself loose solved the problem. Accessing the screw requires only removal of the rubber grip around the zoom barrel. The whole procedure can be accomplished with a single small Phillips screwdriver in less than 5 minutes.

THE REST OF THE STORY

Unfortunately, my failure occurred while attending an air show, where I had little need for the lens at 18mm, rather shooting flying aircraft most of the day at the full 250mm.

If I fiddled with the zoom assembly constantly, it would eventually, but not predictably release. But my habit when carrying the camera is to immediately zoom out to “park” position and lock the zoom as I’m dropping the camera to my side. So as hard as I tried, I accidentally got the lens stuck in wide several times during the remainder of the event.

My initial reaction was that it might be easier to just replace the lens than have it repaired. My research revealed several more interesting “superzoom” lenses which might provide better performance (it’s slowish, not that sharp at longer focal lengths, and autofocus speed on a K-3 is so-so).

I found a low-resolution PDF of an exploded assembly diagram of a Tamron A18 lens in this Pentax Forums post (see post #11). It was too crude to get real information, but gave me some hints as to the location of some internal components.

After disassembling not quite half of the lens from the mounting plate end forward (but before ever moving an optical element), I realized that there would have to be fasteners accessed through the side of the moving zoom barrel.

When I attempted an exploratory loosening of one of the screws, I found it loose. Immediately, I realized I’d found the culprit. The screw passes through a small collar, at it was obvious that the collar and/or screw would interfere with the access hole in the zoom barrel if they extended out from their designed location. Even if the screw never moved further, the collar could sometimes drift outward to lock itself into the access hole.

TOOLS REQUIRED

• Phillips screwdriver size #00

• (Optional) A dull-tipped dowel, perhaps 5-8mm in diameter (a chopstick or disposable ball-point pen will do)

THE PROCEDURE

• If not already in the locked position, zoom the lens fully wide and move the lock switch to “locked.”

• Remove any lens hood from the lens assembly.

• Remove the lens assembly from camera and install front and back caps to protect the lens surfaces.

• (CAUTION: In the case of my ten-year-old lens, the rubber grip was still impressively supple and pliable, but be warned that it's possible that yours could be less so and possibly tear during this procedure. Proceed accordingly.)

  Using a non-scratching, dull-pointed dowel (a tapered wooden chopstick from a takeout restaurant is a good tool), feed the dowel under front edge of the rubber zoom grip. It may be helpful to use a second thinner tool to slide under the rubber to start an opening in which to insert the dowel. Once under the grip, roll the dowel back and forth, letting it grab the inside of the rubber ring, and by applying pressure along the length of the dowel and angling the dowel, work the dowel all the way to the rear edge of the rubber ring. Roll the dowel so that you free up the entire circumference of the rubber ring - we want to make sure it’s not adhered to the plastic barrel underneath.

• Now that the rubber grip is freed up, carefully work the grip off the front of the lens assembly. Grasp it carefully with your fingers and pull toward the front of the lens, then repeat as you work around the circumference of the lens. It’s actually not necessary to completely remove the rubber grip - it only needs to move forward about 1/2” (13mm) to reveal the problem screw.
  
  
• Below, see the location of the screw.
  
  IMPORTANT: This screw is only visible through this aperture when the zoom ring is at full-wide position. Turning the zoom barrel moves the hole out of alignment with the screw.
  
  Its position on the zoom barrel is just counterclockwise of the serial number sticker (which is glued into a shallow relief molded into the barrel). The problem occurs when the screw loosens enough to allow the collar around the screw to catch on the edge of the hole in the barrel through which you can see the screw head in the image below.
  • (I think that this screw is the only likely cause of this symptom, as it is the only fastener which appears to align with a potential interference feature in this zoom position. The other aperture located on this zoom barrel reveals two additional screw heads, but only in other focal length positions.)
    
    
    
    
• Using a #00 Phillips screwdriver (always use the largest possible screwdriver bit that will fit the full depth of the fastener), turn the screw clockwise to tighten it. Snug the screw, but don’t put large torque forces on it. If in doubt, use less force than you think you need. You can always repeat this procedure later if the problem reoccurs.
  • It’s certainly possible that this screw will NOT be loose, and that some other conditon causes this problem. But I’m guessing that this is very likely to occur in other examples of this lens, so I’m pretty confident that you will find this screw loose.
  • If it is NOT loose, do not tighten the screw further. Replace the rubber zoom grip and consider other options to repair or replace this lens.
  • You can now test the lens to see if the problem is resolved. Unlock the lens and zoom the lens in and out. You may find it easier to zoom the lens by mounting it on a camera body.If your testing is satisfactory, restore the rubber zoom ring. The rubber ring will stretch to be wider than the groove into which it fts, so you may have to gently massage both front and rear margins of the ring into the front and rear edges of the groove. That’s it! The lens should now zoom normally.