Tamron 28-200mm 571D Lens - Autofocus Belt Fix

I got this ancient first-generation Tamron 28-200mm zoom lens many years ago, shortly after I switched to Canon EOS DSLRs from the now defunct Olympus 4/3 system. I didn't know much about lenses, but it was cheap (used), sure looked good with the petal-hood, and I wanted something with lots of zoom range.

The labeling says Tamron AF Aspherical LD (IF) 28-200mm 1:3.8-5.6 72mm 571D on a standard Canon EF mount. I have no idea how old it is, but its definitely from the SLR days and probably an early EF adaptation. Build quality is good and it rotates smoothly. (This site says its a 1999-era lens and offers some technical specs.)

It is a twist zoom about 3.75 inches long extending to 6.75 inches (sorry, no metric on this ruler - must get rid of it). Its hefty and worked reliably enough for a couple of years. The images are quite bland, but it let you get the shots you wanted.  It does not have IS and the focus motor is noisy, but always found focus without much hunting and it seems accurate. I'm not really sure how sharp it is, but its been good enough for casual use and posting pics on the web.

In 2012 the autofocus quit as I was focusing a routine photo at an event. The motor just made a slight buzz when you half-pressed, but nothing moved and the focus did not change.  I switched to manual focus to get the shot, but manual focus on a crop sensor camera with a small viewfinder is difficult to do.  I contacted Tamron Canada to ask about getting it fixed. They said given its age, it probably wasn't worth repairing for the cost, so kudos to them for being up-front. Figuring it was worthless to sell, I parked it in a drawer and picked up a Canon 18-200mm EF-S zoom second-hand.
A few years later, I picked it up and figured it was worth another look. With little to lose, I watched a couple of unrelated web videos and decided to open it up and see what I could see. And voila, there it was on my table.

The Fix:  I removed the 4 top screws on the metal mounting plate and gently lifted it up. There is a thin metal gasket under the metal mounting plate that is loose fitting and also held in by the screws. The plate does not come completely off because there is a ribbon cable connecting the lens contacts to the circuit board in the lens, but I was able to lift it gently at an angle and get a look inside.

Luckily, the problem was immediately obvious: its a belt-drive lens, and the belt had slipped. The focus motor connects to the focus mechanism by a small black belt, which had slipped under the pulley. I didn't realize lenses had belts - always thought that given their frequent handling, they'd be all gears, but I noticed in other disassembly pics these old Tamrons do.

The belt is only about 1cm in diameter - I didn't get an exact measurement. Its displacement would explain the short pulse of motor noise when you half-press the shutter button and the lack of focusing action in the lens.

The belt did not feel very loose, but its not very tight either. My sense is that it will probably slip off again over time as it stretches and wears. I do not know what it's exact size or specs are, but if it breaks again, I will just find a large dental elastic (for braces) or something similar. Not too worried about it.

When I tried to grab it with my fingers it fell into a slot next to the rear lens element.  I bent a paperclip into a small hook and fished it out, and re mounted it (see image).  The black pulley popped off the first time, but I just pressed it back onto the shaft - it seems the metal plate sits just above it, preventing it from coming off.

That was it, or at least all I did for it.  I gently lined up the the gasket, replaced the top and refastened the 4 screws.  Simple, done.

I mounted the Tamron to an old Canon Rebel XT (350D), set the focus switch to Auto, switched on the camera, and it worked just fine.
 

Below are a couple of hasty unretouched shots in the backyard after the fix (at 28 and 200mm settings, though on a 1.6x crop sensor, left-most focus point.

The focus was quick and noisy, as it was before. Again, not the sharpest nor contrastiest lens ever, but at least it works for now. There's good, and there's good enough - this one is clearly the latter. The images have been shrunk down to 800px so they are not as sharp here as they were on my monitor in the photo editor, but they weren't particularly spectacular to being with.


And if you found this blog post, now you know.  If you have this old model Tamron 28-200mm LD 571D zoom and the focus just quit, it is very easy and quick to check & fix if its just a slipped belt.  

Cheap and versatile with a good range (especially with a full frame body), though not great optically - there's good and there's good enough. Good luck. 

Next I might try to fix the zoom mechanism on an old Tamron 172DE 70-300mm zoom I have lying around.  Will post again if I figure that one out.