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Repairing a loose aperture coupling chain on a Pentax 6x7

On the older models of the Pentax 6x7 the coupling of the aperture ring on the lens to the metering prism is quite fragile. It is done by a thin chain which can be broken quite easily. It seems that taking off the Prism while the lens is mounted and then REATTACHING the prism with the lens still mounted on the camera can result in a broken, or at least loose chain.

After a horrendous astro-photo experience (car stuck in mud, telescope & camera slamming into the mount, weird encounters with strangers in the dark,...) I noticed, a small golden chain hanging out of my Pentax 6x7. I am not a great fan of jewellery in general, but this sight didn´t make me happy at all!


clicking on an image opens a window with a higher resolution image

For help I tuned in to http://photo.net and started this thread http://photo.net/bboard/q-and-a-fetch-msg?msg_id=0022pW in the Pentax 6x7 discussion forum.

A few weeks later Bill Peifer posted detailed descriptions for the repair:

I had this same problem with an old 6x7 I recently purchased, and I fixed it myself pretty easily. These chains are somewhat fragile and prone to break. Here's how to fix it yourself. You'll need a small dental pick or pen knife, a set of jeweler's screwdriver's, and a bit of light machine oil ("3-in-1" lubricant, gun oil, or sewing machine oil).

(1) Set the body on a clean work surface with the back down and the lens flange facing up. Lens and viewfinder should be removed.

(2) Using a small knife, dental pick, or other small tool, remove the four small pieces of leatherette directly around the outside corners of the lens flange.

(3) Remove the four screws holding the flange in place. ~Carefully~ lift off the flange, and note the number and location of any shimsor washers under the flange. (Check the underside of the flange, since these shims sometimes stick to the flange surface.) Shims, washers, etc, will have to be put back in their original positions when you reattach the flange.

  Shims on the backside of the flange

(4) You'll now see two thin strips of metal, each secured with two screws. (Screwheads may be obscured by a strip of foam lightseal.) These metal strips hold the aperture coupling ring in place. Remove the four screws, then remove the metal strips, then lift out the aperture coupling ring. You'll notice a small wire hook on the back side of the aperture coupling ring -- this is where the aperture linkage chain hooks on.

 

(5) Now you're ready to repair or replace the chain. If the chain "broke" because a split link at either end simply opened up too far, the fix is easy -- just close the split link back up. If the chain broke in the middle, you can try soldering the link, but you may just want to get a 4-cm. length of new chain from a jeweler or watch repair shop.

(6) Hook one end of the chain to the hook on the little slider in front of the ground glass viewing screen, then feed the chain down through the hole, then around the little plastic pulley. It may help if you push the slider all the way over to your left (i.e., toward the shutter release button side of the camera body). Once the chain is in place around the pulley, you should be able to pull the chain and see the slider near the ground glass move back and forth. There shouldn't be any binding while pulling the chain. Now slowly allow the chain back to its relaxed position, so there's no tension on the spring, and so the chain is hanging a bit out of the bottom hole and into the lens flange area.

   

(7) Take the aperture coupling ring in hand, drop a couple drops of oil around its back, front, and sides, and wipe a thin film of oil around it. Remove excess with a clean rag.

(8) Now attach the aperture coupling ring to the chain. Works best to pull a bit on the end of the chain, then loop the end link onto the little hook. Set the aperture ring down into its corresponding groove in the body, hook side facing down. At this point, you want to set the body down on the work surface with the film back facing down and the lens flange hole facing up. Turn the aperture coupling ring and ensure that the chain is moving freely and is still going around the pulley.

(9) It's possible to adjust the position of the pulley so that the aperture coupling ring position is precisely calibrated, in case you use a TTL prism finder. This is simple to do and is covered in the service manual. You can get a CD version of the service manual off eBay for USD $15, or a hardcopy for USD $40.

The reinstalled aperture coupling ring, chain wrapped aournd pulley'

(10) Now reinstall the little metal clips that secure the aperture coupling ring in position, then reinstall the lens flange. Remember that any washers or flanges must be reinstalled in their original positions.

(11) There's a specification for distance from lens flange face to film plane rails, also covered in the service manual. You can measure this with a dial gauge if you have one.

IIRC, the specification is 84.95 +/- 0.04 mm. But don't quote me on that. You could probably just as well install a lens, focus something like a star at critical focus on the viewing screen, then open the shutter curtains as you would for a time exposure, open the back, and check critical focus at the film plane with a knife edge or ground glass. You may find it less bother to just buy a dial guage, though.

  

(12) Clean the old glue off the leatherette and metal surfaces with a little alcohol, then reglue the leatherette pieces with contact cement.


Bill Peifer

© text: Bill Peifer
© photos Philipp Salzgeber

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