Pentax 6x7 metering prism / chain hanging loose

Discussion in 'Medium Format' started by philipp, Mar 11, 2002.

  1. I have an old Pentax 6x7 camera. Obviously I did something wrong with
    it (possibly taking the prism off while a lens was attached...).

    The metering prism stopped working and the chain which translates the
    aperture setting to the prism now is loose on one side.

    Since the repair shop is far away and expensive - can anybody give me
    a hint if there is an easy fix, possibly DIY?

    with best regards

    Philipp Salzgeber
  2. I did the same thing. The TTL prism must be removed or replaced only without a lens on. My chain also broke. Called several places about fixing it and Pentax factory service in Colorado was the cheapest. So I felt good about sending it there! I think it was $165 and about 3 weeks. Good luck!
  3. Maybe I'm just lucky, but on several occasions I took the metering prism off while the lens was still attached without any problems. However, you must take the lens off before re-attaching the prism, or the chain won't be in the correct position.

    If the chain is now broken, the repair shop is probably your best bet. If it just has a little slack on one side, try taking the lens off and re-attaching the prism before remounting the lens.

    BTW, I've had good experience with Pentax in Denver both times I've used them.
  4. There is no limitation I'm aware of on disengaging the metered
    prism with a lens mounted (or unmounted). I've done both often
    with no ill affect. If you do it and you don't unmount and then
    remount the lens it will not meter. I've heard of the chain breaking
    and it seems to me there is a cause, I just can't remember what
    it is.
  5. Hi Philipp,

    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 finder prism 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 shims or 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.

    (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.

    (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 84.96 +/- 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.

    Hope this helps.

    Bill Peifer
  6. To Bill Pfeifer:

    Thank you for your elaborate posting!

    My Pentax was still not repaired, so I gave it a try. With your instructions it was a (quite) painless effort...

    Luckily the chain was not broken, so all I had to do was to attach it to the hook on the aperture coupling ring. Afterwards I compared the metering prism with values from my Nikon FM2, and it seems it works quite good!

    Thank you very much! (the repair would have been quite expensive. A new shutter cost me about 700USD two years ago...)

    During the process I made some documentation images. Would you mind if I post your instructions together with some annotations and photos on my home-page? Of course your authorship will be clearly presented.

    Philipp Salzgeber
  7. Hi,
    My Chain is not broken (yet), but when i change the aperture I feel some kind of tension, and there is definetely something not working properly. Do you suggest I should try to follow this instruction:
    Or do you see any way around opening the body, (since the chain is not broken)
    Thanks, Kai
  8. i figured it out, I followed the instruction, it worked,
    Thanks a lot.
  9. Somebody put a scanned copy of the service manual on the web:

    In figure 4-105 on page 98 it lists the chain lengths as 49mm and 99mm (there are two chains, one on each side of the "cart" that moves). I originally used 85mm as specified in the post (before I found the manual) but that was definitely too short!

    Section 4-4-6 on page 71 shows how to align the chain. When the vertical face of the "cart" that the chain moves is in the center of the camera, the left side of the tab that sticks into the lens should be between 0.2 and 0.4mm right of center.

    That said, I gave up repairing my chain. I first had a jeweler repair my chain but it promptly broke in another location. Then I tried a silver chain from a jeweler and it broke when I was installing it. He said gold would be stronger but it would cost ~$90 which seemed like more than I wanted to pay.


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