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
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.
(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 84.95 +/- 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.
