Contax shutter curtain synchronization

Dez · 07-30-2012

The prewar Contaxes are truly splendid cameras, but the shutter mechanism is staggeringly complex, and very different from any other design. I have recently spent about 15 hours coaxing a cosmetically near-pristine Contax III back to life, something I would not normally take on, but for the facts that this camera is in beautiful condition, and is the only one that I have ever seen with a fully functioning and ACCURATE selenium light meter.

It needed a fair bit of work. The self-timer needed the ultrasonic, the long black lever that activates the shutter from the self timer had a broken-off end and needed to be replaced, the shutter tapes (of course) were broken, the gear trains were generally gummy, and the spring that held the escapement gear train in place was tired and didn't work consistently. Worst of all, the medium speed escapement did not engage properly, and the rocking pawl needed to be bent back into engagement with the toothed wheel. I just cannot imagine how that went wrong.

But one thing has so far defeated me, and that is the frustrating design of the shutter release mechanism.

The trailing curtain releases using a fairly normal sort of mechanism in which the shutter release button pushes on a disc which in turn pulls a pawl away from a wheel allowing it to spin. The leading curtain however is unique. A long lever gradually disengages a hook that holds the curtain up. These two mechanisms are initiated by the same movement of the shutter button, but are otherwise completely independent. There is an eccentric adjuster for the hook that holds the leading curtain, but there is no obvious interlock between the actions of the two curtains, as would be the case for any sane design.

If the shutter button is pushed quickly, there is no problem; both curtains release as they are supposed to, but it is the self-timer that is the real test, as it depresses the shutter button slo-o-o-o-o-o-w-w-w-ly. Within a tiny range of the adjuster, (which can only be accessed when the top casting is removed), the shutter goes from releasing the first curtain and then half a second later the second, to releasing the trailing curtain first so the shutter is already capped when the hook disengages. The hook engagement mechanism just does not look precise enough to make this timing consistent so the curtains release simultaneously. Yet I have old Contaxes that work consistently using the self timer. What am I missing here? Is there some strange hidden mechanism that links the action of the two curtains that I have just been unable to find? Advice would be greatly appreciated.

Cheers,
Dez

Dez · 08-02-2012

There are some good reasons why the very few remaining Contax mechanics can do a good business repairing these old beasts.

They are extremely complex and non-intuitive. The cameras, that is, although it may apply to the mechanics as well. The problem of coordinating the actions of leading and trailing curtains was resolved by getting the friction of the shutter tapes correct. The original material is unobtainable, and there is no really good substitute. I had been using substitute material that was too thin. Amazingly the shutter curtain coordination seems to depend entirely on the static friction of the shutter tapes on the slots in the leading curtain through which they pass. My thinner material had insufficient friction, allowing the shutter to cap if the trailing curtain was released slightly ahead of the leading one. The correct adjustment appears to be to release the trailing curtain an instant early. It will not move due to the tapes' stiction.

The two curtains move simultaneously, and when the leading curtain stops at the bottom of the aperture, the trailing curtain continues, completing the exposure due to the momentum of the shutter roller and the fact that the tapes are experiencing dynamic, rather than static friction. This sound ridiculously fiddly, but it works, and is consistent. The shutter closure is aided by the fact that the mid and slow-speed escapements are disengaged by the incredibly complex gear train when the trailing curtain gets close to the bottom of the aperture. That is if all the tiny springs in the shutter mechanism are pulling just the right amount.

The material I ended up using is 3mm Kiev tape, which is not as smooth as the original Contax material, and is ~0.33mm thick. I find that is does the trick if the slots in the trailing curtain are very slightly relieved with a tiny jeweler's file, not to open them up, but to bevel the sharp edges so the rougher surface does not hang up.

Why did I subject myself to MANY hours of frustrating work for a camera which had many things wrong with it, and is not particularly valuable even when fixed, other than the obvious point that I was too foolish to give up when I should have? This is the best Contax III I have ever encountered. Cosmetically it is near perfect, and the meter not only works, but is accurate over its entire range, the only time I have ever seen this.

There is a moral to the story. Never undertake a repair of a difficult and complex mechanism until you are completely certain that you completely understand its operation, or be prepared to spend many frustrating hours while you learn the hard way.

Cheers,
Dez

Highway 61 · 08-05-2012

Quote:

Originally Posted by Dez

The material I ended up using is 3mm Kiev tape, which is not as smooth as the original Contax material, and is ~0.33mm thick. I find that is does the trick if the slots in the trailing curtain are very slightly relieved with a tiny jeweler's file, not to open them up, but to bevel the sharp edges so the rougher surface does not hang up.

Very nice result, and very interesting tip about filing the trailing curtain slots rather than opening them up.

But according to the photos, I see some black tape - and the Kiev tape ain't black...

Dez · 08-05-2012

My first attempt at fixing this used thinner Japanese material, which is black. I wasted my time with the tape, but did get to the point of overhauling the shutter crate assembly and taking a few pictures for my notes for next time I am foolhardy enough to dig into one of these cameras. There just wasn't enough static friction to make the camera work reliably, so I re-replaced the tape with the Kiev stuff. There seem to be a few versions of this stuff with different widths and thicknesses, but always white to the best of my knowledge.

Cheers,
Dez

KoNickon · 08-05-2012

Fascinating writeup. I've long wondered just how the ZI engineers came up with that shutter design.

My III was overhauled by Oleg Khalyavin not long before I bought it from someone else. While the shutter release is not perfect at all speeds (probably a linkage issue), the shutter speeds themselves seem accurate. My guess is Oleg used the Kieve tape, and probably made that modification to the trailing curtain that you describe. My hat's off to you for even attempting this work.