Contax IIa/IIIa shutter tensioning?

[Grytpype]

A question for someone who knows their Contax IIa/IIIa shutters inside out!

I'm re-assembling a Contax IIIa after a full strip-down and clean. I have had the curtains out of the curtain frame: the ribbons on the 2nd curtain were unequally tensioned, so I tightened one by 1/3 turn and slackened one by 1/3 turn to make them roughly equal. The instructions in the manual for "pre-loading" the curtain springs on assembly make absolutely no sense as far as I can see (maybe they have lost something in the translation!), so I refitted the curtains using photos I took during disassembly as a guide.

As I tighten the adjuster, the 2nd curtain springs start tensioning first, which is correct. With the 2nd curtain disengaged from its channel in the leading edge of the 1st curtain and the adjuster slackened completely, if I tighten the adjuster until the slack in the 2nd curtain is JUST taken up and its springs are starting to tension, it then takes another 10 clicks of the adjuster (just under 1/2 turn) to take up the slack in the 1st curtain. I pulled the shutter out of a IIa (sticky shutter - future project) for comparison, and the equivalent figure was 12 clicks, so I think I have got the re-assembly about right.

From what I have read, I had the impression that it should not take many clicks of the adjuster to get some sort of result at the fast speeds. I took my zero position as the point where the first curtain spring started tensioning. Even at this setting I could actually see some daylight through the shutter when fired at 1/250. I could see nothing at 1/500 until I increased tension to 4 clicks and didn't see light right across the frame at this speed until I got to 7 clicks. At 10 clicks I can still see nothing at all at 1/1250, and I'm afraid to go further in case I am over-tightening the springs.

Starting from the zero position I've described, can anyone say how many clicks it should take to get a result at 1/1250?

Steve.

 

[Highway 61]

Starting from the zero position I've described, can anyone say how many clicks it should take to get a result at 1/1250?

Steve.

This is not a matter of clicks number but rather of how all the shutter components and in particular the curtains shafts and beveled geartrains have been machined, assembled, and how they have hold over time. Also, the black ribbons and the brown cords condition and lengths are of some great importance too.

Some corrosion inside the sealed shutter curtains shafts tubes or on the beveled brass geartrains may cause the shutter to always malfunction at 1/1250 or even 1/500. There is a lot of friction involved there because of the shutter design itself (vertical curtains shutter driven by some horizontal winding and shutter release mechanism, all in all within some very narrow space and no demultiplication enough).

When tensioning the shutter, the goal is to have it work well at 1/25 (I mean, 1/25, not 1/250) with the least noise possible.

On the Contax IIa, if all the cams, rotating disks, shutter geartrains and shafts which are on the horizontal board above the shutter are well positioned, clean, and properly lubed, the shutter must work correctly at all speeds over 1/25 once you get a correct 1/25 by tensioning the shutter with a moderate amount.

If you have to overtension the shutter to have 1/250, 1/500 and 1/1250 seem to work (as a result the shutter will quack like a duck at 1/25), this is not encouraging.

Most of those shutters suffer from very hard to fix problems at high speeds, and only a total teardown of all the components mentioned above, an ultrasonic cleaning, and a proper lubrication, might solve the problem (and when it's the case, the problem is very prone to come back after a while if the camera isn't put at regular use - hence the "shelf test" you will hear about...).

Good luck with yours.

 

[Grytpype]

Thanks for the advice. The mechanism in the top chassis I stripped completely, and lubricated as seemed appropriate with watch oil. The part of the curtain assembly I haven't stripped is the upper spindle, because I could not get the tiny pin out, and decided to quit before I destroyed the whole thing! The pins in the lower spindle were relatively easy. I didn't take out the axle from the tube, but flush cleaned it. It was very clean to start with.

The curtain assembly is a TERRIBLY fiddly and fragile thing to deal with - getting the knots at the top of the cords in position before fitting their retaining plates, and tapping the pins in and out of the ribbons' lower pulleys to adjust them! I seem to remember you saying in a previous thread that you had decided to avoid IIa and IIIa in future, and I have come to the same conclusion, certainly if stripping the curtain frame is involved!

You mentioned the condition and lengths of the cords and ribbons, and I think this could be relevant. The 2nd curtain, when it is un-cocked and in the channel at the top edge of the first curtain has some quite noticeable slack, certainly a lot more than the IIa. This was there before I took it apart. There seems to be no way of adjusting this apart from lengthening the cords on the 1st curtain so that its un-cocked position is a little lower down. I think this would increase the gap between the curtains, which might be a good thing. It looks like a very tricky job, even IF the cords are long enough to give some extra length. I think I may have to give it a try though, if I am to end up with a working camera after all the work.

Also the ribbons are a little frayed, though I don't know if they are bad enough to be a problem.

Steve.

 

[Grytpype]

I checked the shutter at 1/25 with the curtain tension set at my zero position: i.e. no tension at all on the 1st curtain when un-cocked. Amazingly it was perfect - just about 10% fast and with even exposure across the frame! Likewise at 1/50, just 10% fast. At 1/100 it produced an exposure of about 7mS with tapering, and at 1/250 just some exposure at the upper part of the frame. Anyway, nothing wrong with 1/25.

Nevertheless, something is wrong with this shutter. In the uncocked position the lower edge of the 2nd curtain is held at the bottom of the channel in the top edge of the first curtain, so it is not under tension and there is quite a bit of slack in the curtain itself (see picture below).

The picture below shows the IIIa shutter (upper image) and the IIa shutter (lower image), both in the uncocked position.

​

The 2nd curtain barrel is 'unwound' further on the IIIa - hence the slack, but I CAN produce a similar result on the IIa by rotating its 2nd curtain barrel further. I have come to the conclusion that the problem relates to the friction clutch in the upper spindle described in the manual as the "curtain brake".

The slack in the 2nd curtain was there before I dismantled this shutter. I marked all the gears carefully and re-assembled it exactly as found. It looks to me as though the "curtain brake" should engage just before the 2nd curtain reaches its uncocked position (which is not happening) and that it doesn't actually reach the stop between the bevel gears as I thought at first. I think I need to do some experimenting with the mesh of the bevel gears. Whether this will make a difference to the results at the fast speeds remains to be seen!

EDIT 8 hrs later: I may be wrong, but I think that what is supposed to happen is that, as the shutter is cocked, the friction clutch of the curtain brake is turned a little, putting it in position to stop the 2nd curtain when the shutter is released. This is not happening on this shutter. The rivets and stops of the curtain brake are in roughly the same position relative to each other as on the IIa shutter, but I cannot turn them to the same position relative to the curtain roller.

Possibly what I need to do is to take off the left-hand retaining plate for the 1st curtain cord and reposition the parts somehow. If I succeed in getting this right, I'm sure it will mess up my previous immaculate 1/25 result which was probably so good because the curtain brake was not functioning!
​

 

[Grytpype]

I think I've been chasing a red herring. I have found that I did have the shutter correctly assembled. It is very difficult to see the position of everything on the IIa I'm using for comparison, with the curtain frame still attached to the mechanism, but I have managed to confirm that it is assembled exactly the same. Probably the curtain brake on this IIIa is working - the reason this seems more effective on the IIa is that its shutter is sticky and slow. It is possible that the 1st curtain cords should be longer: the edge of the 1st curtain does not quite clear the bottom of the film gate when the shutter curtains are in their lowest position, but this too matches the IIa, so I may leave it as it is. I'll carry on and see what results I can get.