Contax II/III: what are the symptoms of an overstressed shutter spring?

[Grytpype]

I've almost finished reassembling my first Contax II and am tensioning the shutter, but I am getting rather unsatisfactory results. All the speeds from 1/125 down are fine, and 1/250 is usable, but 1/500 and 1/1250 are slow and have considerable shading. At 1/1250 I get 1.8mS at the start of frame, 1.1mS at the centre, and 0.93mS at the end.

Starting tensioning from the point where the centre spring is untensioned and the ribbons are fully extended it seems to take about 2½-3 turns to the point where the ribbons are taken up on their pulleys and start to tension. From there I have tightened another five turns (suggested as a "maximum" by Henry Scherer), so there are five turns of tension on the ribbons and about eight on the centre spring.

Maizenberg's instructions for the Kiev suggest 8 - 10 turns, presumably from the same point, so I am at the bottom end of that range, but he says this should give a tension of about 160gm, and the Kiev Survival Site quotes the same figure (probably taking it from Maizenberg). My shutter, tensioned as above, gives just over 100gm.

Reducing the tension to four turns slows the travel and the times but doesn't affect the shading. Strangely, increasing the tension to six turns, does not noticeably seem to affect either the times or the curtain tension. I'm sure it would take rather a lot of turns to get to 160gm.

I wondered when I took the camera apart if it might have been overtightened; it got away from me when I was detensioning, so I don't know how many turns there were, but it went with a bit of a bang. The centre-spring, pictured below, looks a bit uneven, but no worse than a Kiev 4A I rebuilt, and that works very well!

Could a damaged spring cause my problems, and does this look like one?

Steve.
​

 

[Highway 61]

The spring looks fine. A damaged one would look like a terrible electric wire ball.

Too low 1/500 and 1/1250, plus shading at those speeds, might be due to too much friction between the ribbons and their clutches in the first curtain. What kind of ribbon did you use to service the shutter ?

 

[Grytpype]

Thanks for the reassurance! The ribbons had been replaced already, with a black material, obviously quite a long time ago. The width measures 2.9mm and the thickness, using a one-inch micrometer, measures .012" (0.3mm), compared with original ribbons I've measured of .010" and .011", so I think they are OK.

Would too much friction in the clutches affect the fast speeds, though, because don't the curtains run together at these speeds?

 

[Highway 61]

Check the first curtain release arm eccentric adjust screw too. If something is wrong with it, this might explain the capping.

 

[Grytpype]

I have already adjusted this slightly! The problems existed beforehand. I adjusted it to see if I could narrow the curtain gap to increase the speed. It reduced the start of frame from 2.1mS to 1.8 and the end from 1.2mS to .93, but of course it did nothing for the shading.

 

[Grytpype]

There is something I've just noticed that doesn't seem quite right. When I took the mechanism apart I marked all the gears, and I am pretty sure I put everything back as it was. All the escapement levers and cams were right at each speed according to Maizenberg's excellent description.

At the fast speeds both escapements are obviously disengaged, but there are some idler gears that still turn. I can just see these, even with the shutter installed, and if I slow the second curtain and watch the idlers, I find that they run until the curtain reaches the centre of frame, and then stop. The same happens on 1/1250 and 1/250. Is this right?

I'll pull the shutter out tomorrow and have another look.

Steve.

 

[Highway 61]

If the bottom roller spring is undertensioned, this causes shading, because the main spring doesn't pull the bottom shutter curtain quickly enough. Then if the bottom roller spring is overtensioned, this causes shading, because the side springs, driving the ribbons drums, pull the upper shutter curtain too quickly.

The shutter could be adjusted easily if there were some independent tensioning settings for either the main bottom roller spring or the side drums springs.

But this is not the case.

Be patient and you will find the good tensioning settings.

 

[Highway 61]

There is something I've just noticed that doesn't seem quite right. When I took the mechanism apart I marked all the gears, and I am pretty sure I put everything back as it was. All the escapement levers and cams were right at each speed according to Maizenberg's excellent description.

At the fast speeds both escapements are obviously disengaged, but there are some idler gears that still turn. I can just see these, even with the shutter installed, and if I slow the second curtain and watch the idlers, I find that they run until the curtain reaches the centre of frame, and then stop. The same happens on 1/1250 and 1/250. Is this right?

I'll pull the shutter out tomorrow and have another look.

Steve.

You may have desynchronized the upper shutter curtain drivetrain.

 

[Grytpype]

I can't find any problem with gearing or the escapements. The issue I noticed last night was a red-herring. I've checked a Kiev 3A and a 2A and the gears run for the first half of the frame, then stop (the large escapement gear gets to its cutaway teeth). You'd expect either they would run for the whole frame or not at all, but probably they don't cause any noticeable drag.

Changing tension only affects the speed; the ratio between start and end of frame stays the same. It would never be possible to get the curtains in this type of shutter to run at a steady speed, so there is always going to be some acceleration across the frame, but I had expected it to be better than this. This is rather a well-used camera. Could wear and tear be a factor?

Using the eccentric on the release lever, I've reduced the curtain gap at 1/1250 sec from about 1.5mm to about 1mm so the times are better, though not the shading. I now get 1.2 - .84 - .65mS across the frame. I think this is as good as I am going to get.

The Kiev 2A actually has a functioning shutter, though I don't know if it has been cleaned any time recently. Remarkably it is almost a carbon-copy, performance-wise, of this Contax! The curtain tension measures 105gm, and the timings at 1/1250 are 1.2 - 7.7 - 6.2mS. Also I found I had figures from another Contax II, that was working when I got it (but broke its ribbons before I could load film!). This had nearly as much shading, though slower times: 1.9 - 1.2 - 1.1mS.

Anyway, as you've said, Highway 61, there is nothing to adjust, so I think I will just have to live with it and finish assembly. I really do want to have a working Contax at last. I'd never be likely to use 1/1250, and 1/500 is almost usable. Thanks for your input.

Steve.