Strange pressure plate on prewar Contax back

[tonicito]

Hi! This is my first post on RFF after a long long time as a silent reader, so let me introduce myself first. My name is Toni and I enjoy taking pictures with older cameras (you might want to take a look at my blog and/or flickr photostream, links on my signature below).

I have been badly bitten by the Contax bug in the last year. Due to some happy coincidences and a little good luck, a Contax I is the only one missing at the moment. My IIa, II and IIIa see more or less regular action, and my III is in need of attention due to shutter capping issues.

Now to the topic of this thread. On the first rolls of film that I shot with my Contax II, I experienced frame overlapping. When advancing the film I could notice (one develops a "finger feeling" for such things!) the sprockets missing a sprocket hole and the rewind button not turning for a couple instants. At first I was a little misled, because I thought it had to do with the clutch on the take-up spool side needing cleaning and lubrication (as suggested here https://www.rangefinderforum.com/forums/showthread.php?t=65607). But something told me this was not my (only) problem.

I ran some tests with other films. And I noticed that the overlapping was much improved, or even not present at all, with black&white film and thinner emulsions in general. With color slides, on the other hand, film even came to a complete halt with every sprocket failing to transport film. I had a problem with excessive friction. This was my "aha!" moment. I tried running a film with the back of another Contax (a second II, currently dormant in need of shutter ribbon replacement) and, to my surprise, everything worked as it was supposed to do: no overlapping and film advance and rewind were smooth and not anymore challenging and hurting.

Then I took a close look at both film backs (sorry for the bad and noisy smartphone picture, I increased brightness so that the details can be seen).



On top (serial no. Gxxxxx), the one that caused frame overlapping on thicker emulsions. On the bottom (serial no. Axxxxx), the one that worked fine. The pressure plate on the G-series back is obviously different. Comparing with my Contax III and with pictures of Contax backs on the web, it seems the original Contax pressure plate is the one on the A-series back.

Well, I removed the pressure plate on the G-series back and bent its springs a little so that it exerted less pressure. It got better, but it still was too much pressure. Then I exchanged it with the Contax original and it worked perfectly. The non-original plate exerts noticeably more pressure than the original one, because it has a much bigger leaf spring in the center, between the four "legs". The original plate has no connection between the upper and lower "legs" and, thus, exerts less pressure.

Did anyone experience something similar? I had no idea that the pressure plate could be so critical in operation, it came as a surprise to me. Have you ever encountered a prewar Contax with a non-original pressure plate? Do you recognize the pressure plate on my G-series back? It is neither postwar Contax nor Kiev (at least not the same as in my Kiev 4).

I would be glad to hear your comments. Thank you all for coming along with me on my first thread! 🙂

Cheers,
Toni 🙂

 

[Kevcaster]

Hi Toni, welcome to the forum, I see you have a nice collection of Contax so far. I have a ll with the original pressure plate similar to the one fitted to your camera with the A serial number. I would go with your suspicion that the pressure plate is exerting too much force on the film gate, causing the film to display faulty spacing and stiff wind on. The size looks OK so you could take out the extra leaf spring and see what happens. The pressure plate in my model exerts very little pressure!
Good luck
Kevin

 

[Dwig]

The so called "pressure plates" should not actually be exerting any significant pressure on the film.

In a properly designed 35mm camera, there are two sets of guide rails, one pair abovbe the film gate and one pair below. Sometimes the outside "rails" are only short left and right pads rather than full length rails. The film fits between the two outer rails, riding only on the inner pair. The outer pair are slightly higher and support the pressure plate. This leave a channel for the film. The pressure plate's job is to keep the film flat and in its channel. Changes in the spring tension shouldn't have a significant impact on the drag on the film unless the outer rails are improperly machined leaving too narrow a channel for the film.

I'd suggest carefully checking the roller near the back's serial number to make sure it isn't binding. Also check the inner pair of film guide rails to make sure there is no evidence of corrosion on their surface that would increase friction and also narrow the film channel.

 

[Kevcaster]

Yes Dwig makes the right point here, and the 'film channel' is created by the Contax pressure plate.

 

[Dwig]

Another point:

The OP should very carefully examine the film that came out of the camera when the binding occurred looking for surface scratches on both the base side and the emulsion side. If there are significant fine scratches on either then their location can give a clue to the "evil culprit".

Base side scratches would indicate the possibility of an issue with the pressure plate or the guide roller near the serial number. Emulsion side scratches would indicate otherwise. If there are emulsion side scratches in the sprocket area then the film guide rails would be suspect. If they are over the image area then some other culprit is involved (e.g. edges of the film gate, ...).

 

[Peter de Waal]

I have a Kiev 4A and 4AM. The top pressure plate on your "G" Contax body looks like one of the Kiev varients. The rivets on the centre-line of the plate are the give-away. My 3x Contax II's and 2x Contax III's have them top and bottom in the centre (as per your "A" cover)and the plate springs are bridged near the ends of the plate where they attatch to the back cover.
On the Kiev pressure plate the spring stamping is different. Why that should matter I'm not sure. The Kiev plates lack the relief notches top and bottom where the film perf runs. Perhaps that's the issue?

 

[tonicito]

First of all, many thanks Kevcaster, Dwig and Peter de Waal for your replies! It is a thrill to be able to share my insights and struggles with such engaged and knowledgeable folks.

Now to the Contax issue. No, the roller on the offending camera back (G-series) is not binding, it rolls smoothly. Dwig's observation about the film channel is of course quite true, and I am going to follow this track and check:

1. if the offending pressure plate is, indeed, perfectly plane
2. if the outer and inner rails are smooth and free of rust, dents or any irregularities that cause friction on the film when too much pressure comes from the plate
3. carefully examine the faulty film(s) for scratches and/or marks

Kevcaster, there is no independent leaf spring behind the offending plate (I have to take a picture of its rear side): so to speak, the four "legs" are joined by a rectangular "body" that is, in turn, riveted to the middle of the plate. This more complex leaf structure is what provides much higher pressure than the original Contax pressure plate.

Peter, thanks for the tip on the Kiev 4A/4AM: of course the presence of Kiev spare parts on a Contax should not come as a surprise to anyone these days. I don't know how many Eastern parts are still inside my Contax II but, as long a it works properly (and so far it does!) it is no big deal to me.

About the remark on the relief notches, I recently found a short article from Peter Hennig about the extra roller that the Contax has on the body on the supply side (looking at the camera from behind, the left side. This roller is supposed to help proper position of the film when using reloadable cassettes on the supply side. Mr. Hennig makes the point, though, that modern daylight cassettes have a "mouth" which protrudes a little more than the original prewar Kodak ones. This additional roller, it is his point, exerts quite a bit of pressure on this protruding "mouth", which produces miserable film flatness, to the point that he even suggests removing the roller altogether, if no reloadable cassettes are to be used. Now, my observation was that thicker film emulsions where more prone to bind than thinner ones. In fact, on the ones where binding occurred, there was noticeably more friction in the cassette itself, all other things being equal. That made me think if that body roller in combination with the offending pressure plate might be the culprit, too. It is maybe the exact geometry of the film cassette what makes a difference as well. The ones that worked were definitely different: the kind one finds on rebadged Rollei-branded emulsions sold by Macodirect, for example.

I need to find a calm evening to check my Contax II issues again. I will keep you informed on my progress. But again, many thanks for pointing me in directions I would not for sure find out by myself.

Cheers,
Toni 🙂

 

[Peter de Waal]

Toni thanks for these observations.
That's an interesting problem. I used to spend inordinate amounts of time on film-feed issues when I worked on cine cameras, ARRI's etc. This should be a comparatively simple issue to sort out. The Contax II/III reminds me of an ARRI. They are deceptive cameras. They appear to be a modern production, but in reality they were all selectively-fitted hand made items.

I've never has a problem with film in my ones, either with Zeiss Ikon factory reloadables or modern consumer cartidges. Last year I lucked upon a late Contax II 'G' body locally in mint condition, works well too. It has the serial number duplicated around the edge of the rewind knob and focal distances in feet. Definitely not a recycled Kiev. I have a 1943 ZAMAC bodied 1.5-5cm Sonnar 'T' on it that came from a 1938 Contax II I found in the UK. Do you have a link for those comments from Peter Hennig regarding the film transport?

I will have another look at the film transports on my Contax's and do a few pictures for this post. Once I've fixed the fauly hard disk on my Mac!
Cheers,
Peter

 

[Highway 61]

As far as I can judge from the photos, the G-series back also got some bumps at some point and looks somehow distorted. It may have been in need of some fixing in the past. Someone might have permanently damaged the original pressure plate while trying to remove it for some reason (in general you need to remove that plate to fix a dent in the camera back casting), then tried to salvage the camera by fitting a (too large and too thick) late Kiev pressure plate.

 

[Grytpype]

Mr. Hennig makes the point, though, that modern daylight cassettes have a "mouth" which protrudes a little more than the original prewar Kodak ones. This additional roller, it is his point, exerts quite a bit of pressure on this protruding "mouth", which produces miserable film flatness

You can find this article in the Spring 2005 issue of the Zeiss Historica Journal viewable here. I'm not sure whether the presence of the roller causes a problem with film flatness, as the pressure plate is well inboard of the roller, and probably will hold the film flat unless there is an unusual amount of tension in the film, but I think it might possibly cause an increased risk of scratching if the slot of the cassette is not well formed. I can't say I have noticed anything.

The same problem (if it is a problem) exists in all the Kiev Contax clones I have checked, even up to the late '70s.