Problems with a Bronica SQa spare back

Has anyone had problems with a film back on a Bronica SQa. The one I had with my camera works fine, but I recently bought a very good condition spare so I could swap over B&W and colour whenever I needed.

The spare back has a problem with frame spacing when using colour film. (Kodak Gold 200) It loaded easily and I took all 12 frames then processed it only to find that I had only eight and a half images. The start of the film had a space almost the equivalent of a full frame which was blank. The next frame had a quite wide gap between the first and the 2nd on and the 3rd frame had a slightly bigger gap between it and the 2nd frame with each following frame the gap increased so that between the 8th and the 9th frame the gap was large enough for a complete frame.
The film counter worked normally and the only difference I found was the wind on was a little bit stiffer than for a B&W film.

I put an out of date B&W film through the next day to see if I could replicate the problem, but after processing that film it was apparent the spacing was correct with all 12 frames being equally placed right along the full length.

The film back I bought that gave the problem appears to have had very little use and is unmarked, The dark-slide has no marks where it would show that there had been repeated insertions and withdrawals

I hung both films up and photographed them to show you the difference. (Take no notice of the dark line evident through the complete film, that is just the background behind the film.) The density of the colour negs are not in question either, because they all print very well. Your thoughts please

The 1st frame on each film was the one on the right hand edge

SQ-A owner here .... in general, backs are the Achilles heel of the system, but (so far) my problems have been film winding through in one shot, apparently a consequence of the interlock pins misbehaving, and light seal issues. It's difficult to visualize why color vs B&W would matter, but I suppose backing paper could be different in thickness and provoke problems. Some backing papers these days have a more polished, "plasticky" surface that might affect something. Although as I understand it (always suspect) the film travel is sensed by a fair sized roller that the film wraps around rather thoroughly thanks to the U-bends to place the emulsion side out. I'd think it unlikely roller slippage is involved.

Internally there is a magic disk with slits around the perimeter among the mechanics in the winding system, and there seems to be some sort of slip clutch action between the crank and the actual film wind spindle. I could maybe see misbehavior there causing overlap, although excess travel seems less likely. It's probably something a CLA of the back could fix, but I've no experience in doing it or having it done -- yet!

Alas, some of the scary uncertainties of using 40 year old gear.

I doubt that it has anything to do with color vs B&W film. I believe the back counts frames only as the roller is turning (since it doesn't count unless you put film/paper in). My guess would be that from lack of use, either the roller or other piece like the clutch was sticking, so you had to wind more for it to register a frame. And possibly by the time you used the second roll, it had seen enough action to run properly. It's just a guess though.

For testing backs and checking frame spacing on 120 cameras, I saved backing paper from 120 films and test wind it through without film. On a roll of paper, I labeled the black side of the paper with the frame numbers 1-12 with a magic marker. So by winding this through and looking through the open shutter at these numbers as I take test exposures, I can see if it's approximately in the right place.

This will usually start to fall behind as I get to the end of the roll, because it's only the paper and not paper+film, so the wind spool diameter is thinner and it doesn't take up quite as much per turn. (Two thicknesses of paper might be about right.) However, it gives an approximate test of whether the camera+back are advancing properly.

The backing paper on the Kodak Gold film was similar as you suggest a shiny surfaced plasticised ribbon and is certainly thicker. I wonder if that has something to do with the problem? It is certainly more robust than the paper. The back when I bought it was still in its box and in a sealed bag with the original paperwork. I doubt if it has ever been used although it was sold as 2nd hand it is virtually brand new..

I have always understood the wind on and frame measurement was controlled by the series of gears connected to the gear wheel on the edge of the film back and the corresponding gear on the edge of the camera body.

What I did notice when winding the B&W film on the winding crank always returned to the same position on the body roughly at the 5 o' clock position when looked at directly. When the colour film was wound on the crank always moved forward by around 1/8th turn more. Also looking at the uncut colour negative strip, one clear edge is slightly narrower, but with the B&W both clear edges were equal width. This difference can be seen in the two images I posted.

I spoke to the dealer from whom I bought it and he had never heard of such a fault before. I have not had a chance to use the camera since then but will do so as soon as I can making sure that it is loaded the correct way and that there is no fault on my behalf

Have to admit reddesert's idea about perhaps the first roll through loosened up some gummed mechanics so the next roll worked also crossed my mind quite some time after my post. That could possibly be another explanation; e.g., had you started with the B&W then the color, the color might have been OK (and the B&W messed up). As I suggested, I don't consider it likely that the backing paper differences are significant for an SQx back. The latest shiny Kodak paper on 400TX even seems to work fine in my TLR which uses a knurled wheel on a spring loaded shaft to sense film motion.

Likely a CLA of the back would fix it, but I'm not sure where to look to get that done.

Whatever it was seems to have cleared up because I passed another roll of Kodak Gold through today and then processed it. 12 exposures which all will be printable and as they are on my heavyweight Manfrotto Tripod they should be as sharp as they get.

It was noticeable that the 2nd roll of colour the wind on was quite a bit smoother as well. The 1st roll also sounded rough and at the point where I passed the end of the film the wind on suddenly became easy.

If you look at the edges of the attachment of the color film you will see the edges which are clear are not equal width, whereas with the B&W one the edges are even width. The edges are the same on the 2nd colour film both are the same width. Somehow the film seems to have got miss-aligned when winding on.