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But just for kicks here is the passage taken from the Simplex 5-star soundhead manual:

"THE PROCEDURE for threading film through the soundhead is as follows: Bring the
film down from a correctly threaded projector. Retract the pressure roller and pass the film between the
pressure roller and the impedance drum. Close the pressure roller against the impedance drum, making
certain the film is between the lateral guide flanges. Pass the film to the right over the idler roller, approximately
halfway around the idler roller and to the left over the damper roller, half around the damper roller
and between the opened pad rollers and the film sprocket. Draw the film snug (damper arm pulled to its stop
on the pad roller arm); back off (1) perforation, and close the pad rollers. Closing the pad rollers allows the
damper arm additional rotation and allows the damper arm spring to control the film tension. From the film
sprocket, direct to film to the take-up guidance roller pattern."

My guess is that you have a platter misaligned. Or someone is being sloppy when they are building a print. Horizontal scratches (usually at a slight angle) are most often caused by film riding over the edge of a platter while it is spinning. Check the film return path and your makup tables to make sure that they are aligned properly

When you are lacing the film through the sound head, do not close the lateral guide roller as you lay the film on the drum. Instead, continue to thread but don't pull up the tension roller at all. Don't leave any slack either. Then, when you close the lateral guide roller, a small bit of tension will be applied. This should be enough so that when the film is up to speed and has stabilized, the tension arm will be off the bottom a little bit. At the other end, there will be a very slight belly in the film between the pay-out sprocket in the picture head and the lateral guide roller.

There are several reasons for all of this. We have found that there is quite a bit of variation in the spring tension on the arm, and also the alignment of the roller. This means that the film is being directed one way by one roller and another way by another roller. With more tension this problem is made worse. The real control over the film is coming from the sound drum and its flywheel. It doesn't need a lot of tension to track properly, and more tension from a misaligned roller only disturbs the film path.

If the scratches you are fighting are truly coming from the sound head, I would offer a couple of thoughts. Thread up a film as outined above and after it has settled down, note the belly in the film as mentioned. If it is more than just noticeable, there is a good chance that there is some drag in either the drum or the lateral guide roller. The roller is easy to check. Lift it free and give it a spin. If the trouble is with the drum there are two places to look. First would be the flywheel bearings and the second is a little obscure. We have found in a number of cases that there is so little clearance between the flywheel itself and the cover on the rear of the sound head that there may be some drag. It has also been caused by some of the wires going through that area.