Has anyone a suggestion on what to look for?

By the way, are you experiencing your problem on start-up? Or, are you experiencing this after start-up? How far into the reel are you when it happens? The clutch setting might be just too tight if it is happening sometime other than start-up. Restoring the loop on the fly is very easy to do, but it is still a pain in the rump to do it, especially if you snag your finger on a splice and it drags your finger(s) through the pad roller and sprocket. (Been there)

If you are running 6,000 foot reels, you have to be vary careful about your clutch settings as well as their cleanliness and over-all condition.

RCA had a real crappy clutch in some of their 3D magazine setups. They were not a conventional disk-leather-disc sandwich design. If they got oily or dirty, the projector would come up faster that the clutch could compensate for. This resulted in a little slack in the film. When the take up finally comes up to speed, it would jerk the loop away from the constant speed and holdback sprocket. Back in the old days with Acetate film, it would strip out the sprocket perfs, especially if the holdback sprocket is worn out. Those sprockets took a real beating and wear much faster than any other sprocket.

I'll almost bet your holdback sprocket teeth are worn to the nubs.

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I've just been running loops on this thing so far, this problem popped up as I was heading out the door.

The magazines are Simplex, same silvertone finish as the Simplex 35 & the sound head.

The holdback sprocket teeth seem great.

quote:

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By the way, are you experiencing your problem on start-up? Or, are you experiencing this after start-up? How far into the reel are you when it happens? The clutch setting might be just too tight if it is happening sometime other than start-up.

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Right at startup. Thread, hit the motor, bam, it snatches out the loop.

{ EDIT )
Just saw you guys' replies after sending this post. Thanks!

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I know this may be a dumb question but I will ask it anyway.

Are the takeup reels "easy start?" The center hub and the flanges are not tightly fastened so that when the machine starts and there is a rapid wind in the takeup, the hub turns freely from the heavy flanges. This 'inertia' in the flanges can cause the film to be abused in the sound head exit sprockets. Dirt coupling the flanges to the hub of an "easy start" may also cause this problem.

The solid reels are ok for the feed spindles but they are death in the takeup magazines.

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Just as a passing thought.....is there any binding of the take-up shaft? Take the belt off, put a reel on the spindle and give the shaft assembly a spin. It should spin freely. If not, time to dismantle, clean, and lubricate the shaft and the bore.

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I'll be back there on Friday, & will go through all the suggestions.

I really appreciate this, everything is going MUCH slower there than it should due to rust, age, disuse, deterioration, etc. Being able to picking up some speed on troubleshooting in that booth is excellent.

It is probably dirty stiff loose belts, and some coagulated oil in the spindle bores, as well as deteriorated clutches. All are very easy fixes, and will cost little money (if any at all) except for the cleaning materials and the labor involved. If you have leather belts that are shot, you might want to consider using "Roundthane" belts. I never had any trouble with them, although I still prefer the old leather ones.