This is topic Overcoming Soundhead Inertia in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Mitchell Cope (Member # 32) on 06-24-1999, 07:13 AM:
I'm relatively new to film collecting and I have a projector that you're probably not familiar with, an A.V.E. 35mm Transportable. But the problem I'm experiencing may be common and I wanted advice. When I start up my films I sometimes notice that the loop grew in size, large enough to rub against the back wall of the projector. After fooling around with it for awhile, I figured that that the film is being stripped out of the takeup sprocket by the soundhead on startup. I don't know about most projectors, but I have a huge flywheel attached to my soundhead. Because of trying to overcome this large moment of inertia on startup at the soundhead, a lot of tension exists between it and the takeup sprockets. (Yes, I have the two arms that normally keep the film tension constant between the two points.) This tension is enough to strip film from the takeup sprockets. On polyester based films, the holes aren't torn, but you can see where the sprockets have worn a grove in the film, making it worst on the next startup. I'm sure this would tear sprockets on acetate based films.What should I do? I can only see two options. I could put finger pressure on the takeup sprocket clamp to ensure that it's as effective as possible, but this doesn't do anything about the film tension. The other option is to see if I could spin my soundhead before startup. I don't know how successful this would be since there is a lot of surface area in contact with the film and the drag/friction might prevent me from spinning the soundhead up, even for a short duration.
Is this a common projection problem? Any advice?
Posted by Joe Redifer (Member # 3) on 06-24-1999, 08:07 AM:
Where did you get this projector? Perhaps the flywheel is not matched to your specific soundhead? I really can't say as I am not familiar with that projector. Can you go around back and spin the flywheel manually? Try this at startup. This is a half-assed solution, obviously. Maybe Brad can think of something better?
Posted by Greg Mueller (Member # 17) on 06-24-1999, 08:31 AM:
Is the wrap around the sound drum suppose to be that tight? Usually you get some slippage when you start up and the sound drum/flywheel come up to speed slower than the rest of the machine, same with shut down. Could you maybe have the wrap to tight?
Posted by Brad Miller (Member # 2) on 06-24-1999, 01:31 PM:
I'm not familiar with your model projector, but let me see if I understand what you're talking about. Are you saying that the sound drum cannot get up to speed quick enough and there is a drag to it, causing the film to pull hard against it? Or is it something else? You mentioned a loop getting large enough to hit the projector wall, but I'm unsure of which loop you are referring to...the loop just after the intermittent? I have four immediate thoughts.
#1 If you were to spin the sound drum by hand, does it spin freely once it is up to speed and take a good 30 seconds at least to slow back down to a stop after the film runs out? If not, replace your bearings in the sound drum shaft immediately.
#2 If you are having film slippage resulting in larger/smaller loops, it is probably a pad roller not clamped down close enough to a sprocket. You should be able to place two layers of film between the sprocket and it's associated pad roller, but three layers should create a drag when you try and spin the pad roller by hand. Too much and you can scratch or sprocket damage a print. Too little and the film (usually at the splices) will jump the teeth of the sprocket and change your loops during a run.
#3 What happens if you thread with NO tension? If your sound drum bearings are in good condition, you "should" be able to run with only one sprocket of tension, or one sprocket past zero tension and the sound should not warble.
#4 Are you running reel to reel? How tight is the takeup reel pulling at startup? You may need to loosen it as it sounds like your sprocket damage is coming from there. Do your tests with a junk trailer and no takeup reel to take that aspect out of the equation, and then once you're satisfied the projector isn't doing any damage, repeat the tests with a takeup reel. Preferably the largest non-floating hub reel you have.
Hopefully someone else on the forum has worked directly with these machines before and can give you more detailed advice on things to check. If you can send a picture over, (print or email scan) I will post it and see if I or someone else can assist further.
Good luck.
Posted by Mitchell Cope (Member # 32) on 06-24-1999, 07:02 PM:
OK, I found a solution. My initial options of pre-spinning the soundhead or putting finger pressure on the takeup sprocket clamp did not work. What I did find to work quite well is to lift up the rubber pinch roller, start the projector, then put the pinch roller down after the soundhead is up to speed (less than 5 seconds).I guess with the pinch roller down, the tension on the film is directly related to the moment of inertia of my particular flywheel (5 inches in diameter, must weigh a couple or more pounds). With the roller up, the film can slip around the soundhead until the drum is up to speed. At that point, the roller can be brought down.
The bearings for the soundhead appear to be in great shape as it will turn for literally minutes after a reel.
Thanks for the help.
Posted by Ken Layton (Member # 133) on 06-25-1999, 01:17 AM:
Have you tried a "slow start" kit for the motor?
Posted by Mitchell Cope (Member # 32) on 06-25-1999, 05:35 PM:
I've not tried a "slow-start" kit. Tell me more. What specifically is it? Was it designed to solve this problem?
Posted by Ken Layton (Member # 133) on 06-25-1999, 05:48 PM:
A resistor inserted in series with the motor start winding will soften the start up torque. The old RCA 1040 soundheads had this resistor mounted in an electrical switchbox. The resistor was between 10 and 40 ohms at 70 watts with a slider adjustment on it.
Try one of those OHMITE "Brown Devils" wirewound jobs.