8mm Forum: Elmo ST 800 Repair

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Posted by Greg Polutanovich (Member # 6653) on April 01, 2019, 09:43 PM:

Hello..my Elmo st 800 works good with picture and sound BUT when I rewind the film back on the original reel,it starts fine but towards the end it slows down and even stops before the entire film is on the reel and I have to do the rest by hand..Can anyone please help with this problem..Thanks!!

Posted by Steve Klare (Member # 12) on April 01, 2019, 10:00 PM:

Hi Greg,

My guess is your problem is happening because you aren't using the original Elmo reel as a takeup. (-because this is exactly what happened to me!)

These reels have an inner core more than 4" in diameter: much larger than most other reels. What this means is at a given film speed the takeup reel has to spin that much faster with the smaller-hubbed reel, which multiplies the drag in the film, which eventually overwhelms the clutch in the supply arm, which causes rewind to grind to a halt.

I couldn't rewind even a full 400 foot reel, but after I switched to the Elmo 800 foot reel on my ST-800s I never had this problem again.

Does your take-up reel look like this?

Posted by Greg Polutanovich (Member # 6653) on April 02, 2019, 02:13 PM:

Yesss..I used a smaller take up reel and I do have the original 800 reel and will use that next.I hope that solves the problem and thanks again for your help!!

Posted by Paul Adsett (Member # 25) on April 02, 2019, 03:05 PM:

Steve is exactly right. In mechanical terms rewinding the film is like a continuously variable transmission (CVT). When you first start the rewind the diameter of the film on the feed spool is small and the diameter of the film on the take up spool is really large, so the rewind motor is operating into a large mechanical advantage, which it loves because it only has to exert minimum torque. When you get to the end of the rewind the reverse is true, because now the feed spool is almost full of film (large diameter) and the take up spool is almost empty (small diameter) and the rewind motor is operating into a fractional mechanical advantage which mean the rewind torque is very high which the motor hates. That's why its always good practice on any projector to use bigger spools on the back than you have on the front, whenever possible.

Posted by Steve Klare (Member # 12) on April 02, 2019, 06:39 PM:

Yes,

You look at those reels and it's just common sense to ask "Why did they put that immense hub on these? That space is good for at least another hundred feet of film capacity."

-now we know!

This is more an ST-800 problem than for other machines. My ST-1200HD came with an 800 foot reel with a small hub: doesn't bother it at all.

Posted by Brian Fretwell (Member # 4302) on April 03, 2019, 02:18 AM:

Of course the capability to perform reverse projection means that the torque on the rewind spool has to be low to prevent film damage. Remember the press button on B&H 16m machines to engage a stronger pull for rewinding.

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