Film-Tech Forum ARCHIVE: What would cause a dowser to slowly slip down?

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Posted by Dan Chilton (Member # 2288) on 12-22-2005, 01:23 PM:

This happened for the first time last night: I was watching the film and suddenly I noticed the picture getting dim towards the bottom. At first I thought it was part of the movie (as it was a dark scene), but then it became evident after a scene change that something was amiss. I glanced around and couldn't find anything. It kept getting worse and finally I saw that the dowser was slipping down and cutting off the light. I manually closed it and opened it back up... problem solved.

What would cause a dowser to slowly creep down like that? It's controlled by a Simplex electric changeover which has worked like a champ. After reopening the shutter, I kept an eye on the changeover to see if I could notice any movement in the operating rod that would indicate a problem... nothing. The problem didn't reoccur, but this happened towards the end of our final movie, so I haven't had a lot of time to test it. Any suggestions on what may have caused this? Should I tighten the speed adjustment screw to be safe? [Confused]

Posted by Rick Raskin (Member # 1561) on 12-22-2005, 02:06 PM:

A little more information would be helpful. Is it a Simplex C/O or another brand such as Weaver or Zipper? Are you working with automation or doing changeovers? What type head are you using? The guys here are pretty darn good at nailing a trouble but the more they have to work with the better. My first impression though is see if it happens again. After all, "if it ain't broke, don't fix it".

Posted by Dan Chilton (Member # 2288) on 12-22-2005, 02:14 PM:

quote: Dan Chilton
It's controlled by a Simplex electric changeover

No automation... manual changeover. Simplex XL projector.

Posted by Scott Norwood (Member # 30) on 12-22-2005, 02:37 PM:

What type of changeover--Kelmar, Essanay (zipper), or Weaver? I've seen Essanays that said "Simplex" on them, but I've never heard of a Simplex-brand changeover (maybe someone else has).

Posted by Dan Chilton (Member # 2288) on 12-22-2005, 03:01 PM:

Scott, you're right: it's an Essanny SA5 (117 A.C.)

UPDATE: Yep, it's definitely a problem. Upon opening the dowser, it starts to slide down a fairly good clip. I'd say, if left unattended, it would go from being fully open to fully closed in a little less than 3 minutes. I have a trusty rubber band and pair of vice grips holding it open right now. Any suggestions?

[ 12-22-2005, 04:50 PM: Message edited by: Dan Chilton ]

Posted by Rick Raskin (Member # 1561) on 12-22-2005, 04:45 PM:

If I remember correctly, Zippers don't remain powered after the change-over is activated; e.g., there is no holding current. Therefore, I'd check to be sure the couplings are secure on the dowser itself. If allowed to vibrate, it might just cause the dowser creep you describe.

Posted by Sean Goodrich (Member # 475) on 12-22-2005, 04:50 PM:

Is there a screw on the top of the dowser? Try tightening that.
Sometimes if it's too loose the shaft bounces back when it opens.

Posted by Andrew McCrea (Member # 674) on 12-22-2005, 04:56 PM:

Just out of curiousity, what is a changeover? I always thought that it was the same thing as a douser...

Posted by Dan Chilton (Member # 2288) on 12-22-2005, 05:02 PM:

I was under the impression that a changeover was the mechanism that automatically dropped the dowser of one projector and lifted it on another during a "changeover." I could be wrong, but that's what it's referred to as in the Simplex parts catalog.

As for my malfunctioning changeover, I'll tighten the speed adjusting nut and see if that fixes anything. Wish me luck!

UPDATE: I tightened it a bit, but the dowser is still slipping. I didn't get a chance to tighten the lock, just the adjusting nut. I'm going to wait until the movie ends to fiddle around with it.

Posted by Rick Raskin (Member # 1561) on 12-22-2005, 06:07 PM:

quote: Dan Chilton
I was under the impression that a changeover was the mechanism that automatically dropped the dowser of one projector and lifted it on another during a "changeover."

You are correct. The mechanism is a 2-winding push-pull type solenoid where winding # 1 is used to open the dowser and winding # 2 to close it. When you press the changeover pedal at the incoming projector a current pulse is sent to winding #1 of the incoming machine and also to winding #2 of the outgoing machine. The incoming machine opens and outgoing machine closes.

Posted by Steve Scott (Member # 630) on 12-22-2005, 06:45 PM:

A dowser is anything that hides the light source. Many old-timers will refer to changeover devices, such as zippers or Kelmar units, as dowsers. In my vernacular, the only dowser is the hand dowser in the lamphouse.

Posted by Louis Bornwasser (Member # 3063) on 12-22-2005, 06:54 PM:

When we move, overhaul or install a projector we always change the changeover to one made by Kelmar. There are internal "stops" within the changeover to eliminate this kind of slippage problem.

The older (1940) designs are prone to burn out and misadjustment; the Kelmar is recent and is much less prone to trouble. (Strong makes a seemingly identical changeover; BUT internally certain adjustment are not made and certain clevis pins are not durable enough. Avoid.) Louis

Posted by Dan Chilton (Member # 2288) on 12-22-2005, 07:25 PM:

Looks like I may need a new changeover altogether. I tightened the speed adjusting screw, followed by the locknut, and... snap. The locknut sheered in half! I don't know if I should replace the screw and locknut ($10.00 + shipping) or just get a Kelmar. What will a Kelmar set me back?

Posted by Gordon McLeod (Member # 33) on 12-22-2005, 08:17 PM:

Get the Kelmar far less grief in the long run

Posted by Aaron Sisemore (Member # 145) on 12-22-2005, 09:01 PM:

quote: Andrew McCrea
Just out of curiousity, what is a changeover? I always thought that it was the same thing as a douser...

FWIW, changeover=electric dowser. One and the same.

There is also the 'hand dowser' that is the lever that cuts the light at the lanphouse snood.

-Aaron

Posted by Peter Mork (Member # 1336) on 12-22-2005, 11:34 PM:

There's supposed to be a ball bearing and tension spring beneath that adjustment screw. Too much tension and the solenoid won't be able to move the slide; too little and vibration can cause the slide to creep downward as you saw.

The ball bearing might be missing. I've found a rubber pencil eraser makes a good substitute.

[ 12-29-2005, 03:57 PM: Message edited by: Peter Mork ]

Posted by Robert Throop (Member # 61) on 12-23-2005, 02:36 AM:

Years ago I worked in a theatre with Zipper changeovers and the dowser suddenly started to creep closed after the house had been open for 5 years. Investigating I found that the hole had never been drilled in the brass tube for the ball bearing to ride on the armature. Essanay refused to do anything due to the time lapse. We replaced the coil and I threw the old one in my junk box. A couple of years ago, I dug it out, drilled a hole and used it to repair a burnt out coil. I have no idea why it took so long for that c.o. to start acting up.
Bob

Posted by Michael Schaffer (Member # 1204) on 12-23-2005, 05:07 AM:

quote: Aaron Sisemore
FWIW, changeover=electric dowser. One and the same.

Not necessarily. I worked with a pair of Ernemann VIIB back in Berlin which had dousers operated by a quicksilver pressure switch.
Well, OK, those were from the 30s, but I just wanted to mention that.

Posted by Mark Gulbrandsen (Member # 72) on 12-24-2005, 09:30 AM:

quote: Aaron Sisemore
FWIW, changeover=electric dowser. One and the same.

There is also the 'hand dowser' that is the lever that cuts the light at the lanphouse snood.

Aaron... Not completely the same. Many manufacturers offer electric dowsers on lamphouses and consoles so the dowser and changover are actually two completely different things and should be kept so.

Mark