Film-Tech Forum ARCHIVE: Eprad Mut dancer arm oscillation

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Posted by Ian Parfrey (Member # 5122) on 04-17-2009, 05:12 PM:

Hi Everyone.

My Eprad double mut I have here is doing some strange stuff and I'm hoping someone out there may be able to help.
The problem appears intermittant and random. Both feed and payouts work ok upon starting a show, even with 42" spools, but once going, the takeup dancer arm starts to oscillate for no apparant reason. When the oscillation starts, there is usually a loud "clunk" from the takeup gearmotor on acceleration and the ballast lamp lights brightly for a few seconds. The gearmotor then slows down, then accelerates and the cycle continues, each time the oscillations reducing in amplitude until it settles down and runs ok for about 2-3 minutes where the cycle start over. Payout is not affected and runs fine and dandy.

So, where do I start?
Ive cleaned the variac windings and wiper, tightened up the chain to the variac shaft, adjusted the cam so it activates just on the cam-rise and checked the ballast lamps- both of which are fine. I have found a bit of lateral play in the variac shaft, maybe this could be the cause?

What would cause the dancers to start oscillating? Would the "clunk" coming from the gearmotor indicate a stuffed gearmotor?

Im not sure if the electro-caps have been changed, or even if this could be responsible for this problem. Ill change them anyway.

This things starting to worry me.

Thanks for any advice anyone can give.
Cheers
Ian

Posted by Mark Gulbrandsen (Member # 72) on 04-17-2009, 06:33 PM:

quote: Ian Parfrey
So, where do I start?

Those General Electric gear motors don't last forever, not even 50 years and they are NLA. I've seen both the gears wear out and the comutators wear to the point that the brushes(or whats left of them) are riding in a groove. Your best bet is to install Digital Cinema... At least those now common 2K intermediate files will look good projected digitally... they look dismal when put on film.

Cheers!

Mark

Posted by Gordon McLeod (Member # 33) on 04-17-2009, 06:52 PM:

A local gear shop should be able to make new gears for them

Posted by Ian Parfrey (Member # 5122) on 04-18-2009, 05:03 AM:

So it's fair odds that its the motor itself and not the control gear? It's strange to watch it running fine for a few minutes then have it oscillate for no apparent reason.

Posted by Mark Gulbrandsen (Member # 72) on 04-18-2009, 05:51 AM:

Take the motor to a reputable motor shop and have them check it over... you have to start sosmewhere and the comutator is the biggest wear spot on older itels like this. Also check the brushes on the variac. Poor brush contact or a broken brush can cause it to oscillate like you are seeing... those are still avilable new if it is bad...

Mark

Posted by Gordon McLeod (Member # 33) on 04-18-2009, 12:17 PM:

Disconnect the load on the variac and connect an AC voltmeter and run the dancer up and down and there should be a very smooth voltage change if it has dropouts it is either dirt or a burnt area on the variac

Posted by Steve Guttag (Member # 268) on 04-18-2009, 12:22 PM:

Buy a Kinoton SPT5000K.

Posted by John Wilson (Member # 269) on 04-19-2009, 01:57 AM:

In my long ago past, I seem to recall the lamp inside coming loose (unscrewing) and giving the results you mention. Make sure it's tight in the socket.

Posted by Louis Bornwasser (Member # 3063) on 04-19-2009, 06:35 PM:

The problem is not the motor. Take apart the speed control unit. There are 2 issues:

1. lubricate (with silicon) the variac shaft after cleaning

2. the cam is very asymetrical. The steepness of the "on" is hard to operate compared to the "off." Cut a different angle with a side cutter, then readjust as you have already done. Of course, check the motor brushes, but you trouble is most likely in the speed unit. If in doubt, reverse them and see if problems move. Louis

Posted by Tony Ratcliff (Member # 1196) on 04-29-2009, 09:53 PM:

I also have Eprad Swords.
I had a very similar problem recently.
The problem turned out to the the cam shaft itself.
The shaft had worn down and had become too thin and started slipping when the cam activated, causing an out of time condition like it does when the cam it out of adjustment.
I replaced the whole cam and all is well.

Before I figured out what exactly the problem was, I would have to grab the reel when the arm hit its low point, stopping the reel long enough for it to equalize. But over time, even this didn't work and the arm would just randomly drop in the middle of the show.
The problem was really bad when re-winding. I could only rewind slowly at first, because the arm would not equalize and stay up.

I don't have any more spare cams, but Louis may have some.
Or he may be able to create some.