Film-Tech Forum ARCHIVE: IREM Rectifiers humming really badly

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Posted by Frank Angel (Member # 248) on 06-12-2009, 12:24 PM:

Not quite sure what's going on here, but when we set up the projection tower this year, we did everything exactly the same (or so we think) as we've done every year with the IREM rectifiers down at the bottom of the tower. While they always had what I considered a normal level of 60hz hum, this year we fired up the lamphouse (4500A) last night and the hum from these babies was probably 3 times louder than usual. I mean, we could hear it all over the field.

The lamps seem to be buring normally at the proper 160amps and voltage, nice bright light, no flicker. What is making me scratch my head is that BOTH units are humming loudly. The screws in the side panels are all tight -- anyway the hum doesn't seem to be coming from the metal on metal casing, it seems to be coming from the windings. Short of building a containment box around the rectifiers, any suggestions as to what could be the making these things hum so loudly? I am almost tempted to take a SPL reading a few feet away from them to see what I get and let you know. I know this can't be the normal operating noise because I would have balked about it years ago. This is new. And damn annoying.

Posted by Demetris Thoupis (Member # 833) on 06-12-2009, 01:20 PM:

The Irem rectifiers have adjustment screws inside you can tweeq to silence them and they do work! They are called Magnetic Shunt Adjustment. They are shunts between the toroidal coils or whatever you call them with two adjustment screws and one locking screw (middle one). You unscrew a bit the middle and then try screwing or unscrewing (jeez it sounds bad) the external screws until you can achieve the lowest of hum. Sometimes screwing them achieves a better result so don't hesitate to experiment. Don't worry about it. It can be fixed in no time.
Demetris

Posted by Tim Reed (Member # 137) on 06-12-2009, 01:23 PM:

quote: Demetris Thoupis
Sometimes screwing them achieves a better result

I know that works for me.

Posted by Marco Giustini (Member # 4544) on 06-12-2009, 02:01 PM:

pay GREAT attention when you put your iron screwdriver by the big coils while they're working. The magnetic field will attract the screwdriver toward the coils! Wear rubber gloves and guide the screwdriver with your hands while adjusting. You'd better find a plastic screwdriver, I heard that they can be found.

Marco

Posted by Robert E. Allen (Member # 1351) on 06-12-2009, 02:31 PM:

When I worked the booth at the local CineMark 15 we had one of the Christie units that hummed loudly when started. It eventually quieted down some but the tech said it was the windings on the transformer, said it was nothing to worry about and made no attempt to fix it. But it sure was annoying. I'm surprised the customers didn't hear it in the auditorium.

Posted by John Walsh (Member # 168) on 06-12-2009, 05:27 PM:

There is a written procedure for reducing the hum with IREM rectifiers; I'm pretty sure it's in the manuals section. But, note it was written for the free-standing units, not when they are mounted sideways in a console. I never felt the screwdriver was going to get pulled in to the windings. It does 'buzz' in your hand when you reach in, but follow the usual precautions for working on live equipment will and you'll be OK. When I was working on several IREMs, I finally went and got a brass screwdriver which isn't affected by a magnetic fields.

Note that some IREM just can't be made quiet. The hum will go down, but returns shortly.

Posted by Marco Giustini (Member # 4544) on 06-12-2009, 05:54 PM:

I don't know, in my case I had to keep hard the screwdriver from being attracted by the coils. Perhaps I'm wrong and the screwdrivers wouldn't have been anywhere, just vibrating...

Marco

Posted by Mark Gulbrandsen (Member # 72) on 06-12-2009, 08:51 PM:

Frank,

Just stand next to the Irem thats humming and along with it you hum out of phase and that'll cancel the hum completely... problem solved!

Actually, be careful tightening the screws and tighten them in 1/8 to 1/4 turns... Don't get carried away, tighten just enough to calm it down. You want the same tension on all three cores.

If I had a buck for each Irem I had to take apart and tighten I'd be two years closer to retirement! Not that far from it now actually...

Mark

Posted by Frank Angel (Member # 248) on 06-13-2009, 12:09 AM:

Thanks guys. I'll give it a shot.

And Mark, don't think it didn't cross my mind to put a subwoofer next to them and pump 60hz out of phase. And of course that would work with only 50% of the audience --the other 50% we'd have to give those Bose noice-cancelling headsets....and wash them after each show like the Dolby 3D glasses.

Posted by Louis Bornwasser (Member # 3063) on 06-13-2009, 08:36 AM:

Remember you are shifting the shunt(s). (Loosen one side to allow the other side to "push.") "Tuning" really is the right word. I have never failed to get a 99% decrease in volume. Then go back to the other two and repeat.

Finally be sure the shunt-adjusting-screw (knob) is not frozen as a result of misalignment of the 3 shunts. Louis