Film-Tech Forum ARCHIVE: Popping / crackling / static

This is topic Popping / crackling / static in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Dan Chilton (Member # 2288) on 12-23-2007, 07:25 PM:

Tonight, all of a sudden, our sound system has decided to go haywire. When I started up our first movie, there was a strange, completely noticeable, continual, popping static. We run center/surround, and the static is audible in all channels. I was hoping it might be a scratched analog soundtrack, but when I started a completely different movie a few minutes ago, it was still there.

It's not the speakers, as pre-show music and previews, which run digitally at the same level as the movie, have zero distortion or static. It's audible in both the stereo and mono channels on the processor. I cleaned the solar cell and LED between movies, just in case, but that didn't affect the crackle.

Also: bypass dulls the crackle, but it's still there.

We're running on an Ultra Stereo JS-200. Any ideas? [Confused]

Posted by Randy Stankey (Member # 64) on 12-23-2007, 07:35 PM:

Between shows:

Take the film out of the projector, manually turn on the sound system as if you were going to run a movie and turn on the projector motor only.

Clean all your optics as per the method your supervisor taught you.

Make sure your exciter lamp is lit but the xenon is off. Press the STEREO button (or whichever button you're supposed to press) and run the projector without film.

Do you still hear the noise?

If yes, check your main projector drive belt for cracking, fraying and general wear & tear. If it is found to be defective, have your theater's designated responsible individual change the belt.

P.S.: This assumes that you have checked all the easy stuff first.

Film threaded properly?
No lose or broken wires?
No obviously broken or misaligned parts?
All switches, buttons and dials on the sound processor in their usual positions?
Etc, etc.

P.P.S.: Check all the belts while you're at it.

Posted by Dan Chilton (Member # 2288) on 12-23-2007, 08:25 PM:

Well, I just tried your suggestion Randy, and the crackling was definitely still there. I cut power to the processor, reset the cards (just in case), and cleaned the cell again. Fired the projector back up, and the crackle was still there.

Just then, our tech called me back and told me to unhook the grounds on the inputs into the back of the processor. I did, and lo and behold, the crackle disappeared. His explanation went a bit above my head... anyone know why that helped, and what would've caused the feedback in the first place?

Posted by Monte L Fullmer (Member # 2797) on 12-24-2007, 12:29 AM:

sounds like a good case of ground looping...too many grounds attached to one location from different scources...

Posted by Randy Stankey (Member # 64) on 12-24-2007, 11:46 AM:

So the sound was there whether the projector motor was running or not?

If you turned the sound system on, as if you were playing a movie, but left the projector motor off with no film in the sound head, the noise would still be heard?

Then I agree. A ground loop would be your likely problem.

Posted by Dan Chilton (Member # 2288) on 12-24-2007, 01:12 PM:

quote: Randy Stankey
If you turned the sound system on, as if you were playing a movie, but left the projector motor off with no film in the sound head, the noise would still be heard?

That's correct. There were two grounds attached at the same point in the back of the processor (one for the left, and one for the right channel). We'd never had that problem before. Weird.

Thanks for the help!

Posted by Gordon McLeod (Member # 33) on 12-24-2007, 02:32 PM:

I think you should have someone come out and determine what has changed as ground loops don't come and go with out some internvention

Posted by Monte L Fullmer (Member # 2797) on 12-25-2007, 02:14 PM:

quote: Dan Chilton
We're running on an Ultra Stereo JS-200. Any ideas?

Whoops, missed this one: I mentioned earlier of ground looping, but with this USL,the JFM-20 Format card (the unit with the six buttons on the front with the volume control) COULD be the culprit in this event.

If you have another JS-200 in the booth, swap the JFM-20 cards between processors and see if the buzzing follows.

Another thing is that the bypass button on the powersupply is another possibility for they're not the most strongest animal in that unit and do love to create some sound oddities during operation. They'll fail without even being used.

To check if that button switch is acting up, during film presentation, just touch in the button a few times and if you get cracking and popping from the sound, the button switch has gone to better places and need to be yanked out of there and two jumper wires can be replaced in place of the switch.

I've taken out that button switch completely on a few PS units to eliminate such sound irritations.

-Monte

Posted by James Westbrook (Member # 3690) on 12-25-2007, 03:58 PM:

When a repair/exchange is done with an Ultra-Stereo power supply, the replacements have the bypass button removed with a cover in it's place.
On one of his PM visits, our booth tech went through and removed the remaining power supply bypass switches on the processors that still had them.
As of this writing, we haven't lost one since.