Film-Tech Forum ARCHIVE: Puzzling buzzing on 5Star

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Posted by Michael Schaffer (Member # 1204) on 08-08-2004, 11:42 PM:

I have 2 5Star soundheads which produce a low, regular buzzing. It is not electrical (it speeds up and slows down when you switch the motor on and off), and not the perforation or DTS timecode either. I buzztracked and checked the L/R-alignment, and it is on both channels equally anyway. It does not come from the exciter, it is running on DC, and the other other soundhead is equipped with a red LED. I thought about the sound drum bearings or the friction roller coating, but in my experience, wear on these parts makes itself noticed in wowing rather than this low buzzing.
What am I missing here?

Posted by Mike Babb (Member # 1379) on 08-09-2004, 12:31 AM:

Can you see it at 100hz or so on your analyzer? I had this and it came from stray light hitting the sprockets and landing on the cell, or at least that's the theory.

Posted by Michael Schaffer (Member # 1204) on 08-09-2004, 12:46 AM:

There is no real peak, but it seems to be lower than 100Hz. The buzz covers the audio spectrum broadly up to a little higher than 100Hz. It didn't occur to me when I was a the location to hold the analyzer in front of the monitor to see if I could pick up the frequency I heard most prominently.

Posted by Daryl C. W. O'Shea (Member # 1303) on 08-09-2004, 02:23 AM:

Did you see if it goes away with the xenon lamp shut off? It could be light 'from' the shutter.

Posted by Dominic Espinosa (Member # 2122) on 08-09-2004, 02:26 AM:

If it speeds up and slows down with the motor could it be possible that you're getting some kind of interference from the opperation of the motor itself? Perhaps a loose connection or a wire that comes very close to the motor?

Posted by Stephen Jones (Member # 16) on 08-09-2004, 03:10 AM:

I tend to agree with Dominic on this as the same thing happened to me with a Bauer U3 found that the lead from the cell was rubbing against the stabaliser.This occcured after a service as it wasnt present before.Checked connections found was loose,moved lead from where it was touching retightened and all was well.

Steve

Posted by Michael Schaffer (Member # 1204) on 08-09-2004, 08:51 AM:

No, the xenon lamp was off when I tested it. But it is a good thought. I will remember that when I come across a mystery problem that only occurs when the movie is playing, but not when I am testing it.
I will check the wiring. Though it does sound like something that enters the system by being read by the cell, because it somehow sounds "mechanic" (tak-tak-tak) rather than "electric" (bzzzz). It's not a sine wave, but a rapid buzzy pulse. Is that what your problem sounded like?

Posted by Mitchell Dvoskin (Member # 751) on 08-09-2004, 09:00 AM:

Take a business card or a small piece of paper and block all light to the cell while the film is running. If the buzz goes away, you know it is in some way a light problem (alignment or stray reflective light). If the buzz does not go away, you know it is an electrical problem.

/Mitchell

Posted by Darryl Spicer (Member # 711) on 08-09-2004, 09:03 AM:

check two things. One is do the belts have cracks in them. Secondly has the solar cell wireing come into contact with the belt drive causing the belt to rub up against it. I have had this happen do to the wire coming loose from a plastic wire clamp that is screwed into the frame allowing the wire to make contact with the belt and rubbing off the insulation.

Posted by Michael Schaffer (Member # 1204) on 08-09-2004, 09:03 AM:

I already did that (business card) to establish whether it really is on both channels. Unfortunately it is!

Posted by Gordon McLeod (Member # 33) on 08-09-2004, 11:41 AM:

Connect a scope to the preamp and look at the shape of the wave form
Is it there with the motor not running?
It is possible that it is microphonics of bearing noise on the sprocket shaft bearings

Posted by Brad Miller (Member # 2) on 08-09-2004, 12:28 PM:

Just turn the motor on without film in the soundhead (with exciter/led on) and listen while you turn it off and it coasts to a stop. That will let you know if it's a grounding issue with the motor or other mechanical issues. You can knock 50% of it out right there in 30 seconds.

Posted by Mike Babb (Member # 1379) on 08-09-2004, 07:57 PM:

That 100Hz thing is something I'd probably check with pink noise, or maybe Dolby tone on an analyzer. I don't know if you are seeing the problem I ran into but it seemed to be the stray light picking up the edge of the sprocket holes, 4 x 24 puts it right about there. I "solved" mine once with a new LED (SH1000)and once with taking a thin cardboard spacer/gasket out (RCA), but in both cases I think the LED was not lined up properly with the film. When at Dolby school I was talking it over with Dustin and that was the theory, he had also seen something exactly like that.

I have also heard the "machine noise" being amplified through a Kelmar reverse scan, on a particularly loud Christie. This one you can hear by putting the processor in film format, turning on the motor (without film) and listening on the monitor or in the house. You can see it on the scope too.

Posted by Randy Stankey (Member # 64) on 08-09-2004, 10:16 PM:

I vote for microphonics.

I once sat in front of a projector (Simplex 5 Star) for HOURS just watching it run and listening to the noise and trying to figure out what the cause was.

I finally found out that the drive belts were cracked and frayed. The little pieces of frayed belt were "flinging" themselves against the sides of the pulley sheaves and causing the noise.

Posted by Tony Bandiera Jr (Member # 2365) on 08-10-2004, 01:34 AM:

Check your preamp card out as well. Recently had one go bad and start oscillating at random.

Posted by Richard Fowler (Member # 893) on 08-10-2004, 02:20 PM:

Also check the film roller past the sound drum...if this has too much play it will pull the film out of alinement.

Posted by Josh Jones (Member # 447) on 08-12-2004, 01:47 AM:

Just a note on looking at buzz on an analizer. Shutter noise would show up at 48 Hz, Buzztrack issues would be at 96Hz and if it is a buzz, you are actually hearing a square wave, which produces lots of harmonics, so you will also see signal at 192, 288, 384, 480, and so on. the amplitude of the spikes should also decrease per octave.

Josh

Posted by Michael Schaffer (Member # 1204) on 08-12-2004, 02:00 AM:

A square wave contains only odd harmonics, so in that case, there should (theoretically) be only 96, 288, 480, 672Hz etc. But like I said, that`s not the problem here. I tried the tips about running the motor without film, isolating the reader from stray light, and checking the belt condition, but unfortunately, so far none of these has proven to be the problem [Frown]

But I will check everything again. Maybe it turns out to be something simple that I overlooked.

Posted by Daryl C. W. O'Shea (Member # 1303) on 08-12-2004, 02:45 AM:

Was the noise still there when you did all of the above? Are you sure it's even a problem with the soundhead?

Posted by Michael Schaffer (Member # 1204) on 08-12-2004, 03:21 AM:

Yes, it was, but only with film running. As the next step, I will check the participating modules in the sound processor (USL JS series), and the wiring. The sound is only very faint and I could only just hear it when I routinely checked the sound head alignment. Almost no one hears it - but that doesn't count, it has to be hunted down!

Posted by Dennis M Dow Jr (Member # 1970) on 08-14-2004, 02:51 PM:

Michael
If it the USL JS series see if the bypass button on the format card is pushed in.... I had a problem with low buzz when bypass was on... bad grounding done during installation