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Topic: intermittent hum on optical mono

Marcel Birgelen
Film God

Posts: 3357
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012

posted 05-09-2019 07:42 PM

quote: Anhtu Vu
When I run the projector with NO film, I hear noises...not necessarily low cycle hum but noises from the 35 mm running. Sounds like the noisy acoustic in the booth is getting picked up.

That doesn't really make any sense to me, since it's an optical scanner. How would the sound of the booth end up in your sound processor other than via an actual microphone?

I think that some light leaking into the cell, some form of interference and/or badly grounded equipment are more likely culprits.

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Randy Stankey
Film God

Posts: 6539
From: Erie, Pennsylvania
Registered: Jun 99

posted 05-09-2019 07:57 PM

You have tried operating the projector and sound with no film.

Now try operating the sound with no film and the projector motor OFF.

I had a similar problem, years ago, in a theater I serviced in North Carolina.

If the sound is clean with the projector motor turned off you might have a case where vibrations are traveling through the frame and gear train of the projector, causing interference in the sound due to vibration of the exciter and/or cell.

I spent a whole night going through a projector with such a problem until I gave up and called my boss at 5:00 a.m.

He answered quickly... Change the drive belt.

I was like, "Wha?..."

He said, "Change the damned belt and call me back."

Sure enough, the projector belt had frayed and the rough edges were rubbing against the pulley flanges, causing vibration that propagated through the projector frame and appeared as noise in the sound system.

Changing the belt solved the problem instantly.

I called the boss back and he said, "See! I told ya'!"

The name of the phenomenon, I believe, is "Microphonics."

i.e. Anything that causes vibration in the projector frame or gear train can, in turn, cause the sound reproducer parts to vibrate, causing distortion or noise in the sound output.

I don't know if it's true in your case but it's a simple test.
Worth a try. [Smile]

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Marcel Birgelen
Film God

Posts: 3357
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012

posted 05-10-2019 05:58 AM

You know Randy, your story even seems to make sense. [Wink]

I can get how it could pickup some frequencies that way, although, I doubt it will ever work as a microphone, without a very dedicated setup to achieve exactly that.

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 05-10-2019 09:55 AM

Depending on the make of soundhead you might try running a bonding ground wire from the earth ground of the porocessor to the cell bracket Also if this a solar cell and not a reverse scan with LED change the excitor lamp as the filament maybe vibrating a bit inducing modulation into the cell

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Sam D. Chavez
Film God

Posts: 2153
From: Martinez, CA USA
Registered: Aug 2003

posted 05-10-2019 11:07 AM

I don't know if the soundhead model has been identified but Cinemeccanica V5 had a tendency to shake the solar cell/LED bracket while running due to the bracket being cantilevered around the large sound drum. Makes an odd LF ringing sound.

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Helmut Maripuu
Film Handler

Posts: 25
From: Västerås Västmanland Sweden
Registered: Mar 2004

posted 05-13-2019 01:14 PM

Do you turn off the new LED [Big Grin]

lamp you purchased for the projector boot?

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Anhtu Vu
Film Handler

Posts: 98
From: Montreal, Quebec, Canada
Registered: Jun 2013

posted 05-13-2019 01:59 PM

quote:
You have tried operating the projector and sound with no film.

Now try operating the sound with no film and the projector motor OFF.

I had a similar problem, years ago, in a theater I serviced in North Carolina.

If the sound is clean with the projector motor turned off you might have a case where vibrations are traveling through the frame and gear train of the projector, causing interference in the sound due to vibration of the exciter and/or cell.

I spent a whole night going through a projector with such a problem until I gave up and called my boss at 5:00 a.m.

He answered quickly... Change the drive belt.

I was like, "Wha?..."

He said, "Change the damned belt and call me back."

Sure enough, the projector belt had frayed and the rough edges were rubbing against the pulley flanges, causing vibration that propagated through the projector frame and appeared as noise in the sound system.

Changing the belt solved the problem instantly.

I called the boss back and he said, "See! I told ya'!"

The name of the phenomenon, I believe, is "Microphonics."

i.e. Anything that causes vibration in the projector frame or gear train can, in turn, cause the sound reproducer parts to vibrate, causing distortion or noise in the sound output.

I don't know if it's true in your case but it's a simple test.
Worth a try. [Smile]

Tested sound with no film and projector off. Dead silent.

I will definitely give your suggestion a try.

Thanks

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Randy Stankey
Film God

Posts: 6539
From: Erie, Pennsylvania
Registered: Jun 99

posted 05-13-2019 07:16 PM

It might be a bad belt but it's not necessarily so.

Microphonics can be caused by anything that causes vibration.

It could be a loose screw or bolt. It could be a random part that has come loose. It could even be some external device that's vibrating through the floor and propagating through the projector.

Do check the belt. It's the easiest thing to look for.
However, you should also check to be sure that everything that is supposed to be tight is properly locked down.

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Anhtu Vu
Film Handler

Posts: 98
From: Montreal, Quebec, Canada
Registered: Jun 2013

posted 05-17-2019 02:36 PM

The motor boating problem was fixed by changing the reverse scan assembly reader.

The hum was caused by lateral misalignment. Unfortunately, I still can't get it as good as projector 1 (to the point where the buzz track sound completely disappear) and as a result SR sound is not as focused (a bit smeared).

Maybe after years of wear and tear perfect alignment is no longer possible. Anyways, I would like to change the whole assembly...anyone knows where I can get this piece (pic) minus the reverse scan assembly reader ? It's from a Century sound head.

Thanks

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