Film-Tech Forum ARCHIVE: Sound issue help

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Posted by Chris Herman (Member # 5303) on 08-13-2009, 04:02 PM:

Hey there everybody, Im having a sound issue i was hoping somebody could give me some ideas for.

We have had an on and off buzz/hum in one of our auditoriums for quite some time now. Its high pitched and continuous, no modulation or anything of the sort. It comes from the center channel only, and didnt stop when we replaced the center channel amp. Sometimes turning the processor off and back on will fix it, sometimes not. It runs in non-sync as well as Dolby SR.

We have had a tech look at it multiple times, but as hes never chanced to be here when the sound was going, he hasnt been able to fix it.

The sound processor is a Dolby CP65.

Any ideas?

Thanks for your time everybody!

Posted by Demetris Thoupis (Member # 833) on 08-13-2009, 04:17 PM:

Do you have power running along your speaker lines? Are you using dimmers or HVAC? Try switching non booth related equipment on and off to see if you get the nuzz/hum you refer. on the odd occasion it could also be something in the CP. Try swapping the speaker outputs to see if the problem follows the speaker or stays in the centre channel.

Posted by Tony Bandiera Jr (Member # 2365) on 08-14-2009, 12:21 AM:

Demetris has some good points above, look at those things and here's something else to try:

Pull the center channel CAT64 and swap it with either the left or right one and see if the noise follows it. (Mark all three cards before you pull them so they can go back into the proper places.)

I have had a few CAT64's go bad with the issues you describe.

Another possibility can be a bad CAT150 card, if the problem only happens in any of the stereo optical modes. (Or also in Non-sync if you have the "NS2:4" mode enabled.)

Posted by Demetris Thoupis (Member # 833) on 08-14-2009, 12:49 AM:

Or God forbid it could be a dry join on a the backplane of the CP! Now that's just mean! [Eek!]

Posted by Andy Bajew (Member # 5330) on 08-14-2009, 01:44 AM:

Chris-

We had the exact same problem in one of our theaters. I researched to figure out every possible issue. I messed with everything from the optical reader to the hearing impaired transmitters. The issue was that one of the cases on the motherboard of the processor was faulty-a 500 dollar replacement processor and installation fixed the issue.

Posted by Jonathan Worthing (Member # 803) on 08-14-2009, 05:49 AM:

I agree swaping the Cat64, or even just removing & re-seating.

Some times if the unit has a knock the cards can move.

What type of booth monitor do you have.

One thing you could try is pulling the power to that.

I have had a very similar problem twice before the first time there was a crossover card not seated correctly in the booth monitor.

Posted by Gordon McLeod (Member # 33) on 08-14-2009, 11:04 AM:

Also if the problem isn't the cat 150 or 64 check the little dip relays that do the bypass
Also does your booth monitor allow you to monitor the processor output independently of the amp out
if so listen to it and see if it is on the processor outputs
Also if you are biamped it could be the electronic crossover

Posted by Tony Bandiera Jr (Member # 2365) on 08-16-2009, 12:37 AM:

Wish Chris would check back in and tell us what if anything he found.. [Roll Eyes]

Posted by Chris Herman (Member # 5303) on 08-17-2009, 06:00 PM:

Thanks for the replies everybody! Sorry its been so long, i havent been to work for a few days.

As of know theres no buzz, so i'll have have to wait until it comes back and see if any of the suggestions fix the problem. Hopefully it will just be something simple like reseating a card.

Thanks everyone!

Posted by Eric Robinson (Member # 2966) on 08-17-2009, 06:39 PM:

These are tricky to track down...especially when they disappear when your trying to find it.

For all its worth I had a very similar problem which came from Dolby EX surround adapters in two different theatres. On one of them I just reseated all of the cards and boards. The other one I did a repair exchange on the entire unit. No problems since.

Posted by Mark Gulbrandsen (Member # 72) on 08-18-2009, 09:26 PM:

What kind of projector is involved? Some of the older Christie P-35GPs had a poor ground connection on the reverse scam preamp board. Of course this goes back 15 or 17 years... It's best to start at the beginning weather it be a digital or analog front end.

Mark