Film-Tech Forum ARCHIVE: CP45 making strange noise

This is topic CP45 making strange noise in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=009020

Posted by Francis Casey (Member # 514) on 12-09-2007, 11:29 AM:

Hi
One of our theatres is having a problem with their CP45 making what sounds like pink noise in the surround channels. This CP45 has been modified to run in center surround mode. The problem occures randomly and the noise is only in the surrounds , and can even occure when film is not running. Powering down and then up again usually restores normal operation. I even had the projectionist swap power supplies which made no difference. Does anyone have any suggestions as to the cause and remedy for this.It almost sounds like somehow the internal pink noise generator is randomly sending its output to the surround channels. I will be going to the theatre(about an hour away) in the morning to try and resolve this, and since i have not come across this problem before i would appreciate any help you may be able to give.

regards
Francis

Posted by Gordon McLeod (Member # 33) on 12-09-2007, 06:01 PM:

Is this set up for mono or stereo surrounds?

Posted by Francis Casey (Member # 514) on 12-09-2007, 09:16 PM:

Hi Gordon
It is set up for stereo surrounds if that helps

Posted by Peter David Bruce (Member # 4447) on 12-10-2007, 03:56 AM:

Regardless of the cp45's mods, I have found that due to bad buildup of dust on the control board, the two buttons that we use to trigger pink noise can occasionally 'stick' through normal pressing of the buttons, causing it to go into pink noise.

Although this is accidental, I have not heard of it turning on and off with the pink noise by itself. Seems to me, once it goes to pink noise, you have to TELL it to revert back to normal by hand.

Not entirely sure of the reason for this one I am afraid.

Peter

Posted by Gordon McLeod (Member # 33) on 12-10-2007, 09:22 AM:

If it is in both surround channels then I would think it is the pinknoise generator
If in one then I would suspect that little daughter board as I have had them fail
I too have had the push buttons jam starting pinknoise but it will cycle if that is the case

Posted by Pete Naples (Member # 761) on 12-10-2007, 11:54 AM:

You guys must have some seriously sticky dust if it's jamming CP45 buttons!

Posted by Francis Casey (Member # 514) on 12-10-2007, 09:45 PM:

Hi Everyone
Thanks for your suggestions and help.
I was at the theatre today and discovered the cause af the problem. It was not related to the internal pink noise generator at all as i had first thought. The soundhead is a Balantyne VII with a Kelmar reverse scan reader and the BLACK Kelmar power supply. The LED had dimmed over time to the point where the left channel could not be raised to dolby level even at max pot setting and max LED power setting. This setup uses a microswitch on the changeover to provide maintained formats , either Non/Sync or 05 . The combination of maxed out gains on the left preamp and LED power supply,and the maintained contact, was causing occasional "pink noise like" noise to be generated in the surround channels. No noise was produced if the automation connector was disconnected, and the formats changed manually. I'm not sure why the noise was being produced only when the automation connector was connected. I reballanced the preamp levels to where both channels were just below "green" , and slightly lowered the LED drive level,and this eliminated the problem. I did not have a new LED with me but will replace it as soon as i can "borrow" one from another theatre.

thanks everyone
Francis

Posted by Mark Gulbrandsen (Member # 72) on 12-10-2007, 10:02 PM:

The CP-45 is a strange unit.... it making strange sounds is not strange at all...

Mark