Film-Tech Forum ARCHIVE: CP55 Hum

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Posted by Richard May (Member # 2627) on 02-26-2008, 07:23 PM:

I couldn't seem to find the answer with the search so here goes. I have a bad buzz in L,C,R,and Surrounds. All formats including non sync. I pulled all the inputs out of the amps and it goes away. Sounds like the processor. Problem is, the fader does not control the hum. Stays same level no matter what setting. Mute too. In bypass it stays the same also. Tried ground wires here and there. Diconnected the monitor leads. Turned off Reverse scan. Nothing. Also noticed that the Cat 249 regulator card is very hot. Any other ideas?

Posted by Adam Holland (Member # 4539) on 02-26-2008, 08:07 PM:

I don't know what type of system you have but have you tried jumping your ground. I have had a problem that sounds similar to what you have at several athletic stadiums I have worked at as a public address announcer, both Division I-A and High School. It happens when I plug my computer in then tie it to the sound system via an 1/8 to RCA cable.

When I take a simple three prong to two prong adapter that you can buy at your local hardware store and plug the amp into it then into the wall the hum goes away.

I also had the same problem at the FISKE with our current set up showing a movie. I did the same thing but this time used the adapter to plug the computer in and it worked as well.

Due to waiting to get a state grant for new 35mm equipment we are using a computer and an LCD projector at this time in our project.

Posted by Ken Lackner (Member # 1002) on 02-26-2008, 08:29 PM:

I have run across auditoriums that have had hums similar to what you describe. I tried the same troubleshooting steps with very similar results. I finally figured out what the problem was. The processor outputs were not wired correctly. Instead of individually sheilded pairs, all channels were wired with a single multi-conductor cable. What's even worse (and I could not believe this) was the negatives were not even connected. Removing these cables and running individually sheilded pairs for each channel corrected the hum.

Posted by Richard May (Member # 2627) on 02-27-2008, 07:12 AM:

All the processor inputs are individual cables. I did not try lifting the ground. I will do that next. I am wondering if there is a power supply issue. I thought I remember reading somewhere about power supply issues with the 55.

Posted by John Walsh (Member # 168) on 02-27-2008, 08:19 AM:

My first guess would be the power supply. I assuming this is a problem that just started happening, rather than a new or recent installation?

Posted by Richard May (Member # 2627) on 02-27-2008, 08:40 AM:

Hi John. This unit has been running at this theater for over 20 years now. I was thinking the power supply. I found it strange that the hum didn't change with the fader or mute.

Posted by Steve Guttag (Member # 268) on 02-27-2008, 09:50 AM:

Power supply, possibly, I would look to the Cat 249 regulator board first. It was not the best thing in the world and was replaced by the Cat 249A in the CP-65. I've had many Cat 249s go bad (and almost all overheat...pull it out and I'll wager you have a large blackened area on the board). The cat 249A also uses larger filter caps.

Steve

Posted by Richard May (Member # 2627) on 03-06-2008, 01:52 PM:

Very weird. Found an old 1/4" chassis mount female plug attached to front of console. It was used for the old FM style hearing impaired unit. It was hooked up to center channel out on processor. Disconnected it and entire hum went away. Thanks for all your replies.

Posted by Frank Angel (Member # 248) on 03-10-2008, 09:05 AM:

quote:
I have a bad buzz in L,C,R,and Surrounds.....Disconnected it and entire hum went away.

Very different sounds, usually with different possible causes. Which was it, actually?

Posted by Richard May (Member # 2627) on 03-11-2008, 10:57 AM:

Buzz like a bee...lol. Sorry Frank. You're right. Two toatally different things.