Film-Tech Forum ARCHIVE: CP500 buzzing.

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Posted by Eric Robinson (Member # 2966) on 09-02-2011, 12:40 AM:

I have a CP500 which is exhibiting some strange behavior.

Firstly - with nothing hooked up to it at all, no inputs no outputs, no cables (nothing at all) the processor produces a buzzing sound when nonsync 1 or 2 is selected. I hooked up my own amplified monitor to the Center output to monitor this.

Regular troubleshooting methods led me no where, so I swapped out every single circuit board with a set from another unit. The problem did not go away.

At this point, I'm thinking it might be a bad backplane, power supply, or somehow the front panel is knakering it up.

Any ideas?

As a side note, when dolby SR (format 4) is selected, the dialog comes out of the surrounds and slight crackling sounds from the center speaker. In Dolby Digital, the sound is fine except if you have something plugged into the nonsync input or external 6 channel input - then you get a 60hz buzz on the center channel.

This thing is whacky.

Posted by Ken Lackner (Member # 1002) on 09-04-2011, 07:52 PM:

quote: Eric Robinson
dolby SR (format 4)

A-Type is Format 04; SR is Format 05. [Smile]

quote: Eric Robinson
In Dolby Digital, the sound is fine except if you have something plugged into the nonsync input or external 6 channel input - then you get a 60hz buzz on the center channel.

What would that "something" be? A simple ground loop isolator from Radio Shack may eliminate that hum.

Posted by Eric Robinson (Member # 2966) on 09-04-2011, 08:42 PM:

Yes, thanks for the correction:
04 Dolby Stereo A-type
05 Dolby Stereo SR

I could use ground loop isolation; however, I am more interested in getting to the root of the problem with the processor.

Posted by Mark Gulbrandsen (Member # 72) on 09-05-2011, 09:06 AM:

With Dolby ALWAYS suspect the power supply first! Have you measured the DC rail voltages yet to confirm if any are off? It uses a switching type supply and sometimes when they fail or are failing the small ferrite transformer in the supply can produce loud noises...

Posted by Louis Bornwasser (Member # 3063) on 09-05-2011, 05:03 PM:

You DID connect the short jumper between ground and one of the output LOW terminals, didn't you? Won't work if you don't. Louis

Posted by Pete Naples (Member # 761) on 09-06-2011, 02:46 AM:

Louis! I thought it was only me (us as a company) that ever did that?!

CP500 was notorious for buzzing, we always installed them with a; an extra earth attached to the chassis, b; a link between on of the output - terminals (usually C-) and chassis. That at least gave you a quiet system in terms of processor and amps.

Now what happens with the inputs is another matter entirely, CP500 was irksome in that respect.

Eric, what you're describing about 05 SR, check A chain alignment and also cell wiring. Sound to me like something is throwing the decoder out of kilter, and as you've swapped all the cards that doesn't leave much.

Posted by Eric Robinson (Member # 2966) on 09-06-2011, 02:02 PM:

quote: Mark Gulbrandsen
Have you measured the DC rail voltages

No, but great idea. Ill check them with my scope to make sure they dont have any ripple. I'm assuming the ripple spec should be pretty tight?

quote: Louis Bornwasser
You DID connect the short jumper between ground and one of the output LOW terminals

That is too funny, because I used to do this as a hack at one of our old theatres that had buzzing CP45's. I could do this, but it's putting a band aid on an otherwise serious problem.

quote: Pete Naples
check A chain alignment

Swapping out all the boards may invalidate this suggestion.

quote: Pete Naples
cell wiring

Not an issue since it buzzes with nothing whatsoever hooked up to it, including optical reader.

I'm going to bring this unit over to another theatre whee we have a CP500 and I'll swap out the supply, backplane, and front panel. Somethings got to give.

I tried doing a repair exchange on the whole CP500, but I guess Dolby didn't have any to spare. So, I got to figure it out on site....Bad timing too because I'm swamped with a more important project.

Posted by Pete Naples (Member # 761) on 09-06-2011, 02:52 PM:

Yup, that little wire link works on CP45s too!

"Swapping out all the boards may invalidate this suggestion."

How so? If the A chain is out of whack, enough to upset the decoder, no amount of board or processor swapping is going to make it good! I'm referring to the dialog being in the surrounds here, not the buzz.

Speaking of the buzz, I wonder if you're over complicating this? Perhaps your monitor amplifier that you're using for testing is to blame? Maybe look at the outputs on a 'scope with nothing else connected?

How about mains earth? A colleague had a situation like this year ago, turns out that the mains earth had come adrift in the distribution board, so it was bright and tight in the rack, but floating in the DB and not doing any earthign whatsoever.

You're right though, they are whacky processors. They sound OK when they are working, but can be a real PITA when they fall over.

Posted by Eric Robinson (Member # 2966) on 09-06-2011, 06:06 PM:

quote: Pete Naples
How so? If the A chain is out of whack

Sorry, I was thinking of a CP55 where the A Chain is on the cards. Oooops

quote: Pete Naples
Perhaps your monitor amplifier that you're using for testing is to blame?

Yes...this was my thought too. I disconnected it and hooked up another monitor type of device instead, problem still present. I have had booth monitors cause buzzing though in the past.

quote: Pete Naples
How about mains earth?

I plugged the processor into an extension cord and plugged it into various outlets in the booth, so I think this may be eliminated.

I guess I'll find out when I take the processor to another location. Thanks for all your inputs, they are all definitely areas to consider.

UPDATE
So, here's an update and a closure to the dilemma:
I swapped out the following components:
Power supply
Motherboard
Front Panel
Backplane
All of the internal card cage circuit boards Still Buzzing Now, I know I am insane.
I tried the little jumper from frame ground to one of the output grounds
- killed the buzz.
I don't have any more time or energy to investigate further.
Thanks for all the helpful advice!