This is topic Center Channel Dropping Out ? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Glenn Conatser (Member # 970) on 04-20-2002, 12:39 AM:
Ok here goes....Not sure if this is a SDDS problem, amp problem or speaker problem. While playing SDDS the center chanel will drop out to nothing while all other channels play fine and stay in digital this seems to happen with every print ive put in this house. One thing ive noticed is the center channel amp clips really bad during loud passages. I have read that some QSC amps use a mute function when being overloaded and come back on when things settle down. This is why im thinking its an amp problem. Ive been in the house when the center channel drops out and ive heard no distortion from the speakers what so ever. So i dont thinks its the speaker. The only other thing is the SDDS unit but according to it everything is fine. Could it maybe be a board in the CP200.
Amp in question is a QSC 1400 with a Dolby CP200 and Sony DFP 2000.
If i wasnt clear or left any info out let me know.
Thanks
Posted by Darryl Spicer (Member # 711) on 04-20-2002, 01:57 AM:
Ok, first off you need to run the films for a while in regular analog mode. SR mode is fine if equiped. If you experience the same problem in this mode then the problem is not your digital units.one way to know if it is an amp problem is if possable switch the inputs and outputs with either the amp for the left channel or the right channel. basically you are using the amp that runs the center channel to run the right or left channel and using wiche ever one of those two amps to run the center channel.
If the sound still goes out in the center then the problem is not the amp. If the sound goes out in the left or right then it is the amp giving you the problem.
Posted by Paul G. Thompson (Member # 655) on 04-20-2002, 02:34 AM:
If your booth monitor is capable of monitoring the processor output as well as the power amp output, that should help you solve the source of the problem.
Posted by Don Sneed (Member # 955) on 04-20-2002, 04:42 AM:
I agree with Darryl, first check to see if the same happens in SR & SR-D, if not..."BINGO" it's the DFP-2000, I had at least (4) DFP-2000 lose the center channel last year alone, at (3) different theatres...I would look more at the DFP-2000....but one word of caution...if it is the DFP-2000 & you send it in for repairs...WHOOOOAAA WEEEEEEE exspect to pay $1,500.00 or more for the repairs, each one I had sent in was in the $1,500.00 to $2,800.00 for each DFP-2000 repairs...needless to say the theayre owner was totally shock on each account !!
Posted by Brad Miller (Member # 2) on 04-20-2002, 06:09 AM:
Darryl and Paul have already covered your troubleshooting here nicely, but I just wanted to stick my head in and say "thanks" for giving us all of the facts and specifics of your equipment setup right up front when you posted this message. It's very hard to figure out what's going on when the member asking the question doesn't tell us model numbers. 
Posted by Gordon McLeod (Member # 33) on 04-20-2002, 10:31 AM:
If the 1400 clips for a extended period then the muting will probably pull in and you will have silence. This is because the mute is controlled by part of the clip circuit
The possibilities are you have a bad speaker that is shorting the output of the amp or the amp and speaker are undersized for the room
Posted by Glenn Conatser (Member # 970) on 04-20-2002, 01:16 PM:
If a speaker is bad wouldnt it distort or give you some kinda sign that it was blown ? The speaker is a JBL 4675 with one of the large horns. These houses are biamped and the other house right next to it is identical in every way except the other house is runnig DTS and has none of these problems. Im leaning towards the amp or SDDS but im gonna check a few more things that you guys suggested and ill see what happens.
Thanks again for the help.
Posted by Gordon McLeod (Member # 33) on 04-20-2002, 04:09 PM:
Unless the SDDS is putting out some DC I doubt it is the culprit but the 1400's are tanks and usually something else is the culprit that causes them to fail
Also check the fan is working and the air intakes are not blocked with dust as poor heat transfer can quickly kill an amp
Posted by Don Sneed (Member # 955) on 04-20-2002, 07:41 PM:
Gordon, "You Are Correct Sir", I lost count on the theatres I have installed over the years, I would go back to the theatre just before the warranty runs out to do one final equipment check & notice the amps are running warmer than usual, I always find the cooling fan block with dust, I now check that first before preforming my PM call...
Posted by Michael Schaffer (Member # 1204) on 04-21-2002, 07:27 PM:
Perhaps the DFP is not faulty but badly aligned: to align the b-chain, you should never set the channelīs digital gain to above 0 dB even if the option is given but rather turn up the gain control on the amp to reach the typical 85 dBc SPL. Failing to do so could make the DFP clip hard in the digital domain sending DC signals to the amp which might be the source of the problem.
MS