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Author Topic: Sound channels cut out randomly Connor Kirkwood Film Handler Posts: 25 From: Portland, OR, USA Registered: Jul 2009 posted 03-30-2010 11:15 PM                     We are running CP-50s through QSC amplifiers with ancient monitors in two of our theaters. We have an intermittent problem with the sound in one particular theater. The sound cuts at random intervals. It sounds as if the left channel cuts completely and volume on the center channel greatly diminishes. If left unattended, it will continue to cut in and out throughout the show. It has happened with multiple prints. We had this problem in the same theater about a year ago, but it subsided after about a month. Unfortunately, it does not happen regularly. My projectionist has run multiple prints after hours to try to isolate the problem, but so far is hasn't occurred during his screenings. I had a similar problem with the exact same sound tower in a booth that I worked in years ago in another city. Of course, the problem never presented itself when a tech was present, so the standard answer was that the amps were probably overheating. Which doesn't quite make sense if it's the same two channels doing the same thing each time. You all will say this is insane, but the problem can generally be fixed by turning the booth monitor on and off. This voodoo fix has been verified by three different employees. ?????????????????????  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 03-30-2010 11:23 PM                     Start by checking the fans in your QSC amplifiers. They may be seized up. I've had many go bad on the last year or so. If the fan seizes up the channel will play until it gets too hot then it shuts down until it cools down enough. Hence you can end up with channels going on and off. If the fans appear to be operating fine then calling in a qualified technician to look at the problem is definitely in order... as is an upgrade on the outdated processor! Mark  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 03-31-2010 12:01 AM                     quote: .....but the problem can generally be fixed by turning the booth monitor on and off. This voodoo fix has been verified by three different employees. What make and model of booth monitor? Important info in light of that statement. It is entirely possible that the circuitry of the monitor has a problem which is shorting out or loading down the affected channels. I have had a similar situation some years ago, but I cannot remember what make/model monitor was involved.  |  IP: Logged Jake Spell Master Film Handler Posts: 294 From: Johns Island SC Registered: May 2009 posted 03-31-2010 12:01 AM                     You are verifying that the sound IN the theater is cutting out, not just the on the monitor?  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 03-31-2010 12:09 AM                     Good catch Jake. Conner, what say you?  |  IP: Logged Connor Kirkwood Film Handler Posts: 25 From: Portland, OR, USA Registered: Jul 2009 posted 03-31-2010 12:42 AM                     Mark: The fans were the first thing we went to when this problem arose. We've been checking them regularly and they've been running fine. We'd love to trade in that clunker of a processor, but that might not happen for a while Jake: The sound is definitely cutting out in the theater. I've heard it myself and, unfortunately, so have my customers. Tony: I'm kind of relieved that the monitor could actually be a factor in this. But how is that possible? How can the monitor's circuitry affect the channels when the signals are run through separate lines from the processor? Another thing: This problem never seems to occur when we are running movies on the digital projector. Only 35mm films have been experienced the problem thus far.  |  IP: Logged Jake Spell Master Film Handler Posts: 294 From: Johns Island SC Registered: May 2009 posted 03-31-2010 01:06 AM                     Ok, admittedly not knowing anything about a CP-50 processor, could it be that it simply isn't getting the left track from the reader, where the processor is trying to matrix the right channel only, like a cable going bad? Therefore no left input and a reduced center channel? Again this is only a guess as im not familiar with the processor.  |  IP: Logged Connor Kirkwood Film Handler Posts: 25 From: Portland, OR, USA Registered: Jul 2009 posted 03-31-2010 01:23 AM                     No chance of that, Jake. The left track will play fine for a while, then we'll lose it at some point in the evening. It will then drop in and out intermittently until someone does the little jig with the monitor. Then it comes back until the problem presents itself again, which could be during the next show, the next day or the next week.  |  IP: Logged Jake Spell Master Film Handler Posts: 294 From: Johns Island SC Registered: May 2009 posted 03-31-2010 01:31 AM                     In that case, if it was me, I would leave the monitor off or swap it with another auditorium and see where the problem goes. Is the monitor being used as a crossover by any chace?  |  IP: Logged Ian Parfrey Phenomenal Film Handler Posts: 1049 From: Imbil Australia 26 deg 27' 42.66" S 152 deg 42' 23.40" E Registered: Feb 2009 posted 03-31-2010 04:13 AM                     Connor. Does it happen on non-sync at all? If not then that clears that section of the system. Does it happen with mono? Just an idea here, but when it happens again, try switching to mono and follow the audio from there. Can the symptom be forced in any way? Loose cards can make a similar effect. Does the amplifiers have meters, and if so do they read full program signal when the audio has dropped? It sounds like the monitor may be loading the system in some way. It could be a dry joint somewhere, and the loading of the monitor is heating up the joint, expanding the wire and making firmer contact. I know that when things like this happen, all hell breaks loose in the box- but if you can try these things you may localise the problem and have a better chance of finding a cure. Good Luck.  |  IP: Logged Demetris Thoupis Phenomenal Film Handler Posts: 1240 From: Aradippou, Larnaca, Cyprus Registered: Apr 2001 posted 03-31-2010 06:14 AM                     Connor as others have suggested is definately something to do with your monitor that is causing the problem. The line outputs from the CP50 must surely be paralleled connected to the amps and to your monitor. You still have not provided the monitor model and type. A shortage in either of the two will surely cause these results you mention. When this happens, did you check to see the level displays inside the CP50 that are functioning correctly? This will eliminate anything pre CP50. Check the optical pre-amp card CAT 108 to see if you have two LED's lighting up approximately equaly to indicate whether sound from the soundhead is coming. Check the meters at Cat 109 to see if the sound from the soundhead passes in the meter card for Dolby level. Then check the output card CAT 111 to see if there is indeed an output. Do you also have speaker line inputs from the amps in your monitor? Are you able to switch between Processor input and Amplifier input on your monitor. From the sound of it, it does sound that the problem lies in your monitor and not elsewhere. Demetris  |  IP: Logged Richard Hamilton Phenomenal Film Handler Posts: 1341 From: Evansville, Indiana Registered: Jan 2000 posted 03-31-2010 09:35 AM                     Are the left and center channel on the same amp? When the problem happens, put it in bypass or mono and see if there is any sound. Have you tried swapping amps?  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 03-31-2010 12:20 PM                     The three things coming to mind The monitor even simple ones with the old rotary switch can have issues that load the amps output some of the newer ones i have had the active switching loading the amps also the ones that monitor the processor output they could be loading the proscessors output intermitently if left and centre share the same amplifier i would suspect it also you probably should reseat the cards in the processor as over time the edge strip connectors will make the less than best contact even though they are gold plated contacts  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 03-31-2010 02:07 PM                     quote: Connor Kirkwood Tony: I'm kind of relieved that the monitor could actually be a factor in this. But how is that possible? How can the monitor's circuitry affect the channels when the signals are run through separate lines from the processor? As Gordon mentioned, the monitor can load down the signal regardless of type. Speaking of which you MUST give us more info on the monitor!! The monitor works by a connection through different types of circuity (sometimes simple resistor networks only or with a combination of resistor networks and active electronics) which when working properly isolate the channels from each other but allow them to mix down in whatever combinations you select on the controls to the single speaker in the monitor. If there is a problem with the resistor networks or other circuits it could either short out the signal on given channels altogether or load them down as to reduce the levels. It can even sometimes result in the channels going to the wrong places. Gordon's suggestion of reseating the cards is a good one too. With the rack power OFF, simply pull each card in the CP-50 out about halfway and reseat it.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 03-31-2010 06:25 PM                       Lets assume the monitor does not have crossovers, although this would see the leading cause if it does. QSC had this problem for sure on some DCM's. I've never seen a plain monitor that would load down an amp or a processor enough to short out the audio. So, what if the whole system is breaking into oscillation? This can and does happen if there is no ground back to the monitor from the amp.  |  IP: Logged

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