Film-Tech Forum ARCHIVE: Sound channels cut out randomly

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Posted by Connor Kirkwood (Member # 5371) on 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.

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Posted by Mark Gulbrandsen (Member # 72) on 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 as is an upgrade on the outdated processor!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

Posted by Tony Bandiera Jr (Member # 2365) on 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.

Posted by Jake Spell (Member # 5294) on 03-31-2010, 12:01 AM:

You are verifying that the sound IN the theater is cutting out, not just the on the monitor?

Posted by Tony Bandiera Jr (Member # 2365) on 03-31-2010, 12:09 AM:

Good catch Jake.

Conner, what say you? [Big Grin]

Posted by Connor Kirkwood (Member # 5371) on 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.

Posted by Jake Spell (Member # 5294) on 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.

Posted by Connor Kirkwood (Member # 5371) on 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.

Posted by Jake Spell (Member # 5294) on 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?

Posted by Ian Parfrey (Member # 5122) on 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.

Posted by Demetris Thoupis (Member # 833) on 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

Posted by Richard Hamilton (Member # 321) on 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?

Posted by Gordon McLeod (Member # 33) on 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

Posted by Tony Bandiera Jr (Member # 2365) on 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.

Posted by Sam D. Chavez (Member # 1841) on 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.

Posted by Connor Kirkwood (Member # 5371) on 03-31-2010, 08:33 PM:

Thanks for all the help and info, everybody. I'll post info on the monitor as soon as I get back to the theater.

Ian: The problem never seems to occur in non-sync, thus it has never been an issue for films run on our digital projector. Unfortunately, our amps have no meters to help us isolate the issue.

Richard: Our left and center channels run through separate amps.

We're going to re-seat the cards. If that doesn't help, we were considering just unplugging the monitor. I'm still unclear on the concept of the monitor loading the signal to the amps, but wouldn't it just drag the problem over to another auditorium if we swapped monitors? I understand that this would isolate the problem, but we only have one projectionist on duty at a time to monitor the theaters, so we would have to rely on customer complaints if the problem recurred in the original auditorium.

Posted by Jeremy Weigel (Member # 4208) on 03-31-2010, 08:55 PM:

quote: Connor Kirkwood
but wouldn't it just drag the problem over to another auditorium if we swapped monitors?

That's the whole point. If the problem follows the monitor in to the other machine, then you almost certainly have a bad monitor. You then swap them back to verify that that the problem occurs again in the original machine. If so, then you can be 99.99% sure the monitor defective. If it doesn't follow and still occurs in the original machine, then you can proceed to look into other causes.

Posted by Gordon McLeod (Member # 33) on 03-31-2010, 09:03 PM:

How is your digital wired and to what channels
Also what output card do you have in the CP50

Posted by Louis Bornwasser (Member # 3063) on 03-31-2010, 10:10 PM:

I'm going to guess that the Lt is intermittent, which kills the left and reduces the center. The monitor only seems to be involved. If necessary, remove the monitor.

Look at red led system and the preamp card (108) or cat 22. You could reverse the cat 22's. Louis

Posted by Gordon McLeod (Member # 33) on 04-01-2010, 12:22 AM:

the part that doesn't make sense is it only happens in film mode yet the monitor affects it
if one had similar levels from an external source an issue with the monitor should affect both
is the external source routed to the same channels
I have had many of the smart showman monitors and the rentec ones cause strange loading issues at high levels

Posted by Connor Kirkwood (Member # 5371) on 04-13-2010, 04:35 AM:

I thought I'd give an update on our sound issue, since everyone was so helpful with suggestions.

We ended up swapping the booth monitors with another house in order to isolate the problem. So far the sound problem hasn't recurred in either theater. Our projectionist also cleaned the contacts on the cards and re-seated them, as suggested. Unfortunately, it's impossible to tell if either of these things actually fixed the issue, or if it will return in a few months as it did before.

I'll post any further developments.

For those interested, the booth monitor in question is an MS-55.