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Author Topic: sudden level drop - cp750? Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 05-12-2017 04:33 PM                     A friend just called me from the middle of a festival. Suddenly they had to push up their CP750 to 8 or 9 to get a decent level on their system. They tried with different content, but the problem remained. They usually play around 4. Anyone ever experienced that with a cp750? They said they can rule out the amps. Their server has no digital audio leveling capability. A dying power-supply? Overheating? - Carsten  |  IP: Logged Stephan Shelley Jedi Master Film Handler Posts: 854 From: castro valley, CA, usa Registered: Nov 2014 posted 05-12-2017 06:43 PM                     Have you checked the power amps? I have recently had issues with the vol pots on the OSC DCA amps. They can cause a vol drop or sound cutout.  |  IP: Logged Richard Fowler Film God Posts: 2392 From: Ft. Lauderdale, FL, USA Registered: Jun 2001 posted 05-12-2017 07:08 PM                     I found the same volume issues for a festival where the installer did not allow proper air flow and was running hot  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 05-12-2017 07:33 PM                     I could make a good living just traveling the country cleaning QSC amps... If you clean the pots be sure to also clean the heat sinks.  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 05-12-2017 09:05 PM                     They probably have more than one amp and all of them failing at the same time is highly unusual. They probably have done some power cycling already? I'm not sure how this could be a power supply related failure, since it would be kind of an odd failure only to affect the pre-amp/analog part and not the digital/controller part of the processor. I guess they're using digital inputs? In the unlikely case they're not, they might check the input side of things obviously.  |  IP: Logged Stephan Shelley Jedi Master Film Handler Posts: 854 From: castro valley, CA, usa Registered: Nov 2014 posted 05-12-2017 09:47 PM                     You loose center channel you loose a lot and if it is a 4 channel amp.... Had one of those have dirty pots.  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 05-13-2017 05:52 AM                     They were in the mid of a festival screening when they called. They didn't bother to pull up the CP to 9 as long as they were there. They WERE bothered what would happen if for some reason 'the old' situation would come back and blast peoples heads off. After that screening round, they powercycled, and placed a fan-blower on the CP750 and the issue was gone. Usually analog output amps would be sourced by the +/-15 Volts, so I would have suspected something around this. With some basic measurement gear it would have been easy to diagnose the call, but they don't have the gear and the knowledge to do this, especially not in the mid of a festival. As they are a bit far off the rest of the world, I advised them to get a backup power-supply for the 750. Does anybody know how the output levels of the CP750 are set? Purely digital, or with digitally controlled pots? - Carsten  |  IP: Logged Richard Fowler Film God Posts: 2392 From: Ft. Lauderdale, FL, USA Registered: Jun 2001 posted 05-13-2017 07:31 AM                     The festival situation they had the CP750 stacked on equipment like books on a shelf with zero air flow and hot to the touch. The booth air conditioning was off. The pedestal was mounted backward with rack face to the front wall for non-easy service. They spent 16 hours to install the equipment versus 4-6 hour average for the site. Did the same...relocate....power cycle...cool down with fan and AC. The "installer" company run great ads of their skills in the trade magazines.  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 05-13-2017 08:24 AM                     Whatever creates service calls is good for them... During the gold-rush of the digital rollout, I saw lot's of installations like that. - Carsten  |  IP: Logged Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 05-14-2017 01:07 PM                     The CP750 has one PCB, no adjustments on it. They do not release a schematic so I don't know how the signal path looks, but it's probably totally digital until the final D-A stage. There would be no reason to go analog and insert a fader stage afterwards.  |  IP: Logged Harold Hallikainen Jedi Master Film Handler Posts: 906 From: Denver, CO, USA Registered: Aug 2009 posted 05-14-2017 02:57 PM                        I don't know about the CP750, but USL processors like the JSD-60 and JSD-100 have an SPI controlled analog pot between the DAC and output line driver. Gain trim is done here to maintain maximum dynamic range. If a trim down is done before the DAC, the signal goes down, but the noise does not. With this particular issue, did all channels go down in level? Does the CP750 front panel bar graph show the levels down? Does a booth monitor also reveal the levels down? Good luck! Harold  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 05-15-2017 04:29 AM                     Hi Harold - I told them to check these indications. However, in the mid of s screening series, their willing to do a proper analysis were limited. They were just glad it worked after rebooting and applying the fan blower... - Carsten  |  IP: Logged System Notices Forum Watchdog / Soup Nazi Posts: 215 Registered: Apr 2004 posted 10-20-2018 11:06 AM                 It has been 523 days since the last post.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 10-20-2018 11:06 AM                     Dolby has issued a field Bulletin on this. They claim it is caused by cooking the unit (not enough ventilation) and the repair is changing the motherboard.  |  IP: Logged Ken Lackner Phenomenal Film Handler Posts: 1907 From: Atlanta, GA, USA Registered: Sep 2001 posted 10-23-2018 04:12 PM                     Yep. Had this happen recently. Norm at Dolby confirmed it's a known issue with the CP750 and I had to send it in for repair. The motherboard was, indeed, replaced. I wasn't told that it was caused by overheating, but that doesn't mean it wasn't. I don't see how, though. There's nothing above the 750 in the rack, and there is a blank space between it and the monitor below.  |  IP: Logged

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