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Author Topic: Hum and buzz from nowhere Brad Miller Administrator Posts: 17775 From: Plano, TX (36.2 miles NW of Rockwall) Registered: May 99 posted 01-11-2004 11:41 PM                     I've got an auditorium with a CP200 that has always been dead quiet as far as system noise is concerned. Out of nowhere, the system will start eminating a fairly loud hum and buzzing. Switching to "bypass", then back to "normal" cures the probem. Nothing has changed over the years. I have absolutely traced it down to the CP200 being at fault, for if the 200 is turned off, the noise goes away. Likewise if the system is in it's bitchy mode and I remove a cat64 card, the noise goes away in that channel. Any ideas?  |  IP: Logged Daryl C. W. O'Shea Film God Posts: 3977 From: Midland Ontario Canada (where Panavision & IMAX lenses come from) Registered: Jun 2002 posted 01-12-2004 12:54 AM                        Have you tried swapping the power supplies out?  |  IP: Logged Brad Miller Administrator Posts: 17775 From: Plano, TX (36.2 miles NW of Rockwall) Registered: May 99 posted 01-12-2004 01:27 AM                     Yes that has already crossed my mind, no I haven't had a chance to try it yet. I am going to take one of mine and try it out tomorrow. In the meantime, I am looking for other recommendations on things to check for.  |  IP: Logged William T. Parr Jedi Master Film Handler Posts: 823 From: Cedar Park, TX Registered: Nov 2000 posted 01-12-2004 01:28 AM                     Brad, What kind of connections are on the Amp end? We had a CP 55 do this at my theatre and it turned out there was corrosion building up on and XLR connector going into the amp. After cleaning off the crud, the Hum went away and has not been back since.  |  IP: Logged Brad Miller Administrator Posts: 17775 From: Plano, TX (36.2 miles NW of Rockwall) Registered: May 99 posted 01-12-2004 01:33 AM                     1/4" connections. I've already checked this out. The cables are solid and are actually only a few years old. FYI, the amps are QSC.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 01-12-2004 06:49 AM                     Also check they bypass relays  |  IP: Logged Gilbert Travin Expert Film Handler Posts: 101 From: Villeurbanne / France Registered: Jan 2004 posted 01-12-2004 07:23 AM                        Hi Bradd and all !! It's my first post on this forum. I benefit from this opportunity to greet all the members of this forum which I had consulted for a few weeks! My English is surely not very good, forgive me; with you I will be able to progress! For Bradd : I had knowm a similar problem of hum and noise a few years ago with a CP50 : the output card (I do not remember its CAT number ) had a failure. After replacement, the system functioned again correctly. Perhaps hum appears if a low impedance circuit is broken (wire , grounding, transistor, IC etc.) : the circuit becomes high impedance antenna for power supply induction at 50/60 Hz ... ???  |  IP: Logged Ken Layton Phenomenal Film Handler Posts: 1452 From: Olympia, Wash. USA Registered: Sep 1999 posted 01-12-2004 09:56 AM                     That Dolby unit is certainly old enough that some of the electrolytic capacitors are probably drying out/failing on the cards. Check the power supply first, though.  |  IP: Logged Edward Jurich Master Film Handler Posts: 305 From: Las Vegas USA Registered: Jul 2003 posted 01-12-2004 11:23 AM                     You might check for a loose ground wire (inside a connector) or a ground touching one side of a balanced line if any lines are balanced.  |  IP: Logged Bob Koch Expert Film Handler Posts: 183 From: williams ca Registered: Nov 2001 posted 01-12-2004 01:34 PM                     I had this once with a new CP200 on a many plex installation. I had a hell of a time finding it as it was intermittent and theatre opening was imminent. Turned out to be the bypass relays. Can`t remember details. Long time ago.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 01-12-2004 10:35 PM                     The bypass relays on the 55 65 and the 200 culd be a pain sometimes  |  IP: Logged John Walsh Film God Posts: 2490 From: Connecticut, USA, Earth, Milky Way Registered: Oct 1999 posted 01-12-2004 11:23 PM                     Another FWIW: We've had several problems with corrosion on the 1/4" connections to the QSC 1300/1310 series amps we use; causes weird things to happen depending on which contact becomes a 'high resistance.' We sort of, ah, glad-handle them, if you know what I mean, and the problem goes away for a few years.  |  IP: Logged Rick Long Jedi Master Film Handler Posts: 759 From: Toronto, Ontario, Canada Registered: Nov 1999 posted 01-13-2004 01:42 AM                     Had a similar problem with a 200. Turned out the filter capacitor terminal screw in the PS-1 power supply had come loose. Put a lock-washer under it, worked fine ever since.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 01-13-2004 09:45 AM                     The Filter caps in the CP-200 power supply (PS1 and the PS1B) are notorious for this sort of thing (as can the bypass relays...remember, on the CP-200 the relays are on the inside of the Control Unit backplane. Measure the voltage rails when the hum is there...I'll wager some are low and have ripple Open up the power supply and tighen all of the capacitor screws (some will be loose trust me). Steve  |  IP: Logged

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