Film-Tech Forum ARCHIVE: Hum and buzz from nowhere

This is topic Hum and buzz from nowhere in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Brad Miller (Member # 2) on 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?

Posted by Daryl C. W. O'Shea (Member # 1303) on 01-12-2004, 12:54 AM:

Have you tried swapping the power supplies out?

Posted by Brad Miller (Member # 2) on 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.

Posted by William T. Parr (Member # 677) on 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.

Posted by Brad Miller (Member # 2) on 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.

Posted by Gordon McLeod (Member # 33) on 01-12-2004, 06:49 AM:

Also check they bypass relays

Posted by Gilbert Travin (Member # 2138) on 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! [Smile]

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 ... ???

Posted by Ken Layton (Member # 133) on 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.

Posted by Edward Jurich (Member # 1787) on 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.

Posted by Bob Koch (Member # 1062) on 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.

Posted by Gordon McLeod (Member # 33) on 01-12-2004, 10:35 PM:

The bypass relays on the 55 65 and the 200 culd be a pain sometimes

Posted by John Walsh (Member # 168) on 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. [Big Grin]

Posted by Rick Long (Member # 213) on 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.

Posted by Steve Guttag (Member # 268) on 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