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#160453 - 05/10/06 02:56 AM Short from Mic
Wireless Offline
Member

Registered: 06/09/05
Posts: 73
Loc: Los Angeles CA USA
Someone called me to troubleshoot a P.A. System while we were working on it I got a zap from the mike. I think I narrowed it down to the cable between the amp and the jack on the podium for the Mic. what would cause this?

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#160454 - 05/10/06 05:13 AM Re: Short from Mic
techie Offline
Member

Registered: 05/17/05
Posts: 240
Loc: palo alto, ca usa
First step, check the ground pins on all the power cords.. people frequently cut them off when trying to eliminate hum and buzz, instead of really finding and fixing the ground loops.

It is ok to lift audio grounds, but it is not ok to lift safety grounds.

(there have been entire books written on sound system grounding practices..)

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#160455 - 05/10/06 07:16 AM Re: Short from Mic
hbiss Offline
Member

Registered: 12/16/03
Posts: 893
Loc: Hawthorne, NY USA
what would cause this?

With all due respect, if you have to come here and ask this kind of question you have no business touching that system. People have died because of this exact situation. Further, you could be held legally responsible should someone be injured or killed.

The professional thing to do is refer this to a qualified service company. Sound system work is not normally in the job description of an electrical contractor.

-Hal
_________________________
www.myphonetechs.com

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#160456 - 05/10/06 07:31 AM Re: Short from Mic
Wireless Offline
Member

Registered: 06/09/05
Posts: 73
Loc: Los Angeles CA USA
Hal you are right and normaly I would not but this is for a school I am just trying to help them out.

none of the units have a ground pin (they are only two wire) the mic extension is wired properly but the and there only is a shock when it is used.

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#160457 - 05/10/06 07:58 AM Re: Short from Mic
LarryC Offline
Member

Registered: 07/05/04
Posts: 776
Loc: Winchester, NH, US
none of the units have a ground pin (they are only two wire) the mic extension is wired properly but the and there only is a shock when it is used.


Is it 120 VAC?

Most likely there is an INTERNAL FAULT of the Amp or mixer.

Less likely, there might be a short in the cabinet if it is a permanently installed system.


Is it 24 to 48 VDC?

Then "phantom power" may be turned on. Phantom power is DC sent over the audio wiring to power specific types of microphones that use internal amplifiers.

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#160458 - 05/10/06 02:19 PM Re: Short from Mic
hbiss Offline
Member

Registered: 12/16/03
Posts: 893
Loc: Hawthorne, NY USA
Forget about "phantom power"! You need to look for leakage.

Take your AC volt meter and put one lead on a ground- like the receptical- and then look for voltage with the other probe. Try the amp case, microphone and shield of the mic cable, any place else you can think of. I would start with the mic then disconnect it from the amp and see if the voltage goes away. That way you will know it's coming from the amp or the cable somehow.

This is best done with a analog VOM. Most digital meters present a high enough impedance that they will say there is voltage when it really is just harmless stray pickup, like the hum you get when touching the input of an amp.

The best way to measure for leakage is with a 50 to 100k load resistor across the test leads. Then any meter will work fine.

I would be very careful here and remove the equipment from service until it's fixed.

-Hal
_________________________
www.myphonetechs.com

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#160459 - 05/10/06 05:29 PM Re: Short from Mic
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
We've talked about this in another thread. As Hal has mentioned, check for AC voltage between Earth and chassis. But also look for DC volts. You said that the amps have two wire cords and plugs. Are they polarized or non-polarized? If it is non-polarized, for kicks and giggles, take two sets of voltage measurements, one with the plug reversed. My concern is that you are dealing with a component designed around a "hot chassis". Many designs save weight and money by skipping the power transformer, and going straight to a rectifier. The negative end of the bridge rectifier often becomes the chassis ground. The current that might be able to flow between chassis and earth is better expressed as rare, medium rare, well done, rather than leakage. (does wonders for scope grounds)

If you are dealing with an amp with a hot chassis, plugging it into an appropriately rated isolation transformer, and then earth ground referencing the chassis, is the only really safe bet. I know that this could get a little costly but might solve several leakage and hum problems as well.
Joe

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#160460 - 05/11/06 09:03 AM Re: Short from Mic
hbiss Offline
Member

Registered: 12/16/03
Posts: 893
Loc: Hawthorne, NY USA
Wireless, could you give us the manufacturers name and model number of the equipment in question? Also give us an idea about this system. Is it only a simple mixer/amp or a rack full of stuff.

-Hal
_________________________
www.myphonetechs.com

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#160461 - 05/11/06 09:30 AM Re: Short from Mic
Wireless Offline
Member

Registered: 06/09/05
Posts: 73
Loc: Los Angeles CA USA
Hbiss, I don't know the manafacturer or model no off hand but here is the setup: 8/12 (there is a switch to split 4 speakers)speakers wired in parallel. they are hooked up to a cheap audio receiver. There is a tape deck attached to record lectures (the speakers only work when tape monitor on the receiver is selected). There is a mic mixer ( I think that is what it is called. It is a small blue box with a mic input) which is wired into the receiver in the CD spot with audio wires. the mic jack is plugged into the mic mixer, and the mic into the jack on the podium.

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#160462 - 05/11/06 06:07 PM Re: Short from Mic
hbiss Offline
Member

Registered: 12/16/03
Posts: 893
Loc: Hawthorne, NY USA
Obviously not commercial grade equipment or even equipment intended for use with microphones. Looks like they just threw a microphone mixer on a home stereo receiver.

I think you are going to find a problem is with leakage in the receiver or tape deck. This may or may not be abnormal by the way, considering what this equipment is.

Remember this is home stereo stuff and is not meant for commercial use as a PA system with hand held microphones. Consumer audio equipment is not subject to the requirements that commercial sound equipment must adhere to. Things like three wire line cords and minimal leakage for instance.

Actually being an electrician and being familiar with the code may come in handy. I think you could make a case for this equipment not being listed for the use. Probably the only way you are going to deal with this and make it safe is to get them to replace it with the proper equipment.

-Hal
_________________________
www.myphonetechs.com

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