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Please be gentle with me here lol...

I am wondering exactly when and how an impedance meter might be of benefit to me. Every 70V paging call I have ever been on has been solved with observation and logically breaking things down.

Admittedly, I work on smaller installations of no more than 30 to 40 speakers and I have the advantage of knowing where the legs are and how they are run by either having run them myself or having access to documentation showing how they were run.

So I found myself on a Saturday morning at a large retail store at 5AM to work on a recently installed PA where there was no audio on the sales floor. I know this was a fully working system a few days prior having observed that. The store broadcasts music over the PA so I had the advantage of a constantly playing source while troubleshooting. Having assisted with the installation, I knew how the legs and volume controls were installed.

When I arrived I tested the input to the VC with my test set and had great audio. The output was very faint. This told me there was a short in the cabling somewhere. I went to the point where all the legs on the sales floor were tied together and broke the legs open. Tested and faint audio so I knew the "first" leg had a problem. There were 7 speakers in the leg and spending a short amount of time going from speaker to speaker, I found that the cabling between 2 speakers somehow developed a short to the building up in the truss. I suspect the alarm technician who followed us may have pulled his cabling too vigorously and damaged the PA wiring. So when I can get a lift it is a simple fix. For the call, I just left the offending speaker out of the leg and everything came up perfectly as expected. It was only one speaker not working as it was the "last" speaker in the leg so we left it until a lift can be brought in.

I am always reading that an impedance meter is necessary to troubleshoot 70V but that has not been my experience. Am I lucky? Is it because I usually come into a known installation where I have most of the data needed? Would an impedance meter have sped this up any? Once I found the "offending" area between 2 speakers, a quick continuity test showed a short in the cable and it looks like someone disturbed the wiring in that area.

Here was the process -

Music was being broadcast clearly to the offices, warehouse and the input and output (after pulling the leg off) of the VC for the sales floor so I immediately know it is not the amp or any of the VCs. Not going to waste time on those. I know the taps of all the speakers are within range of the amp because I tapped them. I also know the short would not be between the speaker leads because I took the time to do them correctly. I hate troubleshooting speakers on a 70V and take the time to wire them properly and not expose the copper.

Because the volume was so low on the test set, I was pretty sure there was a short somewhere. This is based on past troubleshooting experience. It is a bit of a PITA to go speaker to speaker to break the leg out and test but it really wasn't that bad.

Where would an impedance meter help me?

Thank you in advance for the explanation.

Last edited by Meyery2k; 03/19/18 05:17 AM.

Michael Meyer
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I have a ZM 1. It comes in handy when you don't know where everything is. We do a lot of older schools and most of the cables are home runs. Over the years people add speakers and don't take into account the wattage rating of the amp. For us it's great to know the issue is 200 watts on a 150 watt amp or by testing individual speakers someone slammed in an 8 ohm speaker. For us it limits the amount of hunt and peck we do.
I hope this helps.


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I think common sense like you have been using will solve 90% of paging troubles The only time is was stumped a little was at a Lumber Jack warehouse where the alarm installer had shorted our wiring with a T25 staple . I also needed a lift to pull 1 staple out , I just used a fork lift and a pallet to stand on . It was a little spookey being over 30 feet up but I survived . So my answer would be only buy one if you feel the need as a last resort . But that is just my thoughts on this after 43 years in telecom .


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I agree with Rotary. Common sense and divide and conquer are your best tools when looking for speaker line problems. The only time I can see using an impedance meter is with an unknown system and you suspect that an improper load is causing amp problems. There, you would want to measure the total wattage required and compare it to what the amp can supply.

I suppose if you had one you would have used it to tell you that there was a short in that speaker line, but I wouldn't go out of my way to get one.You did just fine without it and your method wasn't any slower.

What would be useful is a TDR that would tell you the distance to the short but I don't think it would work very well on a line with a bunch of line transformers hung off of it.

-Hal


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Thank you all. I always read about how you must have an impedance meter to effectively work on paging. I can see the value if you are walking in blind and suspect too many speakers. For finding a short, there really seems to be no other way except to divide and conquer.

@rotary - Yes, I have done the pallet on the forklift on occasion. The older I get, the more frightening it becomes lol...


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I own an impedance meter and actually carry it in the van. I probably have not had to use it for 3 or 4 years, but as has been discussed, it's usually a logical solution to dividing the grid and finding the problem. Now, back in the 25 volt K-Mart days, the amps weren't very forgiving to a rouge 8 ohm speaker. You could tell because the amp was hot enough to fry and egg on it :-)

My favorite is the short one side to the ceiling grid. Thankfully, most new construction leans toward open ceilings and not tiles, bridges, and hats.

As far as the pallet on a forklift. Well, I used to stand on the rails of a scissors lift. All of those practices would get an OSHA violation and worker's comp denials. It's strange to be in a harness on a boom lift, but it feels so much safer.

Carl

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Originally Posted by Carl Navarro
I own an impedance meter and actually carry it in the van. I probably have not had to use it for 3 or 4 years, but as has been discussed, it's usually a logical solution to dividing the grid and finding the problem. Now, back in the 25 volt K-Mart days, the amps weren't very forgiving to a rouge 8 ohm speaker. You could tell because the amp was hot enough to fry and egg on it :-)

My favorite is the short one side to the ceiling grid. Thankfully, most new construction leans toward open ceilings and not tiles, bridges, and hats.

As far as the pallet on a forklift. Well, I used to stand on the rails of a scissors lift. All of those practices would get an OSHA violation and worker's comp denials. It's strange to be in a harness on a boom lift, but it feels so much safer.

Carl

Ah yes, guilty as charged lol...

Short to the grid is common for me as well.

Thanks, again, the impedance meter is offered as this ultimate tool that would solve all my paging problems. I can see the value if I had a system constantly blowing the amplifier or the amplifier was overheating as it would then help me find that likely added 8 ohm speaker or someone thinking if 2 watts is good, 10 watts is better and overloading the amp.


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I worked PA for 40 years and never used an impedance meter (though I did use a Wheatstone Bridge once.

The company I was working for got one when we did the PA system for American Airlines at JFK. About 5,000 speakers in Building 8 (maybe more, I really can't remember anymore) and another couple of thousand in building 9 (later all combined into one big space). For a job that size, you'd be crazy not to have one. Very, very handy when you're coming in to test each cable run.

Did the installers tap the speakers at .75 Watts or did they screw up in a dimly lit space and tap them at 7.5W. Big difference.

I liked working with it. On a small job, I don't think I'd bother taking out.

Sam


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