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Old 01-14-2011, 10:24 AM
Rhythmdvl Rhythmdvl is offline
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How to check for shorts in speakers/wiring?

Receiver went POP, magic smoke came out, and it went back to Onkyo for repairs. It’s due back today and I can’t wait to hook everything up again (nothing like going from 5.1 to television speakers).

The repair ticket says that they “located and replaced shorted components in audio circuit, and located and replaced open components in audio circuit.” It also says:

“Please check speakers and wiring FOR SHORTS, PRIOR to initial hookup.” (caps theirs)


So ... now what? Some of the wiring is in-wall, and I assume I should check the speakers too—but how? I have a DM110 Extech Pocket Multimeter, if that helps, but I’ve never really used it before so have no idea what to do with it or if is relevant at all. I also have a car to go to the nearest Radio Shack or whatnot if I need something else.

Can anyone offer a bit of guidance? If I can use the multimeter, what knobs and settings am I looking to knob and set?

Thanks,

Rhythm

Last edited by Rhythmdvl; 01-14-2011 at 10:26 AM.. Reason: Added link to multimeter
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  #2  
Old 01-14-2011, 10:49 AM
jz78817 jz78817 is offline
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set it to measure resistance (ohms) and measure across the speaker wires. assuming your speakers are 8 ohm, you should read around 6.5-7.5 ohms. if you have 4 ohm speakers, it'll probably read about 3.4-3.6 ohms. if it's much less than that (less than 1 ohm) you have a problem.

ETA: this will only measure woofer/midrange speakers that don't have a capacitor inline.

Last edited by jz78817; 01-14-2011 at 10:51 AM..
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Old 01-14-2011, 10:56 AM
johnpost johnpost is offline
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the multimeter has a resistance function (shown using Greek letter omega).

in these function having the meter probes not touching will show infinite resistance ('open' would be used as an electrical term in regard to a circuit or cable). in this function having the meter probes touching will show zero resistance ('closed' would be used as an electrical term in regard to a circuit or cable, having a 'short' would be having this in an undesired situation).

with the speakers and amp disconnected and the speaker wires not touching measure the speaker cable resistance. if it shows infinite resistance the speaker cables are not shorted.

connect the speaker wires at one end (tape or paper clip), measure the resistance of the cable on the other end. if it shows zero resistance then the cable is closed from amp to speakers.

measuring the speakers for shorts will depend on your speakers, different speaker types will need different methods.
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Old 01-14-2011, 11:57 AM
Machine Elf Machine Elf is offline
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How old is your Onkyo receiver? I had a Sony about 15 years ago that apparently had built-in protection against shorts in the speaker circuit. One time there was a short in one of the speaker circuits (the two wires had somehow made contact with each other at the speaker terminals); as soon as I turned the volume up away from zero, a relay inside the amp clicked off, opening the speaker circuit, and a "FAULT" light illuminated on the receiver's front panel. By cycling the power on the receiver (after eliminating the short), I was able to reset the fault, no damage done.

It would surprise me if your Onkyo receiver did not have similar protection (check the manual?). Nonetheless, it's probably a good idea to check things out before hooking up again.

Last edited by Machine Elf; 01-14-2011 at 11:57 AM..
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Old 01-14-2011, 01:16 PM
Rhythmdvl Rhythmdvl is offline
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The receiver is about a year old (itís an Onkyo TX-SR707). Whether or not it had protection built in, its magic smoke escaped. Fortunately it was under warranty and they took care of it quickly.

Given that the cataclysm occurred about three days after I changed the speakers, Iím inclined to believe thereís a good reason the repair shops suggested checking for shorts.

When I measure each speaker at the wires (except the subwoofer), I get 4.1 for each (the reading jumps around a bit, then settles on that). I disconnected one of the speakers and measured it at the terminals and got 4.1 there too. I connected the wires together and measured from the other end and got .1.

The speakers are a set of Infinity TSS-1200. The product manual says that their nominal impedance is 8 Ohmsóso isnít the 4.1 much lower than expected? Of course, I could be doing something wrong, but does the same reading across each suggest that there is no short?


(I really wish I knew what I was doing. For the long term, any recommendations for a basic beginnersí guide to electronics? Would this be a good excuse to buy one of those kidís electronics labs (like this) be useful for learning the basics?)
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Old 01-14-2011, 01:50 PM
Gorsnak Gorsnak is online now
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Quote:
Originally Posted by Rhythmdvl View Post
The speakers are a set of Infinity TSS-1200. The product manual says that their nominal impedance is 8 Ohmsóso isnít the 4.1 much lower than expected? Of course, I could be doing something wrong, but does the same reading across each suggest that there is no short?
4.1 ohms is somewhat but not much lower than expected. You're measuring resistance and the nominal 8 ohms refers to impedance, which isn't quite the same thing. As soon as you have coils and magnets involved in a device impedance and resistance are going to diverge to some extent. A multimeter isn't really the right tool for measuring a speaker load, but it's the tool you've got and it's giving you adequate information - an actual shorted speaker would return a reading of <1 ohm and you're reading over 4 and all of the speakers the same. So the speakers are fine. You're also reading the same resistance at the other end of the speaker cables, so the cables are fine. If there was a short, it was caused by something like a stray cable strand crossing terminals.
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Old 01-14-2011, 02:01 PM
Rhythmdvl Rhythmdvl is offline
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Quote:
Originally Posted by Gorsnak View Post
4.1 ohms is somewhat but not much lower than expected. You're measuring resistance and the nominal 8 ohms refers to impedance, which isn't quite the same thing. As soon as you have coils and magnets involved in a device impedance and resistance are going to diverge to some extent. A multimeter isn't really the right tool for measuring a speaker load, but it's the tool you've got and it's giving you adequate information - an actual shorted speaker would return a reading of <1 ohm and you're reading over 4 and all of the speakers the same. So the speakers are fine. You're also reading the same resistance at the other end of the speaker cables, so the cables are fine. If there was a short, it was caused by something like a stray cable strand crossing terminals.
Well that explains everything--we just took in a stray!

By the way, what would be the right tool to have used?

I'm going to check, recheck, and inspect everything I can before re-hooking things up. That the magic smoke escaped without the assumed protection circuit tripping, if everything checks out again I'll assume it was an internal fault in the receiver. But I'm going to triple-check just to be sure.

Thanks for the help!
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Old 01-14-2011, 02:12 PM
Gorsnak Gorsnak is online now
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An impedance meter. But it's not the sort of thing you'll find at Home Depot. I've got a Goldline ZM1P at work, which is indispensable when trying to track down which of the 40 ceiling speakers on this loop is shorted.
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Old 01-14-2011, 02:22 PM
Magiver Magiver is offline
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I think the shop is asking you to test your speakers to see if you welded them when the amp departed and also to check for stray cable strands. Using an Ohm meter was sufficient to test the circuit to see what was going on.
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Old 01-14-2011, 02:39 PM
Rhythmdvl Rhythmdvl is offline
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Quote:
Originally Posted by Gorsnak View Post
An impedance meter. But it's not the sort of thing you'll find at Home Depot. I've got a Goldline ZM1P at work, which is indispensable when trying to track down which of the 40 ceiling speakers on this loop is shorted.
40 speakers? Oh, at work. Theater? Intercommissary?
Quote:
Originally Posted by Magiver View Post
I think the shop is asking you to test your speakers to see if you welded them when the amp departed and also to check for stray cable strands. Using an Ohm meter was sufficient to test the circuit to see what was going on.
Ah, that makes sense. In particular, it would keep me from sending the receiver back to them claiming it still doesn't work.
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