a old problem has reappeared - AVS Forum
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post #1 of 12 Old 04-22-2013, 01:03 PM - Thread Starter
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I reported on this forum a few months ago when my home theater system took a large hit, so to speak.
One module of a five channel amp blew and took out the speaker connected with it as well.
It was the center channel.
The protection circuitry within the amplifier and module did not stop the fault from occurring.

I had the speaker and amplifier repaired and tested along with the pre amp processor.
Nothing odd was reported by the repair depot.

I was hesitant to say the least of putting the 5 channel back into service so I auditioned a few different brands of amplifier.
All was well for the last few months. I was only auditioning 2 channel amplifiers as they are easier on the back and wallet.

I decided I liked the most recent two channel and borrowed the corresponding three Chanel from the same manufacturer this weekend.
I setup the processor to accept 5 channels once again and balanced the speakers with audyssey.
I listened to cd and sacd and blu ray.
Then last night I watched digital cable.
Within 5 minutes the amplifier handling the center and surrounds stopped working.
Further investigation revealed blown fuses within the body of the amplifier.
The center channel was the one that blew.

I was watching digital cable when that first fault occurred as well.
I had the cable company come out the first time and check their box and have scheduled an appointment for them to do so again.
I could have been a coincidence, or?

I was lucky it was only the fuse that went and not the speaker again or the on loan amplifier.

What else can I do?
I am going to bring the processor and speaker back to the repair depot that tested it originally for re evaluation.
The cable company is coming back.
Should I get an electrician? What could they investigate? What should they look for?
Maybe an exorcist, I don't know?

So for now I am in a two channel penalty.
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post #2 of 12 Old 04-22-2013, 04:55 PM
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Replace the wires to the center channel, being very careful with the connections, and/or replace the center channel speaker. Sometimes a burr or out-of-round voice coil or magnet assembly can cause the speaker to "hang" or "drag", causing a spike in power (and thus voice coil heating), or there may be a manufacturing defect. If the insulation inside the voice coil has a defect it could be shorting and causing a power spike that essentially shorts the amplifier's output. It is possible you are over-driving the speaker and destroying it, but that won't usually take out the amp (but it is possible).

I doubt it is the TV cable.

I would contact the speaker manufacturer and see if they will take the center back for inspection and/or repair/replacement.

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #3 of 12 Old 04-23-2013, 07:04 AM
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It is pretty obvious that the speaker is shorting out after a period of operation and causing the amplifier to protect itself. There may be a breakdown occurring in the insulation of a voice coil.

I am assuming that you are certain that there is no possibility of a short in the speaker wire or connections (this would probably cause the amp to shut down instantly, which apparently didn't happen). Just to make sure, check the speaker wire for any areas that could have been pinched or deformed which could cause a partial short.

I would get a new center speaker, or send it to the manufacturer for repair. Be sure you send a note describing the problem.

There is no point in sending the amplifier or processor to anyone, because the problem is obviously with the speaker.
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post #4 of 12 Old 04-23-2013, 07:13 AM - Thread Starter
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Although these are good points, I have had the speaker and amplifier reconditioned and tested by a reputable repair shop after the initial failure.
I am going to bring the speaker back to them once again along with the processor.

I am hopeful the more technical minded might chime in on what other type of testing I can do to track this problem down.
For instance I have read in the amplifier manual that it is supposed to protect against DC.
How and where do I measure for DC?
Is DC created within the amplifier or pre amplifier?
What type of measuring device would I use and where to place it?

I also thought to test the speakers for correct Ohms.

Testing the incoming voltage was also another suggestion.
Probably the cables can be tested in some way as well for 75 Ohms i believe .

The suggestion to replace the center channel speaker wire and to check connections is good.
I have WBT terminations so no loose wire is able to contact where it shouldn't.

What I have observed is that i can listen to surround all day and night via CD player.
But a few minutes of TV cable and there is a problem.

There must be a measurement to take somewhere when I switch sources from CD player to digital cable.
The output of the amplifier or preamplifier?

This has been a long ongoing issue that I would like to conclude.
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post #5 of 12 Old 04-23-2013, 08:10 AM
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How more technical minded do you want? The problem I have with getting too technical is that you might not have the knowledge and/or experience to use the advice. For instance, if you do not know how to measure DC, do not understand how it is relevant, will it really help in your troubleshooting? I know this seems harsh, but sometimes you really need to let an expert deal with the problem. In this case, my guess is the speaker itself has an intermittent short, one that may occur infrequently and under certain conditions (such as being played moderately loud for an extended period, or any of a plethora of other conditions), and thus would be a fairly difficult trouble-shooting job if I had the speaker in front of me. Plus I don't have a lot of audio test gear at the house (yes, I have some). Since your local tech did not find it, I say let the manufacturer know. However, you can give the local guy another shot, he'd probably appreciate it and may have the resources to debug it. There are plenty of very sharp local guys, and it's easy to miss something that doesn't show up but once in a blue moon. I have certainly lost some hair over intermittent problems.

BUT, the hooker in this is you only seem to have problems with the cable box. Have them replace the box. It is possible it is putting out some sort of signal that is causing the problem, be it an ultrasonic oscillation or something else.

DC = direct current, such as you would get from a battery. The voltage is 'fixed", there is a defined + and - terminal.
AC = alternating current such as you get from the wall socket, or such as your audio signal. These signals change with time, alternating from + to -, back and forth.
Wikipedia can probably provide pictures and such that will really help.

Speakers depend upon the input voltage alternating so the mean is zero. Putting DC through a speaker will offset the cone in one direction and it will stay there. That reduces the dynamic range since there is less movement available in one direction, and the constant current can heat up and eventually destroy the voice coil. Small amounts of DC are rarely a concern.

Most audio components (including speakers) have elements inside them (capacitors) that block DC. (The LF crossover in a speaker often does not block DC, however.) Components that advertise "direct-coupled" do not, and will pass a DC signal. If the cable box is producing a DC offset at its output, and the signal chain (preamp/amp) is all direct- (DC-) coupled, then hat could cause problems for your speakers, true.

You can measure DC with a voltmeter, which may be part of a multimeter or DMM (digital multimeter). Multimeters typically measure voltage (volts), current (amps), and resistance (ohms). However, only a few mV (thousandths of a volt) at the output of your cable box could cause problems at the speakers, and it is hard to measure that little. If you have a voltmeter (from Radio Shack or other electronics supplier, Wal-Mart, etc.), you could measure across the speaker terminals to see how much DC is present. You'd need to switch the cable box to the setup menu or something that does not have sound. You don't want to just turn down the volume or mute the AVR as that would also reduce the input signal you are trying to measure. You cannot measure (with a DC meter) the DC when there is sound coming out of the speakers because the AC signal will mask the DC offset (you need a more sophisticated instrument for that, like an oscilloscope). I am not sure how much voltage is reasonable, but if it is more than perhaps 10 - 20 mV there may be a problem with DC someplace. You'd then have to begin eliminating things to find the one causing the DC.

It is also possible there is a high-frequency oscillation, one you cannot hear, or very low-frequency oscillation, again subsonic, that is overdriving the speaker.

Or something else.

This strikes me as fairly complicated troubleshooting job for a novice. Again, I know it sounds insulting, but one thing I have learned through the years is to find people with the right expertise. I have screwed up enough things that I have gotten a little better at asking for help, or getting somebody who knows what they are doing to help.

"Experience comes from making lots of mistakes. Wisdom is learning from them. So far, I've lots of experience." - Don

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #6 of 12 Old 04-23-2013, 09:59 AM - Thread Starter
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I was asking in my first post who I could get to troubleshoot my issue.
I did not infer I would be doing anything myself other than what I have listed but I have been unable to find the cause of the problem up to now.
I would like to be at least a bit informed of what to look for if I had an electrician come in to run tests on the wiring and equipment.
Or what to ask the repair depot in regards to what tests they have performed on the products I have given them.

Like I said the pre amp was tested, the speaker that failed was repaired and tested.
The cable box was checked by the company after the first failure and will be again.

I am looking for answers and did not mean to denigrate your responses.
I just don't want this to happen again and would just like to get back to enjoying surround sound and not worrying about another failure.
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post #7 of 12 Old 04-23-2013, 10:10 AM
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Quote:
Originally Posted by kiwi2000 View Post

I was asking in my first post who I could get to troubleshoot my issue.
Not the cable company. And not an electrician.

"All men are frauds. The only difference between them is that some admit it. I myself deny it."
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post #8 of 12 Old 04-23-2013, 01:22 PM - Thread Starter
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sivadselim wrote
Quote:
Not the cable company. And not an electrician.

A preference in these responses would be to highlight people or services that might be able to assist not the ones who cannot.

I am wondering if the problem relates to the method of connection.
I am able to enjoy surround via analog inputs of my turntable.
I am able to enjoy surround via HDMI on my OPPO.
The cable box is connected to the pre ampliifer via toslink.
But that said I have a 5 pack CD player connected via toslink as well.

The speaker manufacturer and preamplifier help lines are snookered as am I.
I do not want to try full surround with my 5 channel until it can be determined what is the cause of these faults.

The speaker help line stated that if it was their product it would simply not work. A hard short.

The RCA outputs from the pre amp might be the culprit but they have been tested an will be again.

The speaker manufacturer and preamplifier help lines are snookered as am I.
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post #9 of 12 Old 04-23-2013, 05:24 PM
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There's no way to get DC across TOSLINK (unless the receiver in the AVR is broken) as it is an optical link. That should also break a ground loop, another common problem with cable. Unless there's another path, like cable box to TV to AVR/pre-pro.

I would try using the RCA digital or analog outputs from the cable box to see if that helps, but that may point to a problem in the AVR and not the cable box. I would ask the cable company to just replace the box, though that may not be the problem.

Cable shows tend to be more compressed, meaning higher average power level. Is it possible you are simply playing the center too loud for too long? There should be some hint of that, like distortion or a (mechanical) rattling sound. Did you notice any symptoms before everything blew, noise, pops, anything?

I think taking the speaker, amp and pre/pro back to the repair place as stated in your first post is a good idea. Be aware this may be a difficult issue to troubleshoot. Of course, it is possible the amplifier is bad and it will be a quick fix. I would also let the tech know how long your speaker and interconnect lines are (BTW, how long are they?) in case there is some sort of oscillation going on. If you have a techie friend, have them bring an o'scope and look at the signal into the speaker to make sure nothing funny is going on before returning everything to the repair place.

HTH - Don

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #10 of 12 Old 04-23-2013, 07:05 PM - Thread Starter
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DonH50
Quote:
Is it possible you are simply playing the center too loud for too long

I was playing all day and the next with the OPPO, CD's, SACD's etc in surround with 5 channels not loud,(too old) . No issues.
I had even calibrated with audussy which is loud no issues.
I forgot about the problem as I thought it was confined to the original amplifier.
That second evening with the loaner 3 channel amp put on the TV and within 5 minutes the surround amp was toast. While listening to below background level sound.
Quote:
Of course, it is possible the amplifier is bad and it will be a quick fix

The original 5 channel blew yes, but when I incorporated different amplifiers, this was 5th different model actually since the fault and added surround, toast.
The original 5 channel has still not been incorporated since the initial fault occurred. As previously stated the 5 channel amp blew bypassing protection along with the speaker doa a costly repair.
I can confidently state it is not the 5 channel amp as there have been too many. It was not involved with this latest fault.
It has Something to do with selecting surround, whether that be the the cable box, cables,processor, OR?.

To commsysman who wrote
I
Quote:
t is pretty obvious that the speaker is shorting out after a period of operation and causing the amplifier to protect itself. There may be a breakdown occurring in the insulation of a voice coil.
I would get a new center speaker

Not so obvious to me, the speaker has been repaired and thoroughly tested. It has worked flawlessly for the past several years as have all other of this brand speakers over the 30+ years I have owned different generations of them without ever an issue.
I thought it a no brainier to replace the amp which is what I was in the process of doing when I connected for a 5 channel setup and have relayed the results.
It is not the amp and I do not believe it is the speaker either.

Actually everything audio in my system has performed without issue until this year, yet nothing has been added or changed in regards to cabling or components.

Cable company tomorrow,repair shop next week for processor and speaker, again.
I sure would like a different, (helpful) suggestion here, I am sick of this, it has been months.
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post #11 of 12 Old 04-23-2013, 07:24 PM
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Sorry I was no help. Good luck, hopefully somebody can help you. - Don

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #12 of 12 Old 04-23-2013, 09:25 PM
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Well, if you are interested in more testing yourself...I would swap the center speaker and one of the other fronts. See if it blows the center channel fuse on the amp again. If it does, then the speaker is fine. If it doesn't then it is the center (or wiring).

The amp doesn't seem to be a problem as you used multiple amps and it is unlikely a problem with multiple amps...

The preamp seems to work fine in surround from it's toslink input and in surround mode. You could switch a "known" working toslink with the cable box and see what happens. It could be a defective input on your pre?

I would try a new interconnect...could be short in the center channel cable? You were plugging and unplugging multiple amps...could be the issue...

I'm not sure how the cable box could be putting out a signal that your preamp would receive and cause your trouble, but I'm not as technical as some...

You can check for DC with a multimeter. Just set it to DC and attach the leads to positive and negative (or signal and ground). DC would usually cause a quick and loud damaging sound...not sure if that is the issue.

Good luck,
B.
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