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What would cause a paging system amplifier breaker to trip daily?

Posted on 2008-10-01
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Last Modified: 2012-06-21
We have two Bogen C100 paging amplifiers in our wire closet with the phone system.  Recently, the unit designated for the shop speakers has been blowing the built-in breaker on the back of the unit.  Every few hours the receptionist lets me know that the shop isn't getting pages, and I reset the breaker.  The amps are both plugged into the "surge protection" side of an APC UPS.  

We have an old Executone phone system connected to the "aux 1" ports on both amps.  A few months ago, I reconfigured the amps into two zones so I could better adjust the volume and the load, and to get ready for zoning in our new VoIP system.  Until last week, it was working fine.  There are many options for the speaker voltage / resistance on the back of the amp, but I just maintained the same terminals that were setup some 15 years ago

I'm not sure if the incoming signal could trigger the breaker overload, the attached speakers (too many? short-circuited), or if the amplifier just needs to be replaced.  I'm looking for possible next steps in troubleshooting.  .  Thanks!
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Question by:wega1985
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warrenbuckles earned 450 total points
ID: 22621661
I doubt your problem is coming from the AC power side, especially since the power is coming through a surge protector AND the problem is recurring.

To state the obvious, a repeated breaker trip is a sign of something wrong.

It could be environmental, a faulty breaker, an internal overload or an external overload.

Sometimes a hot environment can make a breaker more sensitive to load - is the wiring closet hotter than it was before?  Is the problem amp at the top of the stack where it gets more convective heat?

A faulty breaker is possible - you could have it replaced by an amplifier repair shop, but I would look for other causes first.

An internal overload is possible - filter capacitors in the power supply dry out with time and allow more leakage current.  If you can look inside the unit, look for signs of leaking/swollen capacitors.  If you can safely touch the cans, check to see if one is significantly hotter than the other (or its corresponding unit in the good amplifier).

Another source of internal overloads is a failing power transformer - I don't know how the power supply in those units is configured, but it may have an input transformer.  Again, look for signs of heat (scorched paint, for example) and compare the good with the faulty unit.

Resistors can also fail - sometimes cracking from thermal cycling.  They won't cause a short since they usually fail open, but a failed resistor can cause problems in voltage regulator circuits or in the amplifier circuits themselves.  Once more look for heat damage and hot components.

Depending on the output circuit configuration, a failed drive transistor can overload a power supply,  This should result in problems with sound quality, which you haven't mentioned, so I would discount this.

On the output side, it is possible there is a problem with the wiring to the speakers or the speakers themselves.  I assume you have several speakers and checking the wiring could be very tedious, but swapping the amplifiers between speaker networks would give you a pass/fail indication.

Finally, the speaker configuration could be a problem - you mentioned changing this recently.  Again, swapping the amplifiers would indicate a problem with the speaker configuration (or wiring, as above).

Which terminals are you using now and how is the speaker network setup?  A 70V network is common when many speakers are involved.  In this type of network each speaker will have its own transformer (which could be a source of trouble, as well)  but reconfiguriation should be a simple task, provided you havent' put most of the speakers on one amplifier (this will cause severe sound quality problems, too).  On the other hand, a fixed impedance setup ( 8, 4 or 16 ohms) will rely on having a combination of series and parallel speakers that gives an effective input impedance of 8, 4 or 16  ohms.  It's possible the reconfigured speaker setup resulted in a low effective impedance on the shop network.  This will overload the amplifier and, possibly, cause the breaker to pop.  However, your problems didn't shjow up until some time after you reconfigured the system, so this seems the less likely cause.

wb
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by:wega1985
ID: 22623382
Thanks for the comprehensive response.  My first response is that yes, this unit is on the top of the stack, but the closet doesn't appear any warmer than usual.  I also noted that the unit is "buzzing" more loudly than the stable unit -- not sure if that is indicative of any problem.
It is a 70V configuration as you mention.  The speaker networks seem to be fairly even, 15-20 ceiling speakers on the office unit, 10-15 bull horn speakers on the shop unit.  As you mention, there is no notable sound detriment.
I will try swapping the units.
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by:wega1985
ID: 22623456
I also want to note, before the reconfiguration took place, our phone vendor installed a makeshift night ring into place due to limitations of our aging phone system.  It appears that he spliced the ringer/handsfree speaker of a spare phone into the input of the paging system, and setup a forwarded extension that rings over the PA after a certain amount of time.  My concern with this was that the ringer output may be overloading the circuit on top of the existing paging input.  I checked to make sure that the volume was turned all the way down on the ringer.
As the other amp is unaffected, and both were fine for quite some time following the setup, this also seems somewhat unlikely.
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by:warrenbuckles
warrenbuckles earned 450 total points
ID: 22624556
The 'buzzing' you mention is suspicious - it sounds like an overloaded transformer or filter inductor.  This could be caused by a failed filter capacitor or by another component failure in the power supply circuit.  This type of failure is progressive and (should) eventually cause the unit to fail completely

A go/no go test for the power supply would be to disconnect everything but the AC feed from the problem amplifier.  Energize it and see if the buzzing is still present - if it is, I would take it in for service.  Do this before/when you switch the amps, of course.

Wild speculation department: Another possibility is a ground loop caused by the speaker patch you mention - that sounds like an odd setup that might have tied together systems with different ground references.  You could try disconnecting the ringer input for a while and see if it changes the buzz/breaker trip pattern.

I would put my money on the unit's power supply.

wb

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by:knoxzoo
ID: 22629021
Sorry, don't really have time to read through what everyone else has said, so if I'm repeating, I apologize.  When you reconfigured, you likely changed the ohm loading on the amps.  The original install would have been set up so that the speakers were strung in series and parallel to provide the proper ohm load for the amps.  If you changed the setup without knowing which were in series and which were in parallel, you may have inadvertently configured one amp to an extremely high ohm load, all in series, and one to an extremely low ohm load, all in parallel.  

I'd just about bet the one that's blowing is dealing with a very low ohm load, resulting in much greater output power demands.
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Author Comment

by:wega1985
ID: 22633786
That sounds like it might be the case.  I wasn't aware that the series/parallel factor played such a large role.  
The original configuration had all of the office speakers PLUS most of the shop horns on one C100 amp (very old, went bad).  The second C100 amp had only a newer, small portion of the rear shop (6 speakers, maybe).  It made sense to me to put all of the shop speakers on one, and all of the office speakers on the other, when the first amp went bad.  The first amp dying completely is what led to the reconfiguration at all.
At this point, the best option to me would be to buy a third amp, because I do not want the office and shop speakers on the same volume control.  It's either too loud in the office or too quiet in the shop under that configuration.
 
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Author Comment

by:wega1985
ID: 22633922
I just happen to have some of the loads:
Amp 1:  Tripping
Back shop (originally alone on now-failing amp):  78© @ 68w - 6 horns
Front shop (I added to now-failing amp):  190© @ 6W (I'm assuming per)
Amp 2:  No problems
Office:  750© @ 6W, and the label says AT-10 @ full (?)
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by:wega1985
ID: 22633930
Looks like EE didn't accept the symbols; those are ohms.
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by:wega1985
ID: 22634735
A go/no go test for the power supply would be to disconnect everything but the AC feed from the problem amplifier.  Energize it and see if the buzzing is still present - if it is, I would take it in for service.  Do this before/when you switch the amps, of course.
The buzzing was indeed present upon disconnecting everything except AC from the amplifier.
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by:warrenbuckles
warrenbuckles earned 450 total points
ID: 22635355
In a 70 V system such as this the aggregate speaker impedance is not as important as the wiring of the input side of the matching transformers, which should all be in parallel.  It is possible to overload an amplifier such as yours by putting too many (parallel) loads on it but I have never seen one fail due to overloads of this type - the sound quality will be severely degraded but the system should not fail (this is <one of> the problem with subway/transit stations - too many loads).

A speaker connected directly across the 70V terminals will also bring the system to its knees, at least before the speaker blows out - the DC resistance of an 8 ohm speaker coil is typically in the 6 ohm range, and putting 70 VDC across 6 ohms will push more than 11 A through it for a brief and glorious moment, as 11 A through 6 ohms dissipates 11* 11 * 6 ~ 700 W.

On the other hand, an intermittent short circuit in the wiring or the transformers would be hard on the amp, but you should hear a crackling sound when this happens.  A dead short will result in no sound at all, not your problem.

Here's a nice writeup (takes from the manual of a competitor's amplifier) that shows a matching transformer and some system wiring, see Figure 3:

http://www.wescomponents.com/datasheets/100v_guide.htm

Regardless, it sounds like you have a(nother) bad amp - age may be the cause, but a look at the overall system wouldn't hurt.  You might be better off calling in a professional here - there are many companies around that specialize in private telephone/PA systems - look in the phone book under 'Communications Services' or related areas.

wb
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by:knoxzoo
knoxzoo earned 50 total points
ID: 22641591
I agree.  If the amp's buzzing, you've got problems.  Typically a diode gone bad in the power supply or switching leg(s), often caused by overcurrent demands of improper ohm loading.  And, since the loads you listed above are almost certainly parallel connected, that's likely the problem.  Newer amps typically have overcurrent protection that will keep them from cooking internal components when this happens, but older ones will just begin venting their magic smoke.  

As for ohm loading, it's a simple formula set, and can save you a lot of current and future headaches if you set it up right in the beginning.

Series Connection:
    If both of the speakers have an impedance of 4 ohms, the total impedance will be 8 ohms. In a series connection, you add the individual impedances. If there were three 4 ohm speakers in series, the total impedance will be 12 ohms.

Parallel Connection:
    The impedance change with a parallel speaker connection is only slightly more complicated than the series connection. When speakers are connected in parallel, the impedance is reduced. This means that, given the same output voltage, the current demand on the amplifier will be increased. If all speakers have the same impedance, the total impedance is the impedance of a single speaker divided by the total number of speakers. If you have two 4 ohm speakers connected in parallel, the total impedance is 4/2 or 2 ohms. You must be careful when paralleling speakers onto an amplifier. The impedance can quickly fall below safe levels.

You can combine the two simply by bracketing the series group and the parallel group, performing those sums, then doing the sum between the two.

One final note, as speakers age, the surrounds on their moving parts harden at different rates.  When that happens, they tend to pull toward the harder side, offsetting the speaker coil.  Over time, the coil rubs on the magnet surround and wears off the insulation on the coil wiring.  This allows for very short duration shorts within the speaker.  A short announcement, or one made by a female voice, might not short the amp to the point of kicking it off, but a longer one, or one made by a male voice (lower frequencies, like those in the typical male voice, cause greater travel on the speaker cone), or by playing music with more bass in it, might.
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Author Comment

by:wega1985
ID: 22888588
Just wanted to follow up on this issue.  I swapped the two circuits and the problem disappeared.  Possibly the newer amp is more able to handle the larger load?  Either way, thanks for all of your in-depth responses.
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