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Beringer powered PA goes into Over Load

bluebird28

bluebird28

Guest
Feb 25, 2012
497
43
Tacoma. WA
#1
I offered to help a friend that is using a Beringer powered PA that keeps going into Over Load after a short time period. I pulled the amp section to look at the circuit board. There are no burn marks or burned resistors or transistors, the board looks fine as do all the components on the board however the power transistors (4 ea.) attached to the heat sinks are soldered on both the top and bottom of the board the top solder joints are dull gray and the bottom are shiney. Do you think that resoldering the top solder joints would help this situation, or do you have any other suggestions? Your help is greatly appreciated.
 
#3
Yup, power transistors are getting hot. Check his speaker impedance and the gain structure being used.

Before long, somebody will come along and bash powered mixers. Just let them know that if you can't get a workable sound out of a powered mixer, you can't mix.;)
 
#4
I should also mention that while the PA was turned off I plugged my bass cable into the input with no bass attached at the other end, I turned on the amp, it was fine then popped and went into over load mode. Turned off the power, plugged in my bass, turned on the amp and it has run fine so far today. Should I just resolder the top side at the power transistors where the gray solder joints are? There are eleven leads on each power transistor and there are 4 of them.
 
#5
bluebird28 said:
I should also mention that while the PA was turned off I plugged my bass cable into the input with no bass attached at the other end, I turned on the amp, it was fine then popped and went into over load mode. Turned off the power, plugged in my bass, turned on the amp and it has run fine so far today. Should I just resolder the top side at the power transistors where the gray solder joints are? There are eleven leads on each power transistor and there are 4 of them.
Click to expand...

Wait, stop. There can not be 11 leads on a power transistor. You must be looking at something else. Modern solder alloys are not shiny, that's not the problem.

If you do not understand how the circuit works, you really need to recommend that your friend take it to somebody who does.

I assume that you have verified that the speaker loads are proper (nobody "repaierd" a speaker incorrectly) and that there are no shorts in the speaker cables. Both ofthese will cause protection circuits to activate. This is 100x more likely that what you proposed above.
 
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#6
OK it is a powered speaker, the only speaker in the application. The speaker is original i.e. never been worked on or replaced. There are 4 ea. electrical devices only bolted to huge heat sinks on either side of the circuit board. There are 11 pins in each of these devices which are the only devices attached to heat sinks. I have built stereo amps, tuners, equalizers, from kits and a solar charge controller from printed instructions and I used to modify cash registers to have an RS232 port to connect to a computer for point of sale systems. Your right I do not know how the circuit works as I do not have schematics. I was looking for obvious problems like a blown cap, resistor, burned trace or pad on a circuit board. I no longer own a logic comparator. The solder joints on all the other solder pads are shiny including the bottom side of these electrical devices just not the top. So if they are not power transistors what are they? Any power transistors would be anchored to a heat sink correct?
 
#7
You sure seem to want someone to tell you that the solder joints are the problem, but they're most likely not. How have you got the powered speakers attached to the powered board? Your board's power transistors shouldn't be doing anything, so I don't see how they're getting hot. My bet is that something is not connected properly.
 
#8
So now it's not a powered mixer but a powered speaker, possibly with a simple mixer frontend (mic/line input, probably bass, treble, and volume knobs.

What exactly is the product and what is the entire signal chain from player to speaker?
 
#10
Logic comparator is not going to be of any help.

Since there are no power transistors that have 11 pins that I am aware of (I amp aware of just about everything out there power transistor-wise) then you need to consider what kinds of options are there that contain 11 pins and are connected to a heatsink. That leaves just 2 options that I can think of... an integrated circuit power amp like an LM3886, or a class D integrated output module but that's about as far as you will be able to get withour service info unless you are pertty darn familiar with how this stuff works at the circuit level.

If you were to see visually 1 bad part, there are almost certainly others that you can't see. Generally bad parts are not obvious to the eye, that's how this world works.

Don't worry about what the solder joints look like, it has nothing to do with your problem.
 
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#14
It is a Behringer B300 power speaker and Agedhorse is correct they are LM3886's audio amp's. I just got the schematic downloaded. I turned this thing on this morning with just my guitar cable plugged in and after a minute it went into overload. I turned it off plugged in my bass, turned it on played at full volume on both the powered speaker and my bass for 1/2 hour and it did not go into overload. I then unplugged my bass and just left the amp on for 3 hours and no problem. When they use it, it has done the same thing.

I drive an old 1984 Mercedes Benz and the electricals have needed resolder jobs on the speedometer cluster in order to get the gauges working. The heater unit also needed resoldering in order to work properly. The reason I drive such an old vehicle is I drive on Waste Vegetable Oil.

Mass production wave soldering requires resoldering over time in my humble experience. Capicators can blow and the unit can still function in some situations.
 
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#15
If the powered speaker is the only one driven then the ohm load isn't the problem. Some other component that still looks OK visually must have failed. If the warranty hasn't expired, including any add-on warranty from your credit card company (mine adds a year of free extra warranty), take it in for service.

DWBass said:
I hope the PA is not powered and he's sending a 'powered' signal into the powered speaker!
Click to expand...
Someone at my church connected the powered output of a several hundred watt powered mixer to the line input of a multi, multi, multi-thousand dollar DSP unit controlling the house speaker system. The system survived! That was miraculous, really.
 
#17
Hi.

bluebird28 said:
I turned this thing on this morning with just my guitar cable plugged in and after a minute it went into overload. I turned it off plugged in my bass, turned it on played at full volume on both the powered speaker and my bass for 1/2 hour and it did not go into overload. I then unplugged my bass and just left the amp on for 3 hours and no problem. When they use it, it has done the same thing.
Click to expand...

bluebird28 said:
After having only given it the hairy eyeball, touched nothing, I turn it on wait 10 seconds and it now goes into overload.
Click to expand...

Since You even do have the schematic now, why don't you just systematically diagnose the problem instead of still hoping that cycling the amp on and off will blow up something you can then replace, hoping that it'll fix the problem?

My vote would go for the control circuit of one of the LM3886's, or one of the chips themselves.
Very easy to measure/diagnose.

Or the protection circuit itself, which can be a bit trickier to diagnose.
You can very easily bypass/defeat it though for testing purposes if you know what you're doing.

bluebird28 said:
Mass production wave soldering requires resoldering over time in my humble experience.
Click to expand...

Wave soldering has nothing to do with it, it's ROHS and the companies who use cheap PCB material that can't take the elevated heat required for soldering high mass component leads properly.
Especially with lead free solder.
That causes cold solder joints, obviously.

On those cases, re-flowing may help, or it can lift the trace.
The puny legs of the LM chip in question IMO do not fall into that high mass category BTW unless everything else is SMD.

The dullnes You see on the top side most of the wave soldered conventional PCB's is just the flux, oxidation and dross that rises onto the surface when the soldering is done.
Has absolutely no impact on anything but the appearance.

Regards
Sam
 
#18
The LM3886 does not have any kind of protection circuit that works like this. There may be something the manufacturer of the product has designed into the amp but it's not part of the 3886. (I am intimately familiar with this part, INCLUDING the potential for counterfeits in the marketplace). There is something else going on and unless you have proper test equipment (scope, signal generator, load resistors, DMM minimum), know how to use it, and an UNDERSTANDING of the schematic and what it means, you are more likely to create an unrepairable mess that any respectable tech would walk away from.

This is nothing like soldering a wire. Really.
 
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#19
agedhorse said:
The LM3886 does not have any kind of protection circuit that works like this. There may be something the manufacturer of the product has designed into the amp but it's not part of the 3886. (I am intimately familiar with this part, INCLUDING the potential for counterfeits in the marketplace). There is something else going on and unless you have proper test equipment (scope, signal generator, load resistors, DMM minimum), know how to use it, and an UNDERSTANDING of the schematic and what it means, you are more likely to create an unrepairable mess that any respectable tech would walk away from.

This is nothing like soldering a wire. Really.
Click to expand...
Harsh, man, harsh.
 
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