Need help fixing a 3886 based Behringer Studio Monitor amp!

[sylk]

I have searched everywhere but apparently Behringer does not release schematics and none have been posted on the net anywhere I could find.

I have an oscilloscope, how would I check for parasitic oscillation? I use multimeters quite often, however I only use my oscilloscope to check for square waves and so will need detailed instructions.

[sylk]

After reading the manual for the lm3886 chip I realize where the popping is coming from. The chip has built in thermal protection that will shut itself off once it hits 165c and then start operating again once the temperature has dropped to 155c.

The chip driving the tweeter never overheats and so I get sound even though the other two chips are cycling.

So now I guess I need to figure out what is causing the overheating.

If it is an oscillation than I can just order two new lm3886 chips and I should be good to go correct?

[Th3 uN1Qu3]

Just put your oscilloscope between ground and speaker output terminal and look for a high frequency sinewave. It's likely to be over 100kHz.

Though it is likely they simply went bad, and would not oscillate under normal conditions. New chips should fix it. Then find out why they overheat - if the heatsinking is inadequate that calls for some rework. Good old Berry always skimps on heatsinks.

[abraxalito]

Originally Posted by sylk

If it is an oscillation than I can just order two new lm3886 chips and I should be good to go correct?

Well changing the chips might fix up the symptoms of the disease but not address the cause. I've played with LM3886s and I've found they're quite hard to blow up when they're in a circuit that's basically functioning. So there might be some marginal layout (poor decoupling, too much track inductance somewhere) or sub-standard design which is leading to them overheating. Or another alternative is that what's driving them (opamps, active filters presumably) is oscillating and the chip amps themselves aren't at fault. That's why the oscilloscope is invaluable - to spot where the problem starts.

<edit> Do you know if the LM3886s are operating in bridged mode? This means the bass/mid drive unit is connected between the two outputs of the two (overheating) chips. The other alternative is they're paralleled, meaning the two outputs are connected together via some low value resistors.

<later edit> On examining the pics you've supplied (nice to get such hi-res shots!) it looks like they're using a couple of (0R1?) resistors and the amps are paralleled. So one thing to check is - lift one leg of one of the sharing resistors and see if the overheating still occurs.

[DownloaderVideo]

Hi, i have the exact same problem with one of my monitors, i already changed the 3 LM3886T and no change, i have the schematic available, anybody has been able to locate the problem??

Thanks in advance

[DownloaderVideo]

behringer_b2031

[jvande]

I have this same problem with my TRUTH B2031A. The only way to keep them on is to setup fans right behind each one. I'm hoping I can find a better solution to keep these things cool. Seems to me this design wasn't the greatest. I have had these for 5 years and have never had them operate without fans. Sadly I kept them in a closet for 3 straight years because of this.