Help with troubleshooting Behringer BX1200 amp (lm3886)

[dubwoofer]

I'm trying to repair a BX1200 for a friend that produces a loud 120hz buzz when powered on. The amplifier uses two LM3886's in the output.

DC offset at the speaker jack is 40V and the chip on the left side of the heatsink gets pretty warm.

My friend says the problem started after he connected the direct output to a recorder with phantom power turned on.

I suspect that an output chip is bad (the one that gets warm). How can I be sure?

 

[DUG]

...

I suspect that an output chip is bad (the one that gets warm). How can I be sure?

Nothing on input, 40V on output:

Replace it.

 

[stratus46]

Nothing on input, 40V on output:

Replace it.

Maybe not. Blue Sky speakers are similar and main power supply caps are frequent failure items. Suddenly LOTS of 120 Hz buzz. It certainly merits checking.

G²

 

[dubwoofer]

Just checked esr on the filter caps. .03 and .06.

Yeah I'd say it's the IC beyond reasonable doubt...Ordered a new one.

 

[dubwoofer]

No Dice

I just replaced the output IC but the problem still persists: It gets very warm immediately when powered on and there's -42V DC and 5V AC on the output. Did I replace the wrong chip? Was the other one that ran cool the bad one?

I've attached a schematic if that helps

 

[dubwoofer]

I done goofed

Oh dear I'm an idiot...

I isolated the chips at the output resistors and discovered that the cool one was outputting -42V. It makes sense now that I think about it...

 

[martin martinez]

Hi! once i had that problem in the lo freq output of an amplified speaker, it had one 3886 in Hi freq section and two in BTL in lo freq; problem was the big PS caps were breaking from the board pads. It was solved with wire bridges!

 

[LasVegasIRA]

Sorry to resurrect a long dead thread, but I'm trying to repair one of these amps myself and I have a couple of questions.

Here's the story: at first the amp turned on and was putting out a pre-amp signal to the pre-amp out, but there was zero output on the speaker. I thought there was a problem with the power board because the cooling fan was also not spinning.

But after checking the speaker, I figured out that the cable was bad. Easy fix and the amp is now alive and putting out a great tone.

BUT, the fan still isn't turning on.

So, my questions are as follows:

1. Is the fan supposed to engage when the amp is first turned on, or is it supposed to engage once the power IC's reach a certain temp, or...?

2. Does the optocoupler on the power board have anything to do with controlling when the fan engages?

3. Has anyone else had a similar problem and if so, do you have any suggestions?

Thanks in advance for any and all help!!

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