Behringer power amp burnout

[stevekendal]

Just got a nice little second hand Behringer BX1200 bass combo and after just one quiet session it developed a serious hum after I plugged in the footswitch and then no sound at all, with the pong of something burned out. Nothing looks melted inside and I have the schematic, although I am much more comfortable with valve stuff, so can anyone help me out please? There are two 3886 chips , should I just replace those and see how I get on? Steve.

[Enzo]

Maybe. Is there DC on the speaker? That points to failed 3886s, so then I would replace both even if one SEEMS OK.

But you could also have lost a main filter cap. Find the two main power supplies and see if they both have about the same voltage, though at opposite polarities, and that both are free of ripple.

Same with the 15v low voltage supplies.

And you may have lost one of the op amps, usually a TL074. The corner pins on those are outputs, any in the signal path stuck on DC?

[soundguruman]

It will make a "fine" boat anchor.

[J M Fahey]

I guess it uses those LM3886 in parallel, an idea which scratches me the wrong way, ugh!!
Now if they were bridged, that's something more sensible.
Can you post the schematic to check it?
Thanks.

[stevekendal]

I guess it uses those LM3886 in parallel, an idea which scratches me the wrong way, ugh!!
Now if they were bridged, that's something more sensible.
Can you post the schematic to check it?
Thanks.

Its here, in post 6

http://music-electronics-forum.com/t22260/

[stevekendal]

Sorry, I dont know how to do the link, but if you type into the search on this forum "need Behringer BX1200 schematics", you will find it straight away. Steve.

[mozwell]

schematic behringer BX1200, also BX1800 for posterity

[soundguruman]

I guess it uses those LM3886 in parallel, an idea which scratches me the wrong way, ugh!!
Now if they were bridged, that's something more sensible.
Can you post the schematic to check it?
Thanks.

Two pennies in parallel can take way more current, than one penny in series.

[J M Fahey]

Ugh !!
Terrible designs!!
Parallel chipamps are a bad idea, but this implementation is even worse, because each amp has its own feedback network.
Makes matching ultra ultra necessary, and the smallest error sets one chipamp actively shorting the other. Double ugh!!
And the 1800 is not that better, it relies on power Mos providing voltage gain, and there is not local loop, NFB goes end to end "outside" the amp.
Unstable as h*ll.
Well, it's not a moment to redesign, just replace both chipamps and cross your fingers.
And (a lot of) good luck.

[steve in the uk]

What happened to my user name? One chip shows 42 volts on pins 1 and 5, zero on pin 3. Other shows 42 on pin 1 and. 3, zero on 5.

[soundguruman]

What happened to my user name? One chip shows 42 volts on pins 1 and 5, zero on pin 3. Other shows 42 on pin 1 and. 3, zero on 5.

You have been assimilated. Resistance is futile.

[Jazz P Bass]

One chip shows 42 volts on pins 1 and 5, zero on pin 3. Other shows 42 on pin 1 and. 3, zero on 5.

Something is amiss with IC #2 voltages.
Pin ! & 5 are tied together on the schematic.
And pin 3 is two small value resistors (R10 & 11) from pin 3 IC #1.
I would check R10 & 11 for opens.
And verify the circuit trace from Pin 4 to 5 on IC #2.

[stevekendal]

Well I got a pair of power chips but, with so little spare time, I passed it on to another repairer, for him to fit. He just reported back that there are several tracks on the pcb burned through, which is borne out by the pong it made when it died. Says its a considerable job making this good. How likely is it that other components have caused this? Might just get it back and fix it at my leisure

[stevekendal]

Well I got a pair of power chips but, with so little spare time, I passed it on to another repairer, for him to fit. He just reported back that there are several tracks on the pcb burned through, which is borne out by the pong it made when it died. Says its a considerable job making this good. How likely is it that other components have caused this? Might just get it back and fix it at my leisure

Well, I just got the power module back off the other guy and he's made a lovely job of fitting the new chips, but said it still wasnt working and had him baffled. I plugged it back in and hey presto, we're back to the deafening hum, but with no signal output, and no frying either. Filter cap perhaps?

[J M Fahey]

You can test and discard (pr confirm) that hypothesis in 5 minutes.
Just disconnect that poor speaker to avoid firther damaging it, and then read +V and -V rails.
Not indicated in the schematic but I guess should show around +/-30V, or a little less.
Highest suspects anyway are still LM3886, which even if new , are made to fight each other because of the stupid design
*Personally**, I'd remove them and components directly linked to them and mount a regular (discrete) power module there, using the same heat sink, if space permits.
Can you post a picture of the power amp and heat sink area so we know what we have there?
Unfortunately I think the PCB with burnt tracks is beyond repair.