Behringer BX1200 low Output

[oc disorder]

Behringer BX1200 low Output
Would appreciate some help with this.
Was fowarded to me after the power transformer had been replaced.
It still didn't work and the new one was getting hot !.
I found that there was full negative rail on the output.
Replaced both "chip amps" LM3886T and amplifier seemed to
behave normally but.....
Now can only seem to get 13v p-p into 4 ohm load.
See attached 'scope picture Pre-amp out 1 volt per div 2volts p to p
for comparison ...output just clipping on neg half cycle 2v per div
aprox 13volts...
I am a bit amazed to see the chips are not bridge mode which I was expecting
but in parallel , outputs seperated only by a 0.1ohm ww resistor.
A friend suggested so if one dies the other keeps going...;-)...
I presume the mute line for the 3886 is either on or off ie
no half mutes here...! so it must be a fault in the fancy
opto circuit quote from user manual
"Musical opto-limiter for maximum volume without distortion or pumping effects"
The fan comes on after about 20 mins andpower supply seems normal.
I would post the schematic as I don't mind forfiting my first born to Behringer
but I understand that the forum owners/moderators may not feel the same way.
However I know some owners of these amps that are perfectly happy with them
and have not had any trouble and consider them powerful for their size.

[Enzo]

If it were bridged we'd have another horror like the Marshall Mode 4, and it would blow things up left and right. Can't let any part of a bridged output touch ground ever.

I got no problem with the two in parallel and the pair of 0.1 ohm ballasts.

verify both 15v rails are OK for the op amps.

For that matter are both HV rails OK?

Does driving it from the power amp in jack make a difference? How about the preamp out signal?

The optolimiter won't clip the thing, will it? It ought to just turn down the gain when the threshold is reached. I mean the opto works like a trem opto in a tube amp. It can't react to the signal fast enough to clip.

The master stage is between the power amp jack and the amps themselves. Is the signal OK coming out of that master stage? IC10D pin 14 signal HPASS. DOesn;t clip or anything? NO DC offset?

[Gtr_tech]

Mode Four.....heh....snicker......high priced gahbage.....

Word from an informed source is the company may be sold in the near future and Jim's health is not the best these days. Hopefully it ends up with someone interested in building quality stuff, not watching this quarters profit margin report.

[oc disorder]

Thanks Enzo for your interest.
Can't say I've seen any mode 4 amps in this neck of the woods.
Havn't got the P.A.'s to keep up with them.
Look like a glorified valve state..seen heaps of them riddled with dry joints.
The conveyer belt must have been set on full speed through the solder bath !
Back to the Behringer..
Well the supplies are fine + and - 39v and approx + and - 15.5 volts.
However the mute on pin 8 is about - 2.5 volts so it looks like I have a problem with the smd components (shudder).
Have to get some more tooth picks !
On the Nat Semi LM3886 data sheet there is a graph "Mute Attenuation vs
Mute Current" so my presumption was flawed that its either on or off.
I'm going to attempt to disconnect R39 the 1k smd resistor that leads to both
pin 8's the mute pin .
Dunno how I'll go buying 2 different smd transistors ..last time i tried
I needed to buy a roll of 10,000 but I was able to solder in trad packaged trannies onto the smd pads. Not sure if there is enough room here for that.

[jrfrond]

FYI- The LM3886 is meant to run in parallel. You can do this "building-block" style and stack as many as required for the power output of your choice, provided you supply the correct voltage and current to the rails.

[oc disorder]

Yeah thanks John I'm a little out of touch with these latest marvels from Nat Pan.Some friends have been building "Gainclone " style amps and apparently getting amazing results.
Just stumbled upon Nat Pans AN-1192.pdf which details the parallel combination also has a 200W Bridged/Parallel Circuit using 4 of 'em...!
Also found this[CODE]http://www.shine7.com/audio/pa100.htm/[CODE]where some one has built a 100w per ch high quality PA amp similar to the Behringers back end.
Stll having problems with the Behr. Dissconected the 1k now nothing...
Used a 10 k from -'ve supply to mute pins approx 4 volts ..well at least its symetrically clipping..pity Behringer didn't put voltages on schematics..
Guess I better do some reading on the mute circuit .
No DC on the input and the pre out looks fine .
I attached a picture of the input and output traces on my first post.

[QUOTEThe master stage is between the power amp jack and the amps themselves. Is the signal OK coming out of that master stage? IC10D pin 14 signal HPASS. DOesn;t clip or anything? NO DC offset? ][/QUOTE]
Whoops
Yes Enzo just realised IC10 between insert jacks and power amp.
Input distorting way too early so I guess now I have to dissasemble the pre amp and find a smd TLO74.
(Can get a pack of 10 for $25 from www.jaycar.com.au)
Guess some one had a graphic or level booster in the loop
which fried everything after that..c89 100n should have blocked
any DC back to IC 10.
The continuing adventures of the Bell Ringer of Notra Dame...

[Steve Conner]

I had the honour of meeting Jim Marshall 2 years ago at a Marshall amps roadshow. He'd had a stroke and wasn't looking in the best of shape. I wouldn't be surprised if Korg have their eye on the Marshall brand, too.

Enzo, is that why the Mode Fours blow out? Because the sleeve of the speaker jack is hot, so if the body of the jackplug touches off anything grounded it'll short the power amps out?

[jrfrond]

[quote=Steve Conner;73819]....I wouldn't be surprised if Korg have their eye on the Marshall brand, too....

I don't get it Steve. Korg distributes Marshall already here in the States. Are you referring to a complete takeover once Jim passes on?

[Enzo]

Enzo, is that why the Mode Fours blow out? Because the sleeve of the speaker jack is hot, so if the body of the jackplug touches off anything grounded it'll short the power amps out?

Well, I am convinced that is ONE contributing factor. Can't ground anything. But also on the Mode4, it really does seem sensitive to which jack you use. There is a chart on the back showing which jack for what impedance. Also, they seem sensitive to connecting/disconecting the load while running. In short, all the common things we take for granted we can do, we cannot, and amps then blow up.

Just my opinion.

[Steve Conner]

It's just idle speculation. The whole world seems to be clumping into a few megabrands that suck all the smaller ones in like black holes. Fender bought SWR, Gibson and Loud Technologies bought everyone else, Korg already bought the Vox name and stuffed it full of DSPs, and like jrfrond pointed out, they already have a relationship with Marshall. As far as I know, Jim Marshall's daughter is currently running the company.

Enzo, maybe the Mode Four has some sort of current feedback thing going on. I know Marshall used the technique in other Valvestate amps: sensing speaker current with a resistor in the cold speaker line, and feeding that back to the power amp input.

That might explain why you need to use different jacks for different impedances, and maybe certain loads make the current feedback go unstable and cook the amp chips with oscillations? Again this is idle speculation too, I could be as wrong as the time I thought the Valvetronix amps were analog :-O

[oc disorder]

How to repair a Behringer?

[Steve Conner]

Q. How many Behringer service centers does it take to change a light bulb?
A. None, they just replace your whole ceiling under warranty.

Sorry for the off-topic digression

[ABELIS]

complete mudules

.. is there a way to obtain the complete amplifier module??

[Enzo]

Not readily, but where in this world are you?

[oc disorder]

Behringer BX 1200 finale

Well it turned out one of the new LM3886's was faulty.
Between input and ground it measured 1k4 approx and reversing the meter leads got 830 ohms approx.
The amp still worked but the unusual input load dragged everything down.
I had checked each chip amp by removing the .1 wire wound from one of the outputs and confirmed the same output from each chip.
So big lesson there like with some tubes..just 'cause its new doesn't mean it's ok !
Pity I hadn't struck it in an amp I am more familar with.
I guess one of the transistors in the chip input had failed.
False pre conceived ideas of lack of integrity gleaned from the internet didn't help either !
Now I understand the circuit 'spose I'll never see another one !
Murphys Law strikes again !
Thus ends the Behringer experience !
Now where did I put that glue gun...!