HI All,
and Peter D in particular ...
I have been troubleshooting a Behringer BX4410A bass amp ... of course without the help of a schematic thanks to Behringers policy on service.
Anyway, the intial findings were that the amp turns on but the signal cuts in and out ... when it's in, it sounded normal ... then would crap out and sound distorted and just generally crappy. So as you play signal through it it would pop in and out and more so when either a volume or tone control was adjusted up and down.
So after a really good look at the output section for problems ... none seemed apparent.
So then did an isolation of the input section (pcb) and determined 98% that the input brd was good.
Went back to the amp pcb ... still found nothing using a scope to try and trace signal path.
Gave up on it for a week or 2 to come back to it with a fresh eye.
This time there was ZERO sound coming out ... so this was odd. Went and checked output transistors again and almost all were dead shorted. So something finally gave out and cooked most of the output semi's.
After not finding anything on the 1st go round my best hunch was to just replace all the output semi's and now that they are all fried, I went an ordered parts and installed.
Unfortunately, While removing the last component ( one of the driver transistors ), I somehow managed to damage a pcb trace.
So here is where the trouble starts ... looking under very high mag, the trace can only have come separated from one pad. Problem is according to a schematic which is supposed to be a near clone of this amp ( that being the QSC1400 ) that connection 1st of all isn't there and 2nd doesn't make much sense to me.
And during my initial semi-conductor tests ... didn't show up ( as far as I can remember ) as a short between Base and Emitter on this particular driver tranny. If it did, I would have immediately suspected it of being bad ... and remove it to verify out of circuit.
Here's a pretty good pic of the damaged trace in question ...
Hmmm having problems uploading image(s)
I'll post this thread and see if I can edit it for an upload after.
Not sure if it's a size issue ... don't think so as I posted fairly large files in the Yamaha Stagepas thread. Is there a file size limit ? If so what is it ?
Here's the QSC1400 schematic that is supposed to be a near clone.
Notice there is no connection on either drive transistor showing base to emitter direct.
A connection like that in my mind wouldn't be good. The driver tranny in question is the NPN at the top that drives the main PNP's.
and Peter D in particular ...
I have been troubleshooting a Behringer BX4410A bass amp ... of course without the help of a schematic thanks to Behringers policy on service.
Anyway, the intial findings were that the amp turns on but the signal cuts in and out ... when it's in, it sounded normal ... then would crap out and sound distorted and just generally crappy. So as you play signal through it it would pop in and out and more so when either a volume or tone control was adjusted up and down.
So after a really good look at the output section for problems ... none seemed apparent.
So then did an isolation of the input section (pcb) and determined 98% that the input brd was good.
Went back to the amp pcb ... still found nothing using a scope to try and trace signal path.
Gave up on it for a week or 2 to come back to it with a fresh eye.
This time there was ZERO sound coming out ... so this was odd. Went and checked output transistors again and almost all were dead shorted. So something finally gave out and cooked most of the output semi's.
After not finding anything on the 1st go round my best hunch was to just replace all the output semi's and now that they are all fried, I went an ordered parts and installed.
Unfortunately, While removing the last component ( one of the driver transistors ), I somehow managed to damage a pcb trace.
So here is where the trouble starts ... looking under very high mag, the trace can only have come separated from one pad. Problem is according to a schematic which is supposed to be a near clone of this amp ( that being the QSC1400 ) that connection 1st of all isn't there and 2nd doesn't make much sense to me.
And during my initial semi-conductor tests ... didn't show up ( as far as I can remember ) as a short between Base and Emitter on this particular driver tranny. If it did, I would have immediately suspected it of being bad ... and remove it to verify out of circuit.
Here's a pretty good pic of the damaged trace in question ...
Hmmm having problems uploading image(s)
I'll post this thread and see if I can edit it for an upload after.
Not sure if it's a size issue ... don't think so as I posted fairly large files in the Yamaha Stagepas thread. Is there a file size limit ? If so what is it ?
Here's the QSC1400 schematic that is supposed to be a near clone.
Notice there is no connection on either drive transistor showing base to emitter direct.
A connection like that in my mind wouldn't be good. The driver tranny in question is the NPN at the top that drives the main PNP's.
Last edited:
