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Behringer BX4410A Ultrabass Bass amp ... need help.

Discussion in 'Audio' started by Journey11, Jul 25, 2020.

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  1. Journey11

    Journey11

    37
    0
    May 23, 2018
    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 ?
    Damage.jpg
    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.
    qsc_1400 small.jpg
     
    Last edited: Jul 25, 2020
  2. Journey11

    Journey11

    37
    0
    May 23, 2018
    Hello again ...
    After more examination of this damaged area under high magnification and bright LED lighting ... I have solved my problem.
    Seems the broken trace is meant to simply run straight past that pad and around it. It's other broken end somehow got stuck/soldered to that pad and hence looked like it was meant to go there. The trace for the pad was found on the other side of the board. Problem solved on this issue but remains to be seen if replacing the output semi's will fix the amp in the end or there is still an issue somewhere else.
     
  3. PETERDECO

    PETERDECO

    53
    9
    Dec 19, 2019
    Keep me posted.
     
  4. Journey11

    Journey11

    37
    0
    May 23, 2018
    Well got it fixed ! After cutting out the damaged trace portions and running a jumper. I powered up and it still had the same problem.
    Flip flopping back and forth between clear sound and distorted crap. After running a few times, I wanted to check the new semi's to make sure they hadn't blown again and found one of the PNP power trannies had a different reading on DMM than the other two between base and emitter. So I removed it's corresponding 0.22 ohm resistor which I suspected had a bad/intermittent connection. While I was at it, I removed one or two more large 3 watt resistors and found two 150k 1/4 watters were in the way so after removing the 1st one found it was open or well into the Mohms. Replaced it and reassembled and viola ! Good as new !
    Nice amp by the way ... some serious tone control on this baby ... my client told me he only paid $200 for it new (which is what I told him I wanted for the repair and wasn't terribly happy about that ) ... so I kinda called BS on that unless it was at least a 50% sale ! Regardless ... after doing some shopping online ... to replace it with something way less started at around $65 for and old yorkville ... to get something as good started at $300 USD and went way up from there. So I think $200 was fair ... hell the parts costed $50 ! SO I basically was working for less than $5/hr ... but I look at these situations as a learning or experience thingy.
    I'm getting payed to go to school but not very much unfortunately.
    It's too bad I couldn't get a schematic ... sure would love to see where that 150k resistor was in circuit to see how it was causing this condition. If I didn't have the heatsink already re-compounded and remounted, I would have spent some time tracing the path's to see where they tied in. When researching online, I found many DIYers looking for advice and answers to fix these things so it's unfortunate I wasn't able to get more specific about this type of failure and post a utube vid on it.
     
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