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Thread: Behringer Ultrabass BX4500H Issues

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    Behringer Ultrabass BX4500H Issues

    Looks like the schematic for this power amp is hard to come by. Maybe some of you that have access to it can help me out. The preamp is good. I am able to get clear sound out of the line out. The power amp output gives me a DC voltage starting at -9v and slowly drops to -2v and resembles something of a square wave on the scope. When I try to run a signal through it I can see an "attempt" to reproduce the negative half for a split second and it goes away. Turning post gain down and back up, it will do again then die. I have tested all the transistors with my DMM and I haven't found any issues yet. One thing I thought was odd was the optocoupler chip. I was not able to get a diode drop on pins 1 and 2. Pins 4-6 check just like a transistor.

    Heading to check voltages now and will report back. Any ideas?

    One thing to note is a previous repair company in town swapped the fan out for a 120V AC fan. Of course they wired it wrong and it is now dead. As a result, I have checked the power transformer and it is good. Not sure if the DC fan being deleted from circuit will cause any issues. Schematic would help there..

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    Found the 7915A is low at about 9v. Backing up to the main filters I have good connections, good ESR and capacitance. On power up with DMM across C2 I get -55v but it slowly drops to -20v. Likewise, C1 starts at 65v and rises to about 100v. Replaced the 7915 but same readings. All this while on teh dim bulb. Any of this make sense to you guys/gals?

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    Not sure what to do with this. Starting to wonder if one of the caps is going resistive with power applied or maybe I have an output transistor issue even though they test good. Thinking about cutting the traces coming out of the filter caps to see if the voltage returns to normal. I really hate doing that though. Any thoughts appreciated.

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    Are you still on the bulb limiter? If so, you must expect certain readings to act funny.
    You said C1 was going up to 100V, what is it's rating? Are you measuring to ground or across the actual cap terminals? Just offhand, sounds like a grounding issue or a measurement issue due to the bulb.
    Without a schematic there is not much anyone can understand, as far as component designations like C1 etc. I think that accounts for the lack of responses.

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    Sorry yes I should had been more clear. C1 and C2 are the main filters and rated for 80v. I am still on the dim bulb. Naturally I didnt leave it on long in this condition but I think I have that in the right direction now. I changed the filter that was dropping voltage to a 3300/80v I had on hand. There is still an offset between the filters but it's more like 56v vs. 49v instead of 100v vs 20v. Interestingly, I also had to jumper the main filters together. One of the solder joints wasn't making good connection to the other main filter. So I had two issues there. Once I felt comfortable with that, I started signal tracing and it appears I may still have a 7815/7915 regulator issue as I lose signal in the first opamp. Working through that now.

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    Old Timer J M Fahey's Avatar
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    One main cap lowering voltage while the other rises by the same amount stinks of grounded output QSC type power amp, reasonable to find that on a 450W unit.
    Although Peavey has used that architecture even in humble 60W amps, go figure.

    So for starters measure voltage caps twice: once relative to ground, then across each cap (from + to - leg) , and post both sets of results.

    For now do NOT connect speaker or any load there.

    Looks like the schematic for this power amp is hard to come by
    Behringer is very secretive about its schematics, mainly because most are straight ripoffs of known brands, so they minimize the paper trail.

    Preamp stinks of Ashdown designs, with the sub bass generator and the many band EQ; so power amp might either be also an Ashdown ripoff OR same or similar to other Behringer 450W class power amplifiers.

    That said, amps sometimes *can* be repaired without schematic ... by a VERY experienced Tech who has already repaired many units of a similar circuit, so he can recognize certain "landmarks" (instead of a "map" which would be the schematic) and orient himself.

    No schematic and not wide experience? ... forget it.

    Even a "normal" experienced Tech will have big trouble without it.

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    Juan Manuel Fahey

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    Just to clarify, when you say relative to ground are you referring to chassis ground? I wouldn't call me inexperienced but certainly not to the level you guys are. I do work for a local consignment store fixing most of the broke stuff they buy in cheap. I've learned a lot the last 3 years. With all that being said my voltages across the filters have stabilized (not saying that's totally fixed yet), confirmed my 7915/7815 are ok, just not getting rail voltage on the opamps. What I don't know is if the regulators feed the opamps. I also see some zeners..just hard to tell without a schematic and this double sided board.

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    This is a grounded collector design (as Enzo calls 'flying rails') like JM Fahey suspected.
    The QSC MX700 is similar, you can find that schematic here: https://elektrotanya.com/qsc_mx700_m.../download.html
    Yes, chassis ground should work for grounding unless you have things taken apart (boards lifted), then you might need to find circuit ground.

    If anyone has the Behringer P3000 power amp board schematic, please post it.

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    You bring up a good point with the grounds. I do have boards flying around and the heat sink removed. Honestly I am having trouble finding a good power gnd with the boards like this. Maybe gator clip the negative speaker lead to chassis? Unfortunately you can't get a probe on this thing without removing the board and you can see any components without removing the heatsink. I am thinking of passing on this thing. A previous tech removed the thermistor and replaced the fan with (now defunct) AC fan. So without a schematic plus a modified circuit, double sided board, plus the modified circuit possibly being tied to the mute (if it is like the QSC).. the cards are stacked against me. The only clue I have to any component failure is a small black mark on the piece of clear plastic insulator that was screwed to the bottom of the board. This happens to line up to the negative of one of the main filters... not real sure what to make of that.

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    Check that the filter cap solder connections are good. If they get shaky, they can start arcing and make little burn marks like that. Sometimes you have to remove them and scrape the carbon off the leads just to get them to take solder again.

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    I have and even swapped out caps. I have tried grounding the negative speaker wire to chassis and then I tried the collectors to chassis. No matter what I do one the main filters slowly drops toward zero and the other one slowly rises toward 111v. I even tried unsoldering the collectors on the side that is dropping, then I tried the bases. No change. Having trouble wrapping my head around this one.

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    Old Timer J M Fahey's Avatar
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    Try this
    So for starters measure voltage caps twice: once relative to ground, then across each cap (from + to - leg) , and post both sets of results.
    And tell us where do you put your black probe.

    I guess what you call ground is actually not.

    Ground is NOT the wire joining main filter caps.

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    Juan Manuel Fahey

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    Referencing the QSC schematic, I grounded the negative speaker lead to chassis then put my negative probe to chassis. Originally I was measuring across the cap only thinking I could use the wire in between CAPs as you mentioned. I recognize that error now. Not so sure my speaker ground works either. I guess I could use the collectors as ground but I just didn't want to risk shorting them with high voltage a few millimeters away on the emitters. Interestingly, using a resistance measure between collector and chassis, it appears to still be grounded to chassis through the preamp board so these extra ground wires to chassis may not even be needed. I still owe you CAP readings but I likely need to clarify my ground setup first.

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    Lifetime Member Enzo's Avatar
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    No no... speaker lead is not to be grounded. Check continuity from speaker to power transformer center tap ground... I bet you do not find any.

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    I assume you mean the small violet wire coming out of the transformer secondary? If so, I do have continuity to the speaker negative lead.

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    P3000REVE.pdf
    Found it.

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    Last edited by tdlunsfo; 12-10-2018 at 05:25 AM.

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    Old Timer J M Fahey's Avatar
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    Notice that:
    1) collectors are grounded.
    With the side effect that power transistors do NOT need mica or other insulators although they DO need thermal grease (a.k.a. "bird poop")

    2) main filter caps C2 and C7 centerpoint , the copper wire or track joining them is NOT grounded, by any means.
    In fact that wire/track IS speaker out "hot", go figure.

    3) yet that point IS connected to ground through resistors R23 and R24, so you might measure some "continuity" to ground from speaker out.
    That´s why instead of "continuity" which is an undefined value, we much prefer to speak the actual ohm value ... which in this case will be around 410 ohm without speakers and about 4 or 8 ohm with them.

    4) still waiting for main caps voltages measured both ways, so 4 values in total.

    5) amp seems to have 2 sets of ground connections, A and B (just to add to the confusion).

    I guess A is low level / signal / "clean" ground while B is high level / power 7 "dirty" ground.

    yet for no signal measurement both should work the same, and your black probe should go to one of those.

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    Juan Manuel Fahey

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    Lifetime Member Enzo's Avatar
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    "Continuity" is for electricians, we always want resistance. I was being informal when I used the term earlier, and I should not have done that.


    Look upper left corner of schematic, R25 links A and B grounds with 6.8 ohms.

    "A ground" is the power supply common. Like on anything else. "B ground" is the power AMP common, it is not a ground, even when we casually refer to it as such.

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    Enzo, I figured you would want resistance so I did measure it as well, 9.9ohms. This would be X11(2) to chassis. Fahey, I do see the items you listed. As frustrating as this amp is to work on, I have to admire the different design. Looks like that it has separate 15v supplies, one for the preamp and one for the power amp. This explains why I have problems with op-amp voltages as they seem to be "born" from the VCC supplies through zeners. My plan when I get home this evening is to attach a negative probe to X11(2) which is also "A GND" and then take the C2/C7 capacitor readings you guys have been patiently awaiting. Enzo, will it matter that I have the board disconnected from chassis and isolated from any metal? Anything else you like see while I am in there?
    Lastly, is the optocoupler simply a mute/fan circuit? I am having trouble following what is going on here. My X4 has been deleted. I assume this was the DC fan?

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    Last edited by tdlunsfo; 12-10-2018 at 04:56 PM.

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    Board is laying on a towel inside the chassis. All connectors are hooked up. No speaker is connected. Amp power applied and waited 30 seconds. Negative lead is on "A GND" at X11(2). Positive probe:
    C2+ = 9v and falling
    C2- = 7V and falling
    C7+ = -10mV
    C7- = -98V and rising

    Power cut, caps discharged.

    Power reapplied, waiting 15 seconds or so. Probes across the the caps with negative lead on the negative cap terminal and positive on the positive terminal:
    C2 = 3.6V
    C7 = 78V and rising

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    Last edited by tdlunsfo; 12-10-2018 at 11:51 PM.

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    Old Timer J M Fahey's Avatar
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    Wow, that´s weird.
    Basically expected voltages rising/falling relative to ground, but having them wildly shift across terminals, which turns grounding irrelevant, is unexpected, so for now I´m out of ideas.

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    Juan Manuel Fahey

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    Thanks for trying. If it was the shops I would be more inclined to try some more drastic tests. I have already had to repair a few damages traces from desoldering so I think I will send this one on it's way. I'll give it a few days just to see if anyone else has any comments. The only other thing I thought of was attempting to separate the supply from the amp through desoldering or cutting traces to see if the voltages returned...just not sure it is worth it.

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    Don't forget the joker g1's Avatar
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    One more thing I would check. Look at any lifted boards where they mount. Check all screw holes for traces that would contact a screw or any other mounting hardware. If you find any, jumper them to chassis.
    You have all the connectors in place, but sometimes grounding is done through other hardware.

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    Quote Originally Posted by g1 View Post
    One more thing I would check. Look at any lifted boards where they mount. Check all screw holes for traces that would contact a screw or any other mounting hardware. If you find any, jumper them to chassis.
    You have all the connectors in place, but sometimes grounding is done through other hardware.
    I actually tried this yesterday. Only four screws hold the board to chassis. I put the board in place and held it down while I checked voltages. Same result.

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    And zero ohms between power transistor collectors and chassis?

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    Power transformer to chassis is about .3 ohms. Collectors to chassis and power transformer is around 10 ohms.

    With my power transformer secondary disconnected, shouldn't my high voltage winding measured to chassis be pretty close in AC voltage from one side to the other? They are not. In fact there is a huge difference from one wire to the next, like 200v vs. 10v AC. Starting to think I have a power transformer issue... but then again I have looked at this thing so long I may be able to convince myself of anything.

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    Supporting Member The Dude's Avatar
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    If your secondary is completely disconnected, it's not referenced to anything but itself- unless you left a center tap connected to ground.

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    I do have a lower voltage winding on the secondary that has a center tap. I clipped the center tap to ground and got the following:

    Low voltage winding 19.4v on both sides to chassis
    High voltage winding 80v and 11v to chassis

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    Last edited by tdlunsfo; 12-12-2018 at 03:44 AM.

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    Supporting Member The Dude's Avatar
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    Maybe a picture will help. Notice there is no direct connection from one winding to another. Unless there is a center tap on the actual winding that you have grounded, those readings are not relevant. Each winding is it's own "entity". What is it you're trying to figure out? Just measure across the winding in question (outside to outside). If the voltage there is what it's supposed to be, the transformer is fine.

    Click image for larger version. 

Name:	Transformer.jpg 
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    I don't have that unfortunately. I have a preamp schematic and a power amp schematic and none of them show the transformer or any voltages. It took me days to find a version of the power amp schematic. It's a split supply but I have voltage all on VCC- even with the main filters disconnected. I've never experienced this. The voltage across the winding end to end is 96v. Based on what I read we are looking for something around +/-60v DC coming out of the power supply and feeding the power transistor emitters and other ancillary circuits. I see what your saying though.. I cant use the center tap of another winding..I see it and know better..Honestly grasping at straws at this point and probably time to move on. Just stubborn and hate letting one get the best of me.

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    Supporting Member The Dude's Avatar
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    Have you looked through this thread?

    https://music-electronics-forum.com/...992&styleid=12

    The schematic links are all pre-server-crash and dead, but there are a few posts with solutions. Maybe it will help.

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    I have and others on other forums as well. That's how I found my schematic for the power amp. My symptoms are similar but unfortunately all of the previous failed parts test good on mine. Will retest to see if anything has changed though.

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