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Behringer Ultrabass BX4500H Issues

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  • #16
    P3000REVE.pdf
    Found it.
    Last edited by tdlunsfo; 12-10-2018, 03:25 AM.

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    • #17
      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.
      Thatīs 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.
      Juan Manuel Fahey

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      • #18
        "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.
        Education is what you're left with after you have forgotten what you have learned.

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        • #19
          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?
          Last edited by tdlunsfo; 12-10-2018, 02:56 PM.

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          • #20
            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
            Last edited by tdlunsfo; 12-10-2018, 09:51 PM.

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            • #21
              Wow, thatīs 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 Iīm out of ideas.
              Juan Manuel Fahey

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              • #22
                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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                • #23
                  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.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


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                  • #24
                    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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                    • #25
                      And zero ohms between power transistor collectors and chassis?
                      Originally posted by Enzo
                      I have a sign in my shop that says, "Never think up reasons not to check something."


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                      • #26
                        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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                        • #27
                          If your secondary is completely disconnected, it's not referenced to anything but itself- unless you left a center tap connected to ground.
                          "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                          • #28
                            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
                            Last edited by tdlunsfo; 12-12-2018, 01:44 AM.

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                            • #29
                              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

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                              "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                              • #30
                                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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