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Behringer BX4500H Power rail voltage differences

Discussion in 'Amps and Cabs [BG]' started by Scott MacKenzie, Jan 8, 2020.


  1. Scott MacKenzie

    Scott MacKenzie

    Oct 16, 2019
    No affiliations
    Hi All.

    Hope this is in the correct section.

    I'm repairing a BX4500. Power amp (P3000) has a big difference in the voltages of the power rails. I've re-flowed solder to all caps and transistors, checked all diodes, measured caps with an ESR meter. I'm about to throw it against a wall. Anyone have any good tips of where the issues may be? Schematic attached with measured voltages.

    Thanks for any help.
     

    Attached Files:

  2. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Respectfully, my first suggestion would be to understand how the curcuit works before getting in there with a soldering iron.

    First, this is a virtual copy of the QSC 1400 topology which is grounded collector and uses a floating supply. Any imbalance within the amp will cause all of the measurements to skew with respect to ground.

    There are 2 sets of low voltage supplies, the one for the power amp's diff amp is intentionally starved, the additional voltage comes from the amplifier's output itself in a form of bootstrap. In the event of a short on the output, the rails fall which limits the drive and helps protect the amp.

    This is a notoriously difficult amp to troubleshoot without understanding how all these circuits work.

    The point that you circled as no voltage looks like it should be 0V, it's a voltage divider between +/-15V which sets the bias for the non inverting input of the opamp, which looks like it may float to null out minor imbalance if necessary.
     
  3. Scott MacKenzie

    Scott MacKenzie

    Oct 16, 2019
    No affiliations
    Hey Agedhorse.

    No disrespect taken. I work on a lot of tube amps but this is a bit above my head. I assumed that the rails should be balanced but opposite polarities. With the AC voltage coming in fairly equal, but directly after the rectification the voltages become unbalanced, I thought something around that point was whacked.

    Can you point me towards some good info on this subject?

    Cheers
     
  4. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Read up on grounded collector amp topologies, they have been used by several manufacturers, including QSC and Crown.

    Global feedback can really skew what you think you are seeing since the transformer secondaries float from ground without the amp operating.
     
  5. Scott MacKenzie

    Scott MacKenzie

    Oct 16, 2019
    No affiliations
    Ok, thank you.
     
    agedhorse likes this.
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