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Thread: Behringer PMP1280S Power amp problem HCA2400

  1. #1
    Junior Member richardcook's Avatar
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    Behringer PMP1280S Power amp problem HCA2400

    A friend asked me to look at his PMP 1280S. He was told no one would work on the Behriingers. He bought a new Yamaha but really liked this mixer. He had been given a set of speakers and apparently one of them was shorted and when he connected it, the output blew.

    When I got into it, I found all of the output FETs were shorted and the 0.03 ohm sensing resistor was blown off the board. I got new parts and replaced them, but the channel still had -3.67Vdc on the output and the channel would not work. In the process of comparing the voltages between the working channel and non working channel, the probe slipped and took out one of the outputs on the good channel. I replaced it and the previously good side still didn't work correctly but only had about -1.04vdc on the output and would pass some signal.

    I have a schematic and found that the drains of the three output, on both channels were not hooked together. On the previously good channel, the drain connection between T24 and T20 was missing (where the slip occured) so I connected it externally. It now amplifies, but I think there is some other small issue going on that I'm not going to worry about until I get the other channel working.

    The original channel, with the blown sensing resistor was missing the drain connection from the L4 to T25, T22, and T23. I did connect it externally but the channel still has -3.67vdc on the speaker wires and the signal coming from the mixer does not appear on the gates of the mosFets. I'm sure the control circuit is shutting it down but I cannot find information about the 9001 controller and cannot seem to find the source of the imbalance causing the -3.67vdc on the output.

    I did read a posting about the Behringer amp boards having a design flaw and wonder if the original failure caused more damage to an inner layer than just the drains opening.

    Are replacement boards available, maybe a new power amplifier module I can retrofit into this mixer, or some other way to get this back up and running? Any help would be appreciated.

    Current situation

    Bad channel
    T25 gate=-89.2, drain=-3.67, source=-89.2
    T22 gate= 89.2, drain=-3.67, source= 89.3

    "good" channel
    T24 gate=-84.9, drain=-1.04, source=-89.3
    T20 gate= 84.8, drain=-1.04, source= 89.2

  2. #2
    Stray Cap DrGonz78's Avatar
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    Someone is definitely gonna post for you put a link to the schematic.... So just in case here is the link.

    Behringer PMP1280s - Electrˇnica - Esquemas - TV - ┴udio - Digital

    BTW maybe I did not read careful enough but is the good (i.e. previously) channel A or B?
    When the going gets weird... The weird turn pro!

  3. #3
    Supporting Member Jazz P Bass's Avatar
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    When those FET's blow they usually take out components around them.

    Check the zener diodes for sure.

    And the low value gate resistors.

  4. #4
    Junior Member richardcook's Avatar
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    Channel B was the "good" one

    Quote Originally Posted by DrGonz78 View Post
    Someone is definitely gonna post for you put a link to the schematic.... So just in case here is the link.

    Behringer PMP1280s - Electrˇnica - Esquemas - TV - ┴udio - Digital

    BTW maybe I did not read careful enough but is the good (i.e. previously) channel A or B?
    Thanks for the information but I already have the schematic. The original "good" channel was channel B.

  5. #5
    Junior Member richardcook's Avatar
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    Quote Originally Posted by richardcook View Post
    Thanks for the information but I already have the schematic. The original "good" channel was channel B.
    I checked the zeners and they are still diodes so they don't seem to be the issue. One read 0.73 on my fluke 87 and the other read 0.69.

    I'm wondering if I need to verify that all the connections to the drains, gates, and sources of the output FETs are still there since it destroyed several drain connections between the FETs. I did read that this board is notorious for burning internal traces and as we all know once the insulation material has carbonized from an arc its very difficult to recover.

    I've been unable to locate a replacement board after searching the net and e-bay. Does anyone know where to get a replacement board? I found several amplifier boards that would work and I could make them fit but their maximum voltage in is 50V and I have 90V. I don't want to alter the power supply.

    I'm starting to understand why, when my friend took it to his local music repair shop, they charged him $60 and then said "we don't work on Behringer amps." I'm starting to wonder if this is a lost cause????

  6. #6
    Lifetime Member Enzo's Avatar
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    Unless they were a Behringer authorized service center, one major snag for them is not being able to reliably get schematics.

    If they have a policy of not working on Behringer amps, one wonders what the $60 charge was for. If someone brings me an oboe to fix, I surely don;t charge them $60 to say "we don't work on oboes."
    Education is what you're left with after you have forgotten what you have learned.

  7. #7
    Junior Member richardcook's Avatar
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    I agree 100%. I've taken on paying work before and found, in the end, it was not economically repairable after I got all the way into it. Even this type of repair I usually eat and count it as a training cost for me. Not all repair centers are created equal (has anyone ever had experience with the Geek Squad! My experience is that it is a major ripoff), but I digress.

    Enzo, I've read several of your postings. What are your thoughts here? Since I know the inside layer of the board has been compromised, do I prepare my friend for the "its not repairable" reality or do I just need to learn more about the operation of the Behringer power amp and improve my skills in this area. I'm sure you've seen this amp before. Where would you go with it?

  8. #8
    Supporting Member Jazz P Bass's Avatar
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    I would check all of the connections, as you have suggested.

    If the mosfet circuit is sound, then that leaves the IXDD404 driver.

    I have replaced these in the past. The current part# is IXDD604 SI.
    Attached Files Attached Files
    Last edited by Jazz P Bass; 12-08-2013 at 06:43 PM.

  9. #9
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    Are these Behringer pmp1280s's worth fixing I've got one with 250k oscillation output on both channels

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