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Thread: Help!!!, Behringer EPA900 Turns on but it does No sound, NO +-LV/HV

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    Help!!!, Behringer EPA900 Turns on but it does No sound, NO +-LV/HV

    Hello dears/colleagues, I humbly solicit your amiable help with this item Im checking and does not found any other bad functioning component, maybe some of you have had this same problem already:

    1) it powers on and all works fine except there is not any output sound.

    2) speaker cables and their connectors tested fine, speakers tested by separate with another amp set up and are fine.

    3) then I checked the EPA900 output with other speakers and still NO any output sound.

    4) then, disarmed it to start testing inside and the speakers outputs jacks were fine, no any short in them. There is not any evident component or printed circuit traces that looks burned so I began to check the voltages coming from the SMPS and noticed that the +-15 volts to the amp module pcb came fine but not the HV +-80V / LV+-40V voltages.
    Then tested the amp pcb components, I found one output side with one bad output transistor SAP16N and a mosfet, so I decided to replaced the four output transistors (2 SAP16N, 2 SAP16P) and the four mosfets (2 IRFP140N and 2 IRFP9140). Then I tested the rest of components and looks fine, no any other short or open parts. So I assembled it again to tested but followed without any output sound, so checked the voltages and was same with only the +-15V reading good but not any +-LV/HV. Because this, I then disconnected the SMPS X4 connector (8 lines), that takes those voltages to the amp, and this way all 6 voltages reads all fine. Ooooppss I thinked something that I made was bad so I tested the amp pcb components again but all tested good.

    5) believing that something might have gone wrong with the transistors and the mosfets replaced or with any other part, I decided be sure by ordering the AMP PCB new from Behringer, so I ordered it.
    When it arrived, I did a comparative test of the two PCBs, the one I repaired and the one ordered new and all parts/points tested reads same fine.

    6) Then I installed the new one pcb ordered, turning on, correctly and all working perfect except (same as with the repaired pcb) there is NO any output sound, neither no any +-LV/HV, but the +-15V/+48V/+5V are ok. So disconnected the SMPS X4 connector again (and do the same as with the repaired pcb) reading fine all these 6 voltages, plus the +48V/+5V.

    7) Can this problem to be rather a malfunction or a defective part on the SMPS side that transforms to +-40V and +-80V, or with the protection circuit?, the weird thing is why it cuts the +-LV/+-HV voltages only EVEN when the AMP PCB are new???

    Thanks a million for any suggestions/light.
    Edu.

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  2. #2
    Supporting Member Jazz P Bass's Avatar
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    How much do you know about SMPS devices?

    They can be very dangerous to work on.

    It appears that something is triggering the Prot circuit.

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    Thanks for your opinions JPBass.

    I know enough for to be repairing, but like all of us who repair sometimes we can stick on any repair, even when have so much experience or for more that one known.

    Sure, I know its dangerous to work on, so I take all the necessary and adequate care for.

    Correct, something is triggering the Prot Circuit but only for the +-80V/+-40V amp. section., and that is the strangest thing since I have repaired more serious damage in the SMPS with other amps & powered speakers.

    Someone else with suggestions, ideas, light???

    THANKS!!!

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    Supporting Member Jazz P Bass's Avatar
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    I would look at U5/ Pin #10.
    If it is 'High' then the IC is off.
    It must be 'Low' for the high voltage section to run.
    Note: U5/ Pin #12 (and associated connections) is considered to be the Zero Volt Reference that is to be used when probing the Primary side of the power supply.

    Schematic link: https://music-electronics-forum.com/...0&d=1555290863

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    Last edited by Jazz P Bass; 04-15-2019 at 02:54 AM.

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    Thanks a million JP Bass!!!, appreciate your suggestions a lot. I Will try it tomorrow and I will let to yours to know of my testings in the afternoon.
    Ed.

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