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Subwoofer Amp blowing fuses

Discussion in 'Troubleshooting and Repair' started by Ross Edwards, Oct 11, 2016.

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  1. Ross Edwards

    Ross Edwards

    Oct 11, 2016
    Hi folks

    Hoping someone may be able to point me in the right direction, I've got a basic knowledge of electronics but could do with some guidance...

    I have a Mission E8AS1 Active subwooofer which is blowing the 1.6A fuse on the main input immediately on power on, even without an audio source or any volume.

    If I disconnect the 2 boards downstream from the transformer, the fuse stays intact, but with the first board connected (crossover?) it blows. I identified two blown caps on this board, one of which was shorting (8uF 100V +-10%) which were hard to find, but have now been replaced (the two blue ones near the centre of the board are the replacements). However the board still blew the fuse even with the caps removed, and still with the new caps on.

    All other components on the board looks fine to me, no obvious skid marks or bulges, the tracks are clean except for my novice soldering bodge where I replaced the capacitors. There was a bit of goop at the bottom of the blown caps which I have cleaned.

    Can someone advise what I could test/ try next? I have a basic multimeter but nothing fancier.

    Many thanks in advance for any advice, all gratefully received.

    Attached Files:

  2. Alec_t


    Jul 7, 2015
    I'd check for shorts on the varistors (those blue discs in the power supply section), the bridge rectifier and any fat semiconductors. Ideally they should be checked out of circuit, but a dead short should show up even in circuit (though might be confused with a very low impedance in parallel with the device tested).
    Welcome to EP!
  3. Ross Edwards

    Ross Edwards

    Oct 11, 2016
    Thanks Alec - OK so the varistors are 1M and 1.8M Ohm on circuit, which I guess is OK.

    The bridge rectifier (new to me) seems to have current flow from every pin to every other pin, in both directions, which presumably is bad news. Not sure if this is due to it being on the board, going to take it off and check,..
  4. Ross Edwards

    Ross Edwards

    Oct 11, 2016
    In isolation the bridge rectifier does as it should do, but there is no resistance between the + and - rails that it connects to, so I guess this means the problem is elsewhere, but definitely on this board. Is there a clever way of tracking down the duff component in the absence of any visual clues?
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