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Amp Repair - Stuck to negative Rail

Discussion in 'Electronic Repair' started by Ivan Barberis, Apr 14, 2013.

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  1. Hello!
    I have to repair an old Cabotron amplifier.
    The amp is a common class A-B, with MJ15015 e MJ15016 power transistors.
    I have the output stuck to negative rail: checked all the transistor searching for an "opened" one, every transistor is ok (also in the driver part).
    What may cause this kind of fault? Fuses are not blown.
    Thanks a lot for the answers!
  2. N_Cook

    N_Cook Guest

    searching for an "opened" one, every transistor is ok (also in the driver
    Does it have a preamp section ? if so break it at that point and check for
    any DC offset , perhaps from failed opamp in the preamp
  3. Phil Allison

    Phil Allison Guest

    "Ivan Barberis"

    ** As others have said, resistors can go open - film types with values of
    10 kohms or higher and WW types can suffer from internal corrosion when
    large DC voltages are present.

    Of course, check all zeners for shorts and ceramic caps too.

    I use to see a lot of " Cerwin Vega " brand amps which had gone DC and
    taken out expensive speakers because of a faulty batch of 0.1uF disk
    ceramics shorting the + or - 15V rails to the input op-amp.

    ..... Phil
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