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! Ivan.
searching for an "opened" one, every transistor is ok (also in the driver part).
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
I recently fixed an old McIntosh MA5100 with a "stuck to the positive rail" fault in one channel. One PNP transistor in the power stage's input "long-tailed" differential pair had failed open (open base). This had the same effect on the differential pair and feedback loop as if the base had been pulled all the way positive (although it hadn't been) and the feedback loop drove the outputs upwards.
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Dave Platt AE6EO
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** 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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