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Amp Repair - Stuck to negative Rail

I

Ivan Barberis

Jan 1, 1970
0
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.
 
N

N_Cook

Jan 1, 1970
0
Ivan Barberis said:
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.

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
 
P

Phil Allison

Jan 1, 1970
0
"Ivan Barberis"
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?


** 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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