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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Amp Repair - Stuck to negative Rail
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. |
#2
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Amp Repair - Stuck to negative Rail
Ivan Barberis wrote in message
... 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 |
#3
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Amp Repair - Stuck to negative Rail
"Ivan Barberis" wrote in message ... 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. Check that the positive rail is actually present at the NPN output transistor collector, and that the emitter resistor of the NPN is not open. Then check that the positive rail is present at the opamps in the preamp section - assuming that it uses opamps, of course. Even if it doesn't, it will probably still have transistor stages fed from dual polarity rails. The rails here are usually + / - 15 or 12 volts, and they are usually derived from the main output stage rails via some kind of regulator. This might be a three terminal monolithic type like a 7812, or a series regulator using a transistor, or often, just a resistor and zener. Supply to the regulators is sometimes fed via a fusible resistor, and these often fail for no apparent reason. A couple of weeks ago, I had an amp with the output slammed over to one rail just like yours, and that was down to a rail missing from the preamp opamps, and that was in turn due to an open feed resistor. Arfa. |
#4
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Amp Repair - Stuck to negative Rail
In article ,
Arfa Daily wrote: 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! Check that the positive rail is actually present at the NPN output transistor collector, and that the emitter resistor of the NPN is not open. Then check that the positive rail is present at the opamps in the preamp section - assuming that it uses opamps, of course. Even if it doesn't, it will probably still have transistor stages fed from dual polarity rails. The rails here are usually + / - 15 or 12 volts, and they are usually derived from the main output stage rails via some kind of regulator. This might be a three terminal monolithic type like a 7812, or a series regulator using a transistor, or often, just a resistor and zener. Supply to the regulators is sometimes fed via a fusible resistor, and these often fail for no apparent reason. A couple of weeks ago, I had an amp with the output slammed over to one rail just like yours, and that was down to a rail missing from the preamp opamps, and that was in turn due to an open feed resistor. 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. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#5
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Amp Repair - Stuck to negative Rail
"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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