Amplifier troubleshooting

[Firetech]

My current learning project is an old audio amp that was going to be tossed. I managed to bring it back to life-somewhat. I replaced the two power transistors, 2N5686. I also had to replace the relay which was not working. When I connect the two leads from the emitters of the transistors to the primary of the audio transformer, the 12A fuse opens.

My question: Is it correct that the emitters of the two transistors is connected to the primary of the transformer?

Thanks

 

[davenn]

maybe maybe not

do you have any circuit info on the amplifier ? else we are all just guessing

Dave

 

[unclejed613]

what type of amp is it? there are some QSC (as well as others) that use the output transistors to "drive" a floating supply, but i don't think i've seen one that uses a quasi-complementary output stage....

 

[gots2no]

In most output stages, you have one NPN and one PNP driving the output directly. What you're describing sounds like a transistorized version of tube amplifier, where each output transistor drives one leg of a center tapped transformer, where the center tap is connected to B+. Each transistor is supposed to turn on for a brief period of time while the other one is off. If both are on at the same time or either on for too long of a period, it will draw too much current and pop the fuse.

Often when output transistors are damaged, there is a problem in the circuitry driving them and simply replacing the output transistors will result in immediate damage to them. These circuits can be difficult to troubleshoot, especially if you are a novice.

 

[unclejed613]

actually, the collectors are tied to ground, and the emitters to the supplies. the speaker connects to ground, and the center tap of the floating power supply. and there's also two versions, one with the collectors on the supplies, and one with the emitters on the supplies. if you analyze the diagram below, you will see when the top transistor is "on", both output stages have the left hand terminal of the load at a more positive potential than the right. QSC's main technical reason for this odd output stage is that it can be driven from low voltage driver circuits, because the base drive for the outputs never goes much beyond +/-1.5V

QSC usually has a detailed troubleshooting guide in their manuals because this "backwards" output stage throws people for a loop.