This is my first post to this billboard, so if I have done some thing out of order, or need redirection please let me know. I have done some troubleshooting in the past but am not a veteran tech, and would appreciate some direction.
I have a simple audio amplifer - all transistors, an older design, that all of a sudden went DC on me. There are 13 transistors in this circuit, four of them are output transistors that are heat-sink mounted. In my experience often audio amplifiers that go DC (DC on the speaker circuit) have shorted output transistors. So that was the first thing I checked - all of those transistors are fine - I actually pulled those to test. This amplifer has a +44V, 0V, -44V power supply and the speaker circuit is outputing +44V. I have been testing many of the transistors for shorts (mostly in-circuit tests) and have come up with nothing that looks suspicious. I have done the same with the diodes and the capacitors, also with no problems noted. So now I am at a stalemate. In my experience resistors, usually blow with a big stink and a lot of fanfare, and I don't see any of that. The ones I've tested are fine.
I have attached a schematic of the amplifier, the input is on the left, the speaker output on the right. The power transistors are the four MJ15015 on the right. Any guidance on the best course of action to follow in terms of voltages to test at certain points, or expected oscilloscope waveforms at certain stages would be great.
I have a simple audio amplifer - all transistors, an older design, that all of a sudden went DC on me. There are 13 transistors in this circuit, four of them are output transistors that are heat-sink mounted. In my experience often audio amplifiers that go DC (DC on the speaker circuit) have shorted output transistors. So that was the first thing I checked - all of those transistors are fine - I actually pulled those to test. This amplifer has a +44V, 0V, -44V power supply and the speaker circuit is outputing +44V. I have been testing many of the transistors for shorts (mostly in-circuit tests) and have come up with nothing that looks suspicious. I have done the same with the diodes and the capacitors, also with no problems noted. So now I am at a stalemate. In my experience resistors, usually blow with a big stink and a lot of fanfare, and I don't see any of that. The ones I've tested are fine.
I have attached a schematic of the amplifier, the input is on the left, the speaker output on the right. The power transistors are the four MJ15015 on the right. Any guidance on the best course of action to follow in terms of voltages to test at certain points, or expected oscilloscope waveforms at certain stages would be great.
Attachments
-
155.3 KB Views: 24