I'm working on this amp that appears to have a bad channel, the amp powers up but it cycles power and never completely builds rail voltage.
Thermal camera shows one pair of output transistors (Q303/309) getting slightly warm when the amp tries to power on, all of the others are cold.
Measuring the components for that channel and comparing it to the others, I find that Q301, Q310 (mbsa06/56) read <20kOhms across emitter and collector - I remove them and my transistor tester show acceptable values. With Q301 and Q310 removed and measuring the same points, it still reads <20kOhms. The other 3 channels read >200kOhm for those two parts.
I've checked the remaining components for that channel and they all compare close to the other channels. Any suggestions on what to check next?
Thermal camera shows one pair of output transistors (Q303/309) getting slightly warm when the amp tries to power on, all of the others are cold.
Measuring the components for that channel and comparing it to the others, I find that Q301, Q310 (mbsa06/56) read <20kOhms across emitter and collector - I remove them and my transistor tester show acceptable values. With Q301 and Q310 removed and measuring the same points, it still reads <20kOhms. The other 3 channels read >200kOhm for those two parts.
I've checked the remaining components for that channel and they all compare close to the other channels. Any suggestions on what to check next?
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Got it working, one of the output transistors on that channel was bad, but not shorted. Measured with a hfe of 4 🙂