dly66
Super Member
The basic check are the power supply voltages stable and around the correct value? Seldom exactly what the SM says but usually close. Part of that may have to do with your power company and what voltage you are getting at your location. Often it tends to run a bit above 120 Vac.
If you can easily get voltmeter probes on the metal cases of the output transistors with one probe on the transistor case and the other against the metal chassis you can check the main power supply rails. The main power supply rail feeds the collector and with those metal can output transistors the case is basically the collector. That is why they have to be electrically isolated from the heat sink by a mica insulator with thermal paste or a silicone thermal pad. One set should be driven by the positive power rail and the other set the negative rail. Playing music at normal levels if the voltages are near spec I would expect all is okay in the power supply. The older cap coupled amp designs (1960s into the early 1970s) are a bit different, one pair of transistors normally operates at about double the voltage of the other pair.
My Yamaha was the first time I redid the thermal paste on the output transistors. Got too much on the bolt threads of one transistor isolating it from the power supply which led to a blowing a replacement driver transistor and fusible resistor. I had put the amp back in the state it was when I got it. Took me some time to figure out the problem. After that I always check the continuity between the case and collector connection on the other side of the heat sink to make sure I don't do that again. Either that or power it up and check the voltages on the cases.
If you can easily get voltmeter probes on the metal cases of the output transistors with one probe on the transistor case and the other against the metal chassis you can check the main power supply rails. The main power supply rail feeds the collector and with those metal can output transistors the case is basically the collector. That is why they have to be electrically isolated from the heat sink by a mica insulator with thermal paste or a silicone thermal pad. One set should be driven by the positive power rail and the other set the negative rail. Playing music at normal levels if the voltages are near spec I would expect all is okay in the power supply. The older cap coupled amp designs (1960s into the early 1970s) are a bit different, one pair of transistors normally operates at about double the voltage of the other pair.
My Yamaha was the first time I redid the thermal paste on the output transistors. Got too much on the bolt threads of one transistor isolating it from the power supply which led to a blowing a replacement driver transistor and fusible resistor. I had put the amp back in the state it was when I got it. Took me some time to figure out the problem. After that I always check the continuity between the case and collector connection on the other side of the heat sink to make sure I don't do that again. Either that or power it up and check the voltages on the cases.
