SA-GX370 Repair

The SA-GX370 amplifier uses power transistors (2SD2374 and 2SB1548), and  there were signs on the PCB that these transistors had overheated, hence, the first thing to do was to check them. I noticed that some of the junctions were showing resistance in forward and reverse bias and they generally seemed screwy. Therefore, the best course of action was to replace them, and luckily, the replacements were cheap, so it was worth the effort. This amplifier has a very sensitive overload protection circuitry, which checks for correct voltage levels and even the slightest change in supply voltage will cause it to trip into protect mode and show "OVERLOAD" on the display. Therefore, replacing the old and worn out power transistors should help the circuit somewhat.

When replacing these transistors I noticed how thick the original thermal compound was, and in addition, the metal bracket that pushes it to make contact with the heatsink barely provided any pressure. This might be the reason why they were overheating; therefore, I applied an extremely small amount of compound and bent the bracket a little so that it placed greater pressure to make contact with the heatsink.

The 2SD2374 (Q701) is an NPN triple diffusion planar power transistor with collector current Ic rating of 3 A. It has collector power dissipation rating of 25 W, and maximum operating junction temperature of 150 °C.

The 2SB1548 is a PNP power transistor with collector current Ic rating of 3 A. It is complementary to the 2SD2374.

I also checked the voltages of the SVI2304 pins to make sure the IC was not shorted.

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