STK465 Transistor Equivalent Circuit Diagram

As you can see from the circuit diagram, and the pinout, this chip uses power transistors arranged in a Darlington configuration. Pin 6 and Pin 7 are the output pins. Pin 7 provides the positive phase of the signal, whilst Pin 6 provides the negative phase, in a push-pull arrangement. Typically, a 0.33-ohm resistor joins these output pins together.

In this Hybrid IC, if the Darlington pair tr7 and tr8 go short, then as you can see, +Vcc would make a path through pin 7 and to ground. Normally there is a speaker load between this pin and ground, and therefore the speaker coil burns out, if there is no protection circuitry to cut the current path.

If the Darlington pair tr9 and tr10 go short, then –Vcc makes a path through pin 6, and again it spells bad news for the load if there is no protection circuitry.

From an amplifier servicing view, this makes the job of diagnosing easy, because if the voltage at the output pins is either of the rail voltages then the Hybrid IC will need replacement.

The same arrangement applies to the other channel where Pin 10 and Pin 11 are the output pins.