[Argus25]

Ignore the remark in my post #19 about the voltages across the emitter resistors, there is no point in comparing the voltages across the two output transistor emitter resistors in the Hacker circuit, because they are effectively in series, so the voltage drop across them will be equal (and indicate the current you measure in the test link as well), regardless if one transistor if unwell. (that test or comparison only works in output stages with transformers where the emitter resistors are returned to the supply ground).

Looking at the Hacker circuit though, which I now have in front of me, due to the fact that all the stages are direct coupled; one of the most important & simple tests is the voltage between the output transistor emitters at the junction of the two 2.2 Ohm emitter resistors, it should be very close to half the total power supply voltage (the voltage you would measure between the two output transistor collectors) if all is well with every transistor stage.

This "half rail" voltage comes about due to the fact that the input transistor T1 (a bc108 on my circuit) is biased so that its collector voltage controls the base voltage of T2 (the fine adjustment for this is the 50k pot RV3).

A voltage from T1's collector, is coupled directly to the base of T2 another BC108. T2 also receives, at its emitter, a sample of the output voltage via the 1.8k feedback resistor from the output at the junction of the two 2.2 Ohm emitter resistors.

Therefore, T2's base emitter junction threshold voltage (about 0.6V) acts like a comparator in a negative feedback loop to stabilize the DC output voltage at the junction of the two 2.2 Ohm resistors, to close to half the supply rail voltage. For example as the output voltage rises, so does T2's emitter voltage, turning T2 off, T2's collector voltage rises, this starts to turn off T4, T4's collector voltage drops and so does the output voltage at the junction of the two 2.2 Ohm resistors. So it self stabilizes.


From that you can see that if all is well with every transistor's DC bias conditions at least and all the resistors are ok and there is no capacitor leakage anywhere, then with the 50k adjustment somewhere near the center of its range the output voltage at the junction of the two 2.2 Ohm resistors should be 1/2 the supply rail.

One way in this sort of circuit to check if the complimentary output transistors are likely similar and both ok, is to compare their base to emitter voltages, they should be fairly close and of course opposite polarity. The base of the npn (AC176) being more positive than its emitter perhaps be a few hundred mV and the base of the PNP(AC128) being more negative than its emitter.

hope that helps.