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Argus25 · 30th Jul 2018, 10:31 pm

A bit more information might help you too:

You will notice if you look at T3's (OC71) circuit, there are two 150R resistors in series with the junction of them on the base and the ends of them on the collector and emitter.
When enough current passes via those 150R resistors, the voltage developed across the one between the emitter and the base rises to around 200mv (0.2V). This is the typical base-emitter threshold voltage for a germanium transistor (unlike 0.6V for a silicon type). This causes the OC71 transistor to pass collector current, which then shunts the voltage across the two 150R resistors downwards again. So it is an enclosed DC negative feedback circuit set up to generate very close to 0.4V across the two series 150R resistors (between the bases of the two output transistors). It was done this way because as the OC71 heats up, this generated 0.4V drops downward, preventing the output transistors getting thermal runaway.

It still is possible that the collector current in T4 is too low, and T4 is faulty. It is a bit unusual that its base-emitter voltage is a bit low. But, aside from the cross over distortion you are hearing, which one would expect from the very low output transistor bias, the amplifier is still working, which would be unlikely if T4 had partially failed, but not impossible.

30th Jul 2018, 10:31 pm	#51
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Hacker Sovereign 11 A bit more information might help you too: You will notice if you look at T3's (OC71) circuit, there are two 150R resistors in series with the junction of them on the base and the ends of them on the collector and emitter. When enough current passes via those 150R resistors, the voltage developed across the one between the emitter and the base rises to around 200mv (0.2V). This is the typical base-emitter threshold voltage for a germanium transistor (unlike 0.6V for a silicon type). This causes the OC71 transistor to pass collector current, which then shunts the voltage across the two 150R resistors downwards again. So it is an enclosed DC negative feedback circuit set up to generate very close to 0.4V across the two series 150R resistors (between the bases of the two output transistors). It was done this way because as the OC71 heats up, this generated 0.4V drops downward, preventing the output transistors getting thermal runaway. It still is possible that the collector current in T4 is too low, and T4 is faulty. It is a bit unusual that its base-emitter voltage is a bit low. But, aside from the cross over distortion you are hearing, which one would expect from the very low output transistor bias, the amplifier is still working, which would be unlikely if T4 had partially failed, but not impossible.