Quote:
Originally Posted by kirstyd
All measurements were taken with the meter set as it is in the photo
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It is obvious from these figures why your amplifier has severe cross over distortion and sounds awful at low volume.
The base to base voltage of the output transistors, T5 and T6 is only 60mV. It should be around 400mV to get both of the output transistors into initial conduction. (200mV per transistor) So the question is why ? This voltage is developed across the two series 150R resistors as a result of T4's collector current. From your measurements T4's base-emitter voltage is about 150mV (it should be around 200mV but its not ridiculously low), so it should be generating some collector current in the 150R resistors to get at least more than the 60mV that is there.
The 150 R resistors are shunted by the OC71 (T3) which is the "vbe multiplier" transistor. Its job is to control the output transistors inter base voltage downward to about 400mV and also to temperature compensate the voltage downwards.
Assuming the two 150R, the 680R and the 330 R resistors in T4 collector circuit are ok (and T4 is ok..probably) I would suspect that T3, the OC71 vbe multiplier transistor had emitter to collector leakage, or is near shorted out.
With the amp de-powered, what is the resistance from the emitter to the collector of the OC71 (T3) ? And check the resistors.