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tony brady · 16th Oct 2019, 9:44 am

Have you checked the voltages across R808 & 813? These should both be about 0.65V. If these are wrong the bias will also be wrong. Also check the voltage across R805 - this should be about 0.4V. If the resistor is high in value or the current limiter transistor VT801 is faulty the current though the next stage will be too low so you will not be able to get the bias current high enough.

I presume you have checked the signal (off load) with a 1KHz signal and scope? Does the waveform look basically OK except for some crossover distortion kink being visible? What is the DC offset voltage at the speaker output? For this sort of amp it should be less than 100mV and ideally less than 50mV.

16th Oct 2019, 9:44 am	#2
tony brady Hexode Join Date: Mar 2014 Location: Peacehaven, East Sussex, UK. Posts: 278	Re: Goodmans Model 150 Amp Fault Have you checked the voltages across R808 & 813? These should both be about 0.65V. If these are wrong the bias will also be wrong. Also check the voltage across R805 - this should be about 0.4V. If the resistor is high in value or the current limiter transistor VT801 is faulty the current though the next stage will be too low so you will not be able to get the bias current high enough. I presume you have checked the signal (off load) with a 1KHz signal and scope? Does the waveform look basically OK except for some crossover distortion kink being visible? What is the DC offset voltage at the speaker output? For this sort of amp it should be less than 100mV and ideally less than 50mV.