Hi all! I'm close to a mental breakdown cause of a car audio power amp which I have for repair. It's a rockford fosgate model 201s which is a fairly cheap unit producing only about 2x50W or so. But I spent a few hours on finding the problem and the case has gone more or less to the state "I will find the problem - if it will be last thing I do on this planet". It would probably be easier to have a discussion with another audio pro. (I actually repairing about 20pcs of power amps a month). I've uploaded a schematic at:
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Let me describe the problem: The LEFT channel does have a problem on the negative side of the output waveform. The positive side seems to work well. The left channel is on PAGE 2 of the schematics. Something is restricting the drive capabilities of the negative part of the signal. The gate to source voltage of output transistor Q115 is limited to 5 volts, even when the output clips. This is compared to over 15V gate to source of the OK channel. I measure upstream in the circuit and check the signal at collector Q111 which is the negative driver in the voltage amplifier stage and there's some strange behaviour. The collector voltage does not go any negative when the output calls for maximum swing (when driven into clipping). The OK channel does swing about 10V negative compared to its steady state fully driving the output stage correctly. The thing is that I've changed both output FET's, pre-drivers Q112, Q113, voltage amp stage Q111, Q109, current mirror Q104, Q105, long tailed pair Q102, Q103, current generator Q101. Also the 6 transistors in the overcurrent protection block was changed and the protection was also defeated so it cannot limit, without any improvment of the signal. During steady state, without any input signals, all currents, and dc levels is exactly the same as the working channel. A dc offset at output of 1 mV only. I measured through all resistors which appears to be good. Anyone familiar with this modell who can give me some advice where to go next???
Thanks a lot,
Best regards The poweramp freak