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: http://www.etanoldax.se/RF201s.pdf
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

A very, very, very stupid suggestion...

If (as it seems) you've replaced practically all the parts in that channel,
then what's left?

Perhaps a short somewhere on the PCB?