Sony TA-FB930R: Bias current can't be set high enough(?) on one channel

[klausman]

So I have a Sony TA-FB930R that I replaced one output MOSFET on. The manual says that the test point for both channels (warmed up, no signal, no load) should be ±10mv. On the left channel, I can do that just fine. But even riding the amp hard for 10m (50% load through power resistors), I can't get the reading to go higher than 6.5mV or thereabouts. I figure "can't set it high enough" is better than the opposite, but....

Now the question is whether a) that's a problem that needs solving and b) if so, how to solve it. I haven't done THD measurement since I don't own that kinda gear.

My good Mexican friend Service Manuel is attached.

[TimFox]

What audio gear do you have?
Perhaps a DSO with FFT?

[klausman]

What audio gear do you have?
Perhaps a DSO with FFT?

I have a Rigol MSO5xxx (with all the bits), but I am not sure it would be up to snuff for this kinda stuff. The FFT functionality seems woefully coarse-grained if I only sweep 10Hz-20kHz. (Or I am using it wrong).

[TimFox]

What audio gear do you have?
Perhaps a DSO with FFT?

I have a Rigol MSO5xxx (with all the bits), but I am not sure it would be up to snuff for this kinda stuff. The FFT functionality seems woefully coarse-grained if I only sweep 10Hz-20kHz. (Or I am using it wrong).

As a quick check, if you have a decent generator for 1 kHz, you could scan from 500 to 5000 Hz and see the fundamental and at least the 2nd and 3rd harmonics.
First check the generator itself, then the amplifier output.
Obviously, one could get better results with a dedicated distortion analyzer.
The old rule of thumb for an oscilloscope measurement was that 5% THD was visible on the trace itself.

[Jeff eelcr]

Dc measurements between channels, offsets to uV, assuming replacement part was a match to its counterpart.
If not, both should have been replaced.
Jeff