I've inherited this A/V receiver from a former housemate. The device
powers up and all the controls appear to work, but there is no output
from any of the eight speaker outputs. (Speakers A&B both Left and
Right, Subwoofer, center, and surround.)

The headphone amplifier does work, and I have tested that all of the
audio inputs work by connecting an external source to each of them and
piping that sound through to the headphone output.

I am a relative novice at repair, and I am interested in taking this
on as a learning project. My background - I had several EE courses in
college a decade ago and know my way around soldering, DMMs,
oscilloscope, and have these tools available, but I've never probed a
PCB board looking for busted components before. I've repaired things
with loose connections, bad solder joints, obviously blown capacitors,
and mechanical components.

Advice on where to look for this particular problem, or thoughts on
how to approach the daunting task of tracing signals through a PCB for
the first time, are much appreciated. Thanks in advance!

Evan