Chris F. wrote:

Just thought I'd see if anyone knows of some common problems and fixes,
before spending any great deal of time or money on this. Sometimes the set
starts for a while then quits, other times the switching supply will sputter
for a few seconds and fail (the red diagnostic LED then lights brightly). No
cold solder to be seen, and the caps all check good. I'm leaning towards a
bad switching regulator IC (STR-M6511) and/or optoisolator, but I'm really
not sure how to confirm this without replacing anything. Disconnecting the
loads from the secondary side of the supply (one at a time) does not change
the symptom.
Thanks for any advice.

Even though the caps check good they are I'm sure dried out and failing
change out all the large caps near the regulator , or fet it the primary
of your supply I doubt the fet is bad they either work or don't and
short out