In article .com,
"mikem" wrote:

Good Going, Jonathan!

I feel good that I actually might have provided a clue when I wrote:
"I'm thinking a wiring error, or short which occurs only when headphone
plugs are pushed in, or an internal fault inside the SPA400."

Always very satisfying when you can find a real problem and fix it.

Mike,

Your expertise and insistence (along with others) did motivate me to
look again for the ground loop. It actually didn't have anything to do
with the plug being pushed in, but you wouldn't believe what a freak
event it was--the panel hole sits right under a rear lip on the panel,
and the way the jack was mounted (turned) in the hole caused the tip
terminal to make contact. All of this mess was right above the
electrical bus bars, right behind the cabin heat control cables, and
about halfway up the back side of the panel, so it was nearly impossible
to see. I almost gave up before I decided to give it one last look, and
the light finally hit in the right spot. The dental pick was about the
only thing that I could get up in there to pull the terminal away from
the airframe, because I really, really didn't want to take the jack out
of the panel.

Thanks again to all who responded.


JKG