The final photo in this series is taken from the opposite side,
and shows what happens as the power plug is fully inserted into the AC power jack.

Here, one can see the spring contact being pushed down and away from the sensor contact,
thereby telling the Empeg firmware/software that it is operating on AC power.

The jack is slightly distorted here, from having its plastic shell cut-away
to reveal the innards for these photos. As a result, the spring is actually
pushing the plug up higher than would be possible with the shell in place.

On an intact AC power jack, the plug is forced down more than shown,
and in turn forces the spring contact even lower than shown.
This makes it impossible for the spring to be in contact with
the sensor while a plug is inserted.

There are two primary failure modes for this AC power jack.

• The spring contact could fatigue and stop springing back into
  position when no plug is inserted. In this mode, the player
  will be permanently stuck in AC power mode.
  
  
• The spring contact may break off completely,
  in which case AC operation becomes intermittent or impossible.
  The metal fragment could then also short out the (+) and (-)
  conductors when in the car, damaging the player's main circuit board.
  

Cheers