"A" is just the power switch. If you pull that cover off, all that should be under it is just back of the switch.
"B" is the AC supply transformer that rectifies the the supply to 10.3VDC for the sound amp. It is also the isolation xfmr that supplies AC to the monitor.
"C" is the Atari ARII board that is the game board power supply where the AC input from the xfmr is rectified, and it is also your sound amp. With a little work you can replace the ARII with a switcher, but then you will need an amp for the sound. On some games like Centipede, the ARII board supplies a +20VDC for the high score save, so unless you replace the EA-ROM on the gameboard with an updated +5V one, it won't save your high scores. Not sure if Space Duel needs that, though.