If the power brick is the catfish then the Audio/Regulator
might be the barnacle. You’ll find it attached vertically behind the brick by the
coin bucket. Its notable features include bulbous capacitors and a large heatsink in the middle. It’s also the more troublesome of
the two.
The Audio/Regulator (or AR for short) was produced in three
versions, AR-I, AR-II, and AR-III, and several revisions. The AR-I was the
earliest and used for a good many Atari black and white games. The AR-II was
designed to be a superset of the AR-I, adding a few new features while still
being pin-compatible with the AR-I. After that Atari came to their senses and
produced the AR-III which was a complete re-design from the two previous and
not at all pin-compatible.
If all of the voltages check out then
it’s time to move on to testing the Audio Regulator (AR) board.