Acer AT4220A Service Manual page 49

Release the four screws holding the I/O board.
11.
Then detach the I/O board as the arrow indicates.
12.
At first disconnect all the cables that connect to the main board.
13.
Release the four screws fastening the main board then detach the main board.
14.
Release the eight screws fastening the power board and the grounded wire.
15.
Then remove the power board.
16.
Chapter 2 43