Remove the nut and washer holding the 6.25mm guitar input jack to the chassis

Disconnect any wires going from the board to other parts of the chassis, such as to the power switch or power transformer

Identify and remove any other screws that might prevent removal of the board. On my Ibanez, two screws held the power amplifier IC heat-sink to the chassis. Removal of the screws and their accompanying lock washers allowed the board to be lifted off with the heat-sink