Once the BIOS has been fully initialized it then loads the BIOS program into Program RAM, makes communication with the CD drive and then turns on the green LED (and red when accessing the disc) as well as animating in the BIOS text and starting to read a disc.

Once you see the animated logo you are in Second Stage Boot.

At this stage the drive should also open if there is no disc in to insert one, and the disc should operate and spin.

The things that need to be working including all traces to and from them are:

If the Bus Translator near the CD ribbon connector is bad, or any traces from it to the power board, Sound Source or Mega CD Engine are bad, it can prevent getting to Second Stage Boot.

Trace and bell out with continuity every single pin, including power, ground and all bus pins to their locations, and ideally replace with new chip if in doubt.

If the Program RAM is bad, or any traces from it to the 68k or Mega CD Engine are bad, it can prevent getting to Second Stage Boot.

Use the 240p Test Suite to test this.

If the Mega CD Engnine is bad, or any traces from it to the 68k, Video RAM, Bus Translator, BIOS, Program RAM or other ICs are bad, it can prevent getting to Second Stage Boot.

This IC is super difficult to test, and you best bet is looking for damaged traces, reflowing the pins, or hot swapping it from a known working Mega CD to test the IC fully.