Check if the PGOOD signal from the 3V/5V controller or the CPU controller drops out instantly. If one regulator fails to output PGOOD , the EC terminates the power sequence for safety. Symptom 3: Fans Spin, Caps Lock Blinks, No Display
The MB-252 facilitates power transfer through screw terminals or direct wiring depending on the specific radio model revision it is paired with. mbx252 schematic full
Searching for "MBX252 schematic full" should yield a document containing roughly 40-50 pages. If you find a 2-page document, it is likely a power sequence summary, not the full schematic. Check if the PGOOD signal from the 3V/5V