OSC1
53.203424 MHz oscillator
53.203424 MHz master clock oscillator (PAL) — every clock on the board is divided down from it.
Other components on Sega Mega Drive II (Model 2) VA1 — PAL
About this component
The 68000 runs at this ÷ 7 (about 7.60 MHz) and the Z80 at ÷ 15 (about 3.55 MHz); the video and sound timing come from it too. NTSC boards use 53.693175 MHz instead, which is why swapping it is part of some 50/60 Hz mods. A dead oscillator gives a completely dead console with the power LED on. Sources: https://www.copetti.org/writings/consoles/mega-drive-genesis/
Pinout & readings
No pinout or reference readings yet — these show what each pin does and what a healthy part should measure.