Sonic 2 used a Sharp SM-511 MCU (die ID N86)
ROM array with top metal removed
melody ROM array with top metal removed
Buttons:
K1 K2 K3 K4 Beta BA
S2 -- -- Up -- -- --
S3 -- Right Left -- -- --
S4 -- Down -- -- -- --
S5 Action -- -- -- -- --
S6 Pause -- -- -- -- --
Batt+ -- -- -- On -- --
Batt- -- -- -- -- Off --
Batt- -- -- -- -- -- Sound
Piezo:
transistor with emitter to batt- and collector to piezo; other piezo wire to batt+; coil across piezo
base of transistor to cathodes of two diodes; one diode connects to R1 via 120K resistor, other diode connects to S1 via 39K resistor