Work in progress.
This guide will show you how to install a Dual Frequency Oscillator into a Sega Mega Drive.
When region modding a Mega Drive the console will run with an out of spec master clock frequency and video refresh rate when using the forced region. The Dual Frequency Oscillator corrects this by changing the master clock frequency to the correct one depending on what region is selected.
The Sega Mega Drive model 1 has a pair of jumpers called JP3/JP4 which are connected up to the PAL/NTSC selector pin of both the CXA1145P Video Encoder (Pin 7) and the SEGA 315-3513A VDP (Pin 52).
These jumpers are hard set to either +5V or Ground, which sets the VDP and the Video Encoder to the correct region. This makes the Mega Drive an almost universal console, it is only the master clock frequency that needs changing per region.
(DIAGRAM OF CLOCK MASTER CLOCK CONNECTIONS HERE)
PAL models use a 53.203Mhz master clock oscillator
NTSC models use a 53.6931Mhz oscillator
Master Clock Oscillator
Pin 3 Clock Out to
JP3 / JP4 (Video Jumpers) connected to
VDP
JP1/JP2 (Language Jumpers) connected to
In order to make the Mega Drive fully compatible with PAL and NTSC three things need changing together.