Sega Megadrive 1 Overclock

Hi guys, this one's for the vintage folks

o1s1n asked me about modding his MD1 with region & language switches and to overclock it. He did some research, and found 12.5Mhz the optimum speed to reduce slowdown in games without causing any glitches. The only other clock signal the MD carries is 13.3 (from the cart slot) and is widely reported to cause a lot of glitches...this meant a 12.5Mhz oscillator was needed.

The mod is fairly straight forward, the only mistake I did make was leaving the wires carrying the clock signals too long, therefore the clock signal degraded and the system locked up. This was alleviated by reducing the lenght of the wires to just a few inches.

The result is impressive, going into debug mode on Sonic 1 and filling the screen with the jumping red fish causes massive slowdown at stock speeds of 7.5Mhz. When running @ 12.5Mhz all slowdown is eliminated, in the MANY games that are affected by this. 2 player Sonic 2, all 3D games, Ecco 1 & 2 etc etc.

Basically in involves two switches, 1 for the HOLD function (which pauses the CPU allowing you to make clock changes during operation), the other is the CLOCK switch (7.5Mhz & 12.5Mhz). The HOLD effect is achieved by grounding Pin 17 of the CPU. Pin 15 is the clock pin, all of the tutorials state you have to cut the trace going to pin 15. I HATE cutting traces, so I decided to lift the pin up off the board instead. This had the upside of leaving the stock clock solder point exposed and easy to solder to, and with the CPU leg up in the air, it was easy to solder to this a wire which would carry the new clock.

The MD1 (Original High Definition Graphics Model, with no Sega Bios Screen)



Under the hood



The Jumper points for setting region & language



The default bridges removed which normally make the console PAL



The switch wires in place - Grey to Green (5V) = English language. Grey to Black (Ground) = Japanese. Orange to Black (Ground) = 50hz display, Orange to Green (5V) = 60hz display.



The CPU. Here you can see pin 15 lifted from the board.



Now the wires are soldered into place, the white wire onto pin 15 carries whatever clock signal I choose. The blue wire will carry the stock clock speed of 7.5Mhz to one side of the Clock switch. The black wire onto pin 17 will be connected to ground via a different switch to HALT the cpu.



Spaghetti



Souffle



Gunshot wounds



Temp oscillator position for testing. The green wire supplies it with 5V, the black wire for ground, and the purple wire (12.5 Mhz) will go to the other side of the CLOCK switch giving me the choice of speeds.



The finished article. You will notice only 3 switches instead of four. This is because I mistakenly made the holes without testing the system Upon realising the clock wires have to be so short, I had to re-route that switch to the back of the console. Ideally all 4 switches should be at the back, but there is a lot of shielding & plastic lugs in that area so I went for the left side. This means the unit still has Mega CD functionality.



Lonesome, but special



Ok o1s1n, operating instructions. All switches in the down positions give Stock settings. English, 50hz, CPU not paused, 7.5Mhz.

Switches on the left x3:

Closest to the front of the MD is the language switch. Down is standard.
The middle one is the display setting. Down for 50hz, up for 60hz.
The third switch, closes to the rear is the HOLD switch. Down for normal operation, up to hold.
The switch on the back is the clock switch. Down for 7.5Mhz, up for 12.5Mhz.

Enjoy