Game Gear TV Output Modification

The Sega Game Gear was one of my favorite systems growing up.  With a library of hundreds of games plus backwards compatible to the Master System, it smoked the original Game Boy competition.  Even today, it holds a special place in the realm of portable 8-bit games, surpassing the Game Boy Color's palette and boasting many original titles that can be played no where else.

In 2007 someone named Xavier from the SMSPower forums figured out the video DAC circuity to finally bring the Game Gear to a Television.  The hack came about when someone posted a dev model of a Game Gear showing pictures of the ICs used to produce a TV signal.  Here is a pic of the mod I first finished back in 2007:

Here is an improved and simplified schematic for Xavier's circuit (minus the RGB to composite encoder): There are 7 points that must be pulled from the GG into this circuit: D0, D1, D2, D3, pin 10 (of the right ASIC chip), pin 30 (of the right ASIC chip) and C Sync.  Don't forget that D0-D3 are only connected to 1k resistors each.  This a pull down resistor and is not repeated for each chip input (as you can see in the schematic).  Keep in mind that this modification only works with Game Gear systems that look like the picture above, containing 2 ASIC circuits.  For a 1 ASIC Game Gear, use Viletim's mod here.  Here are some finished pics of my Game Gear modification:

I have since done a lot more reverse engineering of the GG, thanks to the service manuals that were dumped originally on the Sega-16 forums.  Here is the worlds first region-free GGTV completed for a customer:

This unit is the 1 ASIC mod that Viletim perfected.  I used socketed logic chips just to make repair easier since this thing was a beast to build.  To the right is a CXA1645 chroma encoder I built.  The JP mode switch is used by ground pin 143 on the Sega ASIC (+5v is US).  Pin 119 (on a single ASIC 315-5535) is labelled as "SECAM" in the service manuals. Simply cut the trace to ground and apply +5v through a 10k resistor and...GG jumps into 50hz PAL mode! Now we can enjoy the full library of SMS games on a GG.  The Genny controller port does not have start but otherwise does everything else. And the last switch is for full screen SMS mode.