Sega Genesis / Mega Drive 1 RGB Bypass

PLEASE READ:  Genesis / Mega Drive 1 consoles do not require a modification for RGB-output, simply a cable.  This page is an "experts-only" page that shows how to bypass the Genesis' internal RGB amp with a different one.  All details are below, but this mod requires you to make irreversible modifications to your Genesis system. Beginner and intermediate modders should not try this mod!

Please make sure to read the main Genesis RGB Bypass page before proceeding to make sure that this mod is something you'd really like to try!!!  You must read the sections about sync requirements and RGB cables, as it might change the way you approach this mod!!!  Also, there are many different revision Genesis / Mega Drive 1's and each will look different inside.  The model shown in this guide is a North American VA3 system and while the basic ideas are the same in all revisions, each mounting location and RGB points will be different.  Use these instructions as a basic guide, but you'll need to double check each detail on your console.


Tools / Parts Needed:
You'll need a few tools for this mod (more info on the tools can be found in the tools section):
- Genesis RGB Bypass board
- Excellent soldering skills.

- Philips head screwdriver
- Soldering iron / solder
- Thin gauge wire
- Small, sharp, cutting tool
- Small pick or "dental tool" if you're lifting pins

- Solder remover; A de-soldering iron and solder braid is recommended
- Multimeter



Removing the RF module:
- Start by completely disassembling your console, including removal of the heat sink.  Now flip the motherboard over and de-solder the RF adapter. I always find this to be a major pain!  You'll need to be patient, use a de-soldering gun and some de-soldering braid. Be very careful not to damage the motherboard when removing it.  Also, make sure the solder pads are very clean on both the top and bottom of where the RF adapter used to sit:


- Next, add some non-conductive tape to the bottom of the RGB bypass board:


- Position the board where the RF adapter used to sit as shown below. You'll then want to run thick wire through the three ground holes, then bend and solder them on each end (I forgot the top ground hole in the picture below). I used wire about the thickness of a resistor and it holds the board very solidly in place. Also, connect a small wire to the 5v hole on the RF adapter's mounting hole. I strongly recommend double checking with a multi-meter that it's the correct 5v location:



Tapping RGBs from the video chip:
This is the part that you should be most careful of. Depending on your model / revision Genesis, you might be able to find easy places to pick up the RGBs signals that don't require soldering directly to the pins, or modifying the motherboard. I suggest using a multimeter and seeing what you find; You might find via's that are easy to tap, or possibly different traces on the motherboard. I strongly suggest trying the easiest and safest method first and see how it works for you.

In this guide, I lifted the RGBs pins of the video chip, to eliminate any possibility of interference. This may not be necessary on your Genesis, so please try a "safe" way first before making irreversible changes to your console. Also, lifting the pins disables composite video and if you've done an S-Video mod, that will no longer work as well.

- Locate the following two chips: The Sony CXA1145 and the Sega 315-5313. As a note, this is a VA3 motherboard and the chips may be in a different place (or rotated differently) on your console. Click for full-sized picture:


- Now locate the Sega 315-5313 chip (or whichever video chip is on your board) and cut four wires that cover the distance between it and the bypass board. Add a tiny bit of solder to the ends of the wires and each of the following pins, then solder the wires to them. Once again, if you don't have a 315-5313 chip, these pins will not work!!! You'll need to look up the RGBs pins for your chip:

CS -> Pin 42 Red -> Pin 27 Green -> Pin 28

Blue -> Pin 29

As stated above, I still got a bit of jailbar-like interference on the model Genesis systems I tested, when soldering directly to the pins (most likely a result of the other components connected to the signal). Once I lifted the pins from the motherboard, the interference went away. Please remember this disables composite video out (and S-Video if you've done that mod) and is most likely irreversible. I don't recommend doing this on your Genesis unless absolutely needed (click for full-sized)!:


- Next, add a small amount of solder to each of the pads on the bypass board and connect the wires from the Sega 315-5313 chip:



Connecting the multi-out
If you've decided to use a different output connector then the built-in multi-out, simply solder the RGBs outputs to the matching pins on the connector. If you chose a Genesis 2-style connector, you can add stereo audio as well. For an easy way, check out the stereo audio mod guide and see if your Genesis has the same audio chip I pulled L/R audio from (although I suggest getting L&R from the top of the board, so you don't have to run wires around the side). If not, you could always install the Mega Amp and connect that instead. This (from a cable perspective) turns it into a Genesis 2, which makes 32x compatibility a bit easier. If you have SCART cables that require power, remember to connect the 5v pin as well (shown as "V" below). 

I've found that this conenctor works well, but will be testing other alternatives soon (click for full-sized):

If you're using a separate connector, please skip to the end! If you're using the built-in multi-out, please keep reading:


Warning: If you plan on using the original multi-out, cutting is required!
If you decide to use the built-in multi-out, you'll need to cut the existing traces on the Genesis motherboard, or cut the pins on the CXA chip. It's my personal opinion that much more can go wrong when cutting traces on the motherboard, which is why I suggest cutting the CXA. You can do whichever you like, but please do so at your own risk!!!

- If you're soldering to the stock multi-out, start by using a multi-meter to confirm pins 23, 22, 21 & 11 on the Sony CXA1145 connect to RGBs pins 7, 3, 8 & 1 (respectively) on the multi-out. Here's links and a picture for reference:

This is both to make absolutely sure you've located the correct pins on the CXA1145 (the chip will be oriented differently in each version of the motherboard, but the pins are the same) and to make sure there's no other components between them and the multi-out. For example, if you're using a Japanese SMS, you might find resistors and capacitors between the CXA1145 and multi-out. In that case, its much safer to simply remove those components, as nothing is "cut" and if you'd ever like to put it back to a stock configuration, simply install new components in their place.


- If there's no components between the multi-out and the Sony CXA1145, cut and separate pins 23, 22, 21 & 11 from the CXA1145 chip. I found it easiest to cut the top of the pins right where it meets the chip, then use a small flathead to separate them. Only the RGB pins are shown in this close-up picture; The csync (pin 11) is located on the other side of the chip (click for full-sized).
WARNING:  Once again, this is most likely irreversible!


- Finally, connect the outputs to the corresponding pins on the multi-out.



Summary / Tips:
- If the screen is too dark, your RGB cable most likely has 75ohm resistors in the console-side of the cable.  If that's the case, simply remove the resistors on the bypass board and bridge the connections (as shown in the above pic).

- If you're still getting any kind of jailbars or screen noise after performing this mod, there's some kind of interference.  You can try using shielded cable for the mod, but on some model Genesis systems, there's just too much interference on the board itself.  In this case, I've lifted the RGBs pins from the Sega 315-5313 and connected them directly to the bypass board (as shown above).  This is an irreversible modification that should only be done if all other options have failed!  Also, doing this will disable composite video, or S-Video (if you've done that mod too).  I'd only do this as a last-resort option.



If you're done, please head back to the main Genesis page.  If you'd like info on mods for other systems, head to the Getting RGB From Each System page or check out the main page for more retro-awesomeness.