Sega Master System RGB Bypass

PLEASE READ: The SMS 1 does not require a modification for RGB-output, simply a cable.  The SMS2 does require a mod, but a "bypass" isn't required, you just need to connect pins and add an output jack.


This page is an "experts-only" page that shows how to bypass the internal RGB amp with a different one.  All details are below, but this mod requires you to make irreversible modifications to your system. Beginner and intermediate modders should not try this mod!

NOTE:  This page will soon be updated to reflect the latest jailbar fixes / RGB bypass solutions available for the Sega Master System consoles.  The basic instructions (where to tap RGBs, sever connections and solder to the multi-out) are accurate though and the only things that will change are specifics related to the boards themselves.

 

Why perform an RGB-bypass?
There are three reasons someone might want to perform this modification:

Jailbar fix:  As shown in the picture above, performing this bypass removes most (but not all) jailbars from Sega Master System consoles. Please note that there are many different ways to reduce jailbars and doing an RGB bypass is just one alternative that I prefer.

New RGB Amp: All SMS systems use a Sony CXA1145 chip to amplify the RGB signal. The CXA1145 is an excellent chip, however this bypass uses the newer Texas Instruments THS7314, which has a high-quality low-pass filter that removes unwanted noise. Using the THS7314 isn't a clear "better or worse" choice, it's mainly a preference. Some might argue that the "ringing" the CXA1145 produces creates the specific look of the SMS that we're all used to and shouldn't be changed. I'll leave that up to you.

SMS RGB issues:  I've had many people email me regarding SMS RGB issues, using the stock multi-out. There are a few solutions for this, but performing this bypass will guarantee that your SMS will be compatible with all RGB setups.

 

 

Installation:

Here are instructions on installing a bypass in an SMS 1.  You can follow the same instructions for an SMS 2, however since there's no multi-out, you'll need to mount your own.

 

Tools / Parts Needed:

You'll need a few tools for this mod (more info on the tools can be found in the tools section):
- SMS 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

 

Determine your output solution:
If you plan on using a custom output connector, you can skip this entire section and go right to "removing the RF module". 

Sever RGBs to the multi-out:
If you're using the stock multi-out, you'll need to sever the existing RGB lines on the board. If you're using any other output connector (or have an SMS 2) you can skip this.

I recommend cutting the traces underneath the multi-out on the bottom of the motherboard to ensure the RGBs signals are no longer connected. I actually recommend two cuts on each line and then test each with a multimeter. Click the image for a full-sized view:

Alternatively, if you have a motherboard that connects directly from the CXA1145 to the multi-out (most NTSC's do), you can cut or lift the CXA pins. Also, if you have an MK-2000, you can try cutting components between. I definitely prefer the above method though.

 

Removing unneeded components from the bypass board:
If you plan on using the stock multi-out, or another multi-out that uses Genesis / SMS RGB cables (such as a Genesis 2 connector), you'll need to remove components on the RGB Bypass board:

RGB:
If you're using a stock SMS / Genesis cable, you'll need to remove the RGB caps on the board.  You can then connect RGB on the multi-out to the "input" side of the capacitor pads.  Alternatively, you can remove the caps and just solder directly to the THS7314.  You can leave the resistors on, as removing the caps will bypass them altogether.

Csync:
As always, with SMS and Genesis, csync can be problematic.  You can either use sync the way it was designed on this board, or use luma.  If needed, you can connect a sync stripper to either method:

If you'd like to use luma as sync, just solder a wire from pin 16 on the Sony CXA1154 chip to CSI on the bypass board.

Whether you decide to use csync or luma, I'd start by leaving the sync resistor & capacitor on the board and connecting the "CSO" pad to the multi out.  Then if you have issues after installation, you can remove the components one by one until the signal works properly (click for full-sized):

 

 

Removing the RF module:

- Next, completely disassemble your console, including 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 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:

Locate the Sega 315-5124 chip on your board. Tin the RGBs pins (26, 27, 28 & 29, respectively), as well as the RGBs-inputs on the bypass board and connect the two. Click for full-sized picture:

 

- Now connect RGBs-out to the multi-out. I actually prefer soldering to the top of the multi-out, to keep the wires from getting pinched. This picture shows the bypass board with the capacitors removed (for use with stock SMS cables) and RGB-out coming directly from the chip (the B&G wires aren't connected, as they would have blocked the view in the picture).

Please note that if you're using a different output connector, you'll also need to connect ground and possibly 5v. Click for full-sized:

Alternatively, you can solder directly to the pins on the bottom of the multi-out (composite video pin is just shown for reference):

That's it! Now give it a try!

 

 

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).

- There will always be some noise or jailbars on SMS consoles, but after performing the mod, it should be greatly improved!

 

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.