Repair & Schematics
Sega Master System

Clean RGB Mod (Master System)

8min

The Master System already outputs RGB out of the AV port, however it has a DC offset which makes it incorrect voltage levels to most monitors.

Most people solve this with making custom RGB cables with AC coupling capacitors and resistors. This is ok, but it's far from perfect. It introduces all problems that AC capacitors add (field droop, tilt, common mode noise coupling from the receiver side and more.

All the RGB mods I personally make are called the CleanRGB. It aims at correctly fixing the console itself to output IRE correct voltage level and signals, and the cable is then a pure DC cable with direct connections, no hidden capacitors, sync strippers, resistor or other magic.

The benefits of this are pure, perfect RGB signals not affected by your cable, receiver, or noise.

Overview

This install will involve:

  • Getting RGB and Sync inputs from CXA1145 to CleanRGB
  • Severing traces from CXA1145 RGB and Sync outputs to AV Port
  • Wiring RGB and Sync outputs from CleanRGB back to AV Port
  • Tuning the CleanRGB for perfect IRE levels

Wiring RGBs Inputs

Start by installing a CleanRGB Universal board into the Master System. Tap the RGB and Sync inputs directly from the CXA1145.

CXA1145 Pinout
CXA1145 Pinout
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The R, G and B In go to the CleanRGB inputs, and we tap 5V from the CXA pin 19 and ground from the nearby ground.

CleanRGB Inputs
CleanRGB Inputs
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Disconnect AV Port from CXA

Next, we need to disconnect the AV port from the existing connection to the CXA1145.

Master System AV Pinout
Master System AV Pinout
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The R, G and B output pins usually travel in a set right by the output pins. Test continuity to confirm or follow visually. On this revision you can see them here and severing them all is easy.

RGB Trace Cuts
RGB Trace Cuts
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Finally, we need to do the same for the CSYNC pin 10 (it uses the CSYNC In not Out pin) of the CXA1145 to the CSYNC pin of the AV port.

Document image
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Connect CleanRGB Output

Now wire up the RGB and Sync outputs to the AV port pins.

AV Port Pinout
AV Port Pinout
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RGB Output Wiring
RGB Output Wiring
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Tuning CleanRGB

Checkout the setup guide for tuning the CleanRGB, or for quickness set your dials to the following positions and then tune by eye.

Master System Pots
Master System Pots
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Results

Using our CleanRGB SCART cable, here are the results on a Master System.

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Document image
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