# Sunthar RGB mux board

# Overview

The objective of this board is to simplify the RGB mux mod performed on the CRT. It has few benefits.

  1. No need to spend time connecting all the grounds together
  2. Easier to solder the RGB in-line resistors, diodes and 75ohm resistors (this can get quiet messy)
  3. Ability to disconnect the back of the CRT TV when needed (10 pin IDC connector)
  4. Adds stability to the mod. No need for hot glue etc. to hold things in place
  5. Tested with various CRTs (Note: not all CRTs are RGB moddable. Tested CRTs are listed on this site)

# Buy this board kit

# RGB mux board

This is a two layer board. The best way to explain this board is through pictures. Below is an image of the latest board revision.

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# 3D view back (board B)

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# 3D view front (board B)

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# Design summary

This was taken from OSHPARK

# Board top & bottom

This shows the final manufactured board as if you held it in your hand. Design shows gold copper, purple mask, white silk, black drills, and the board outline. Internal cutouts are indicated by a black outline but are not filled in.

Below are the resistors and descriptions for them

Resistors Value Notes
R1, R2 and R3 75Ω R, G, B terminating resistors
R4, R5 and R6 ? R, G, B inline resistors (value for this needs to be calculated)
R7, R8 1kΩ resistors for audio left and audio right ( mono and stereo )
R9 -- short this if you need 5V, or use it for voltage dividing
R10 ? optional blanking voltage divider resistor
R11 0.7v diode to prevent current from the CRT going back to the device
R12 ? optional CSYNC 75 ohm termination. see tip below

R9 and R10 forms a voltage divider. This is optional and only to be used in cases where < 5V is needed.

Typical setup would look like the below

# Example: 5V blanking signal with just diode

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  • R9 is shorted
  • R10 is left open
  • R11 has the diode (pay attention to the direction)
  • R12 left open (in this case the CRT csync was already 75ohm terminated)

TIP

How to check if R12, 75 ohm termination is needed? Once you wire your CRT, check the resistance between R12 without any resistors. If your multi-meter reads 75Ω, you can leave this open.

# How to use the voltage divider on the board

TIP

This method only makes sense if you don't already have any grounding resistors on your CRT main board's blanking line. Check your CRT schematics and layout of blanking to be sure.

img Plan: At point A you will have 5V from console. At point B you want 3.7V, so that at point C you can have 3V after the diode voltage drop. Point C then goes out as a blanking signal directly to your CRT via the brown wire on the ribbon cable.

If you want 3V blanking signal, you can generate using the below method.

# Example: 3V blanking signal

  • R11 = 0.7V diode
  • R9 = 1kΩ (let's keep this at 1kΩ for simplicity)
  • R10 = 2.8KΩ (calculated - see below)

Calculate R10 using ohm law, voltage divider calculator

  • R10 = (Vout x R1) / (Vs - Vout) = (3.7V x 1KΩ) / (5V - 3.7V) = 2.8KΩ

Other values for R9 and R10 can also be used to achieve this. Example: R9 = 350Ω, R10 = 1KΩ. You can then use a voltage divider calculator (opens new window) to find what resistor values would give you 3.7V

# Testing

Connect the SCART cable to console, ribbon cable to the IDC adapter, turn on console. Keep the CRT off. Measure voltage between the brown wire (blanking) and black wire from the ribbon cable to confirm if you are getting 3V

You can test to see what voltages you are getting from the mux board. Make sure the mux board is not attached to your CRT when taking the measurements.

For the above exampe:

  • A should read ~5V
  • B should read ~3.7V (after the voltage division)
  • C should read ~3V (after the 0.7V diode voltage drop)

# IDC rainbow cable

The board was designed such that it took advantage of the IDC rainbow cable colors to be intuitive.

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You can buy the necessary parts and ribbon cable separately and also crimp your own cable. All you need is a plyer that can apply an even force not to break the plastic tabs. Pay close attention to the orientation of the cable in relation to the notch. You want to make sure this is correct, otherwise you will be feeding 5V from console to the wrong pin.

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WARNING

Make sure the cable is crimped exactly as shown on the above image. Black cable needs to be on the left side of the notch, when looking into the cable.

Below table provides an overview of the color of the cable and how it translates to RGB mod.

Cable Color Purpose
Brown Blanking
Red Red
Orange Ground (opt)
Yellow CSYNC
Green Green
Blue Blue
Purple Audio Ground
Grey Audio Right
White Audio Left
Black Common Ground

# Top & bottom layer

# Top & bottom silk screen

The bottom layer should appear 'mirrored' as if you were looking down on it through the board from the top.