TMS9929 RGB Adapter

TMS9929A / TMS9928A RGB

A RGB demodulator for TMS9929A / TMS9928A chip, mostly used on MSX computers and Colecovision.

This board outputs RGB+CSYNC from the original YPbPr chip channels of the TMS99 chip. Output from each color channel can be adjusted using a trimmer. Based on the Universele interface RGB op MSX1 en SV.328. Mine fixes the emitter follower transistors of this version. This repository contains the KiCad project and the Gerber files, the PDF schematics and the BOM file.

Features

• Each channel can be tuned
• Provides audio output pin
• Provides 5V output pin (if you want to use it with an RGB Scart cable for "modern" TVs)
• Fits into the MSX composite modulator slot
• Extremely cheap!

Video

Sample output (photo taken on BVM 20DU)

Assembled PCB

The PCB in a Phonola VG-8020

How to assemble

• Y, Pb, Pr signals must be taken directly from the TMS chip (pin 35,36,38 on TMS9928A or TMS9929A)
• Audio is from pin 4 of the AY38910
• 5V pin is optional: wire it only if you need to connect the computer to a scart TV.
• Removing the original composite modulator (on MSX) is not mandatory but encouraged: unfortunately it creates noises also on the RGB interface. Keep in mind that removing the modulator will disable the composite video and the RF video out of the machine.
• On the Phonola VG8020 I removed the RF modulator to make room for a 8 pin mini din connector. I used the 8pin Mini DIN to EuroSCART PACKAPUNCH cable for RGB modified consoles. It's possible also to desolder the original composite out connector and use a 8 pin DIN connector to make it compatible with a Sega Genesis 1 SCART cable.

External view of the mini din connector (RF modulator has been removed)

Perks

This board provides an RGB output for the TMS9929A / TMS9928A chip. Although it's impossible to have 100% correct colours, it's possible to obtain a palette very close to the correct one in this way:

• Set each RGB trimmer to 0
• Turn up a little the RED and GREEN potentiometer until something visible appears on screen
• Turn up the BLUE potentiometer until the background becomes blueish - keep in mind that the blue channel is the most problematic one because of the TMS99x non-standard design
• Execute the command color 1,1 to set the screen and the text to black
• Notice how the black is blueish, and turn down the BLUE potentiometer until the background becomes less blueish ;)
• Execute the command color 15,4 to restore the screen to the default colours
• Repeat until you get a satisfactory result
• adjust the GREEN and the RED potentiometer accordingly

In this way the palette should be close to the original.

Compatibility

This board has been tested with TMS9929A (PAL) chip and TMS9928A (NTSC).

• PCB shape is the same of TMS9929A RGB and Component adapter
• Special thanks to iz8dwf