To understand what we're going to do, we start by looking at the CTG-15 schematic on the beforemario blog. Inspecting the image, it's readily apparent that the video game controller IC is sending component video to the RF modulator.
Now, I don't want component video from a pong console, and i would prefer to use composite or Y/C. It's easy enough to tap the luma signal directly from the PCB and get a black and white image, but to get the chroma signal for S-video we'd need to either demodulate the RF output or modulate the color difference signals with the NTSC clock. Searching the terms "chroma modulator chip" leads us to the MC1372P, a Motorola chip that appears to do the same thing as the Mitsubushi M51342P and has a baseband chroma signal output, which is what we are looking for.
To replace the Mitsubishi chip, we would have to adapt the MC1372P's footprint and do some level-shifting, and this is where i was headed until i read a section on page 7 of the Motorola datasheet which states:
Seeing how similar these chips are, it would be silly not to try this, and i can attest that -- basically -- it works. There are just some pitfalls and technical issues to work through. The composite output from the modulator chip is weak and requires a buffer, and since we're not doing RF we need to separately tap the PCB for audio and buffer it. The Block Kuzushi units require an additional diode to drop the DC voltage on the Luma input to the modulator.The RF modulator may be easily used as a composite video generator by replacing the RF oscillator tank circuit with a diode as shown in Figure 3.
Ingredients:
- 1x PNP transistor (2N3906), 1x NPN transistor (2N3904), 1x silicon diode (1N4148).
- 1x 330Ω, 1x 10kΩ, 1x 100kΩ, 1x 2.2kΩ, 1x 22kΩ, 1x 27kΩ.
- 1x 220uF, 1x 4.7uF, 1x2.2uF. all rated for 16V.
- 1x 3.5mm audio jack
if working on a Block Kuzushi unit, you'll need an extra 10k resistor and another diode.
if working on a Color TV Game 6, you may want to mount a 5.5mm OD x 2mm ID barrel jack to use your famicom AC adapter with the console.
Working backwards from the RF modulator chip, remove the components circled in red:
Install the new components in the video path to make the composite output circuit. Refer to the following figure.
Note that the schematic labels the power supply lines in an unusual way. One can think of -9V as GND so what's labelled as GND becomes +9V.
You can tap audio from the small capacitor between the video game chip and M51342P pin 15.
Keep the grounding on your output connectors consistent. The machine is wired to use +9V for the video signal return path, so you need to do the same thing for the audio return path or risk damaging stuff, which i will not be liable for.
On a Block Kuzushi unit, the finished PCB should look sort of like this:
The solder side is mirrored. not all the changes in the image are relevant to this mod.
Before testing the console, make sure the channel selector switch is set to channel 2. You may need to adjust the potentiometer on pin 13 of the modulator to get a picture.
Previews:
(HDMI capture card with 480p output from the Retrotink-2X Pro)
Block Kuzushi
Color TV-Game 6
Hopefully this information will make some part of video game history more accessible.
