Master System:SMS II Video Mods
The Sega Master System II was a cost-reduced successor to the original Master System, and was introduced during the early 1990s as a budget alternative to the 16-bit Genesis/Mega Drive. In addition to eliminating the reset button, expansion port and Sega Card slot, it also did away with the AV and RGB outputs in favor of only having RF out. While this may have been acceptable at the time, especially since the SMS II was marketed as a budget console, nowadays this makes it more difficult to connect the console to monitors such as Sony PVMs or modern upscalers.
Fortunately, the SMS II is capable of being easily modified for composite, S-video or RGB outputs.
Overview
Despite only having RF output, the Master System II generates its video the same way as all other Master System consoles. Its VDP outputs RGB, which is then wired to a video encoder chip. The encoder then outputs a composite video signal which is fed into an RF modulator alongside the audio signal. On the NTSC SMS II, the video signal going into the modulator is already suitable for outputting directly to a display, so composite video can be simply tapped from the appropriate RF modulator pin after removing a pulldown resistor. A similar process also goes for the audio signal. Both of these can be done without removing the RF modulator, though the modulator can be removed in order to facilitate a no-cut mod. On the PAL SMS II, a separate circuit will need to be created if the modulator is to be retained; if not, only some slight circuit changes will be needed to get a proper composite video signal.
Getting S-video output requires more work, especially in the case of the CXA1145 encoder used in all NTSC SMS IIs and some PAL models. While it generates separate chroma and luma signals necessary for S-video, these were intended to be fed back into the encoder in order to generate composite video and were not designed for connection to an external display. Nevertheless, it is possible using external circuitry to get a usable S-video output from this encoder. The Fujitsu MB3514 used in later PAL SMS IIs is capable of outputting proper S-video, requiring only a 75 ohm resistor on both the chroma and luma signals.
RGB output is even easier to attain since both the CXA1145 and MB3514 also output amplified RGB. Depending on the connector, no extra components are needed.
If you have a PAL SMS II and wish to have it output NTSC video, refer to this procedure here.
Note that the SMS II sold in France differs in that it only outputs RGB, and was bundled with a special SCART cable. It lacks a video encoder entirely as well as all of the associated support circuitry, so it cannot be easily modified for composite or S-video output. Because it shares the same motherboard as the NTSC SMS II, it is possible to repopulate these components to get composite, S-video and amplified RGB, but that is beyond the scope of this procedure.
Materials and Tools
Materials
- Leaded solder
- Hookup wire
- Flux
- Output connector(s) of choice (see procedure for details)
- Additional components (see procedure for details, this will vary depending on the particular installation)
Tools
- Soldering iron
- Flush cutters
- Wire strippers
- Desoldering iron or desoldering braid
- Cordless drill with step bit (depending on output connector selection)
- Multimeter
Procedure
Composite
Video is taken from Pin 20, and requires a 75 ohm resistor and 220uF capacitor in series, with the positive lead towards the chip, and the negative towards to RCA socket.
Audio is mono only, and is taken from Pin 9 and requires a 10uF capacitor with the positive lead towards the chip.
S-Video
You will require a standard NPN Transistor and a 27 ohm resistor for the Luma line, and a 75 ohm resistor and 220uF capacitor for Chroma.
Chroma is located on Pin 15, and requires the resistor and capacitor (positive to chip) in series to the S-Video port (Pin 4, top left).
Luma is a little more tricky, and requires Pin 16 to be connected to the middle pin (Base), the left pin of the transistor (Emitter) is connected to a 27 ohm resistor to the S-Video port (Pin 3, top right). Power for the right leg of the transistor (Collector) can be obtained from Pin 12 of the CXA chip (bottom left)
The following diagram is made by raelgc:
