Sega SC-3000

The Sega SC-3000 computer 

This is an interesting and rare machine. It is Sega's only computer. Originally launched in 1983, it predates the MSX standard, although it has comparable specs and game quality. Like the MSX machines, it is based on a Z80-compatible CPU, uses a TMS9929A (a version of the TMS9918A) video chip, and it has 16K video RAM. Curiously, it has very little RAM, only 2k. The Basic language is available as a cartridge that also adds more memory. The computer is compatible with Sega's first home console, the SG-1000.

My Sega SC-3000 after some superficial cleaning. This is a PAL version, I don't recognize the keyboard layout.

I got mine from a vintage computer vendor, it came with the Basic Level III B cartridge that adds 32K RAM. It did not come with a power supply, so I'll have to figure out how to power it.

Disassembly

The unit can be opened by removing 3 screws in the front of the case. Then the keyboard can be removed.  The top of the case is held in place by the keyboard and plastic tabs.

After removing 3 screws, the keyboard comes off. The top of the case is secured with tabs.

Close view of the keyboard connectors. I really dislike these kind of connectors. They are used in many other contemporary computers such as the SVI-328.

The motherboard is encased in a metal shield with top and bottom parts soldered to the motherboard. It is hard to remove the motherboard from the case: it is necessary first to desolder the on/off switch from two leads that connect it to the motherboard; there are slots in the bottom of the case, towards the back, were a flat screwdriver can be inserted to free the board from plastic clips, then the board can be removed, it is a tight fit.

To access the motherboard, I removed the top and bottom shields from the board by melting each solder point with a large iron tip, then bending the shield away from the board. After removing the shield, I used a vacuum desoldering gun to remove solder from the board and the shield tabs. I plan to mount the shield back in the motherboard but not solder it.

Motherboard, after removing the shield and cleaning some dust. There is a daughter board over the video processor. Amazing that the designers were able to route this single sided board.

View without the daughter board. The heatsink is quite impressive, I wonder what is under it ?

The motherboard is curious in that it is a single sided board. These kind of boards are more often found in VCRs, power supplies, and other high volume products. The only other computer in my collection that uses a single sided board is the Panasonic FS-A1, a later MSX-2 machine. 

This board was made in 1984, judging from the later dates on the chips.

The SC-3000 has 16K of video RAM, but instead of the common 4116 16k x 1bit DRAM Chips, it uses the MCM4517, also a 16k x 1bit DRAM, but one that only requires 5V, instead of (-5V, 5V, and 12V). Since only one voltage is required, the power circuit is quite simple.

View of the underside of the motherboard. The heatsink is for the 7805 linear voltage regulator mounted though a hole.

According to online sources, the power supply provides 9V and it is center negative. I verified the polarity, and it seems that the 9V are used as input to a 7805 linear regulator (mounted flat, on the back of the board, and attached to a heatsink through a hole in the board). The linear regulator generates the 5V needed by most components.

View of the video daughter board. The main IC is the LM1889N video modulator. It requires 12V that are generated here.

I used this opportunity to install my usual polarity reversal protection: a power diode, inline with the circuit, installed right after the power socket. The voltage drop from the diode should not pose a problem, because the 7805 will work well with an input voltage as low as 7V.

Trace cut between the fuse (large pads) and the power switch.

2-Amp diode installed for reverse polarity protection.

First Test

Now that I knew how to power the computer, it was time to test it. I also needed to get a composite video signal. The computer has a monitor port, I got it from there.

Setup for the initial test. I added a short adapter to reverse the polarity of a modern 12V power supply (central positive). I am using alligator clips (yellow and black) and a copper wire, to tap the composite video signal from the output din connector. I connected the Basic III-B cartridge. The red alligator clips are used in place of the power switch.

And it is working!

Resources

• Story of the SG-1000 and SC-3000 (Wikipedia).
• General Information about the SC-3000 at SegaRetro.
• Sega SC-3000 Review on YouTube.
• Sega SC-3000 30 Games in 3 minutes, on YouTube.
• Lots of Sega documentation.
• Sega SC-3000 Pinouts.
• Sega SC-3000 Schematics.