Sega Mega Drive / Genesis (Model 2)
The Sega Genesis Model 2 – sold outside North America as the Mega Drive II – is the first major redesign of Sega’s 16-bit console. Introduced in 1993, it condenses the original two-board architecture into a single cost-reduced PCB that integrates more logic into the 315-5660 “M5” ASIC, deletes the headphone amplifier, and replaces the large 8-pin DIN with a mini-DIN-9 A/V port.
 | |
| Sega Genesis MK2 with 6 Button Controller | |
| Specifications | |
|---|---|
| Manufacturer | Sega Enterprises Ltd. |
| Type | 16-bit home video-game console |
| Released | April 23, 1993 (JP) October 1, 1993 (US) |
| Discontinued | 1998 (superseded by Genesis 3 & licensed clones) |
| Intro price | JP ¥12 800 • US US$99 • EU £99 |
| CPU | Motorola MC68000 @ 7.67 MHz (NTSC) / 7.61 MHz (PAL) |
| Memory | 64 KB 68k WRAM • 64 KB VRAM • 8 KB Z80 RAM • 128 B colour/VSRAM |
| Storage | ROM cartridges ≤ 4 MB (40 Mbit†) • Sega CD & 32X add-ons† |
| Display | — |
| Sound | ASIC YM3438-core 6-ch FM + DAC TI SN76489 PSG Left/right line-mix to AV multi-out |
| Dimensions | 220 mm W × 210 mm D × 55 mm H |
| Weight | ≈ 700 g (VA1 board + shielding) |
| OS / Firmware | Minimal TMSS ROM v2.x (8 KB, inside ASIC) |
| Predecessor | Sega Mega Drive (Model 1) |
| Successor | Sega Genesis (Model 3) |
| Codename | Takatoku / “Mark VI” compact |
| Model no. | HAA-N0 (JP) • MK-1631 (US) • MK-1631-50 (EU) |
Historical Context
edit- Development began mid-1992 to cut manufacturing cost (fewer PCBs, chassis tooling shared with Model 2 Sega CD).
- Launched 23 April 1993 in Japan bundled with *Sonic CD* coupons; reached U.S./EU retail in October 1993 at a launch price of US $99 (console + Sonic 2).
- Model 2 rapidly replaced remaining Model 1 inventory and remained Sega’s primary 16-bit SKU until 1998.
- Total Genesis/Mega Drive family sales surpassed 30 million units worldwide; ~40 % are estimated to be Model 2 boards (VA0-VA4, Majesco VA7/GOAC).
Hardware Architecture
edit| ASIC | Functions Combined |
|---|---|
| 315-5660 “M5” | VDP core • YM3438 FM synth • PSG • I/O (pads, TMSS) • Bus arbiter |
| 315-5684 | RGB DAC • stereo mixer • master clock PLL • TV encoder controls |
| External silicon | 68000 CPU • Z80 CPU • 64 KB SRAM × 2 (VRAM) • 64 KB SRAM (WRAM) • ROM & IO glue |
What Changed vs. Model 1?
edit- Removal of discrete YM2612; FM engine now inside ASIC (= YM3438 core) → slightly cleaner FM but different DAC ladder, causing famous “tinny” Model 2 audio.
- Mini-DIN-9 A/V with C-sync no longer routed; composite chroma/luma fed from the encoder inside 315-5684.
- On-board RCA RF modulator omitted – external RF switch box used in NTSC territories.
- TMSS boot ROM updated to v2.00 (“Produced by or under license…”) / “Sega v2.00 1993”.
- 32X compatibility assured: side expansion edge identical to Model 1 (though PCB cut-out shifted 2 mm on VA0).
System Clocking (NTSC)
edit- Master crystal: 53.693 MHz → /7 = 7.670 MHz (68k), /15 = 3.579 MHz (Z80).
- Pixel clock: 53.693 MHz / 1 → VDP; colour sub-carrier generated internally by 315-5684.
Memory Map (abridged)
edit| Address | Size | Device |
|---|---|---|
| $000000-$3FFFFF | ≤ 4 MB | Cartridge ROM |
| $A00000-$A0FFFF | 64 KB | Z80 bus window |
| $A10000-$A10FFF | 4 KB | I/O, TMSS, version register |
| $C00000-$C0003F | 64 B | VDP data & control |
| $FF0000-$FFFFFF | 64 KB | 68k Work-RAM (batteryless) |
🔌 External Connectors
edit| Pin | Signal | Notes |
|---|---|---|
| 1 | +5 V (75 mA) | RGB SCART auto-switch |
| 2 | Audio right | 1 Vpp line-level |
| 3 | Composite video | 1 Vpp, 75 Ω |
| 4 | Audio left | – |
| 5 | Ground | signal ground |
| 6 | Y (luma) | For S-Video on rare VA3-S boards (unused) |
| 7 | C (chroma) | – ″ – |
| 8 | Blue (0.7 Vpp) | RGB direct from DAC |
| 9 | Red (0.7 Vpp) | – |
- Green shares composite video on model-2 harness.
- CSYNC no longer present—mods tap VDP pin 11.
Motherboard Revisions
edit| Rev | GPU/Audio Core | Notes | Audio Quality |
|---|---|---|---|
| VA0 | YM3438 + discrete pre-amp | Early JP only; poor S-video traces | Mild hiss |
| VA1 | YM3438 inside 315-5660 | Global; “M5” silkscreen | Noticeable PWM noise |
| VA2 | Same ASIC; cheaper DAC | Wide RF shield removed | Slightly darker |
| VA3-S | Adds 315-5684-A encoder | Provides true S-Video in Asia PAL | Best stock audio |
| VA4 | Majesco/AtGames | GOAC “315-6123” (everything on one chip) | Muddy, hissy |
Errata: VA1 consoles can exhibit “rainbow jail-bars” due to shared RGB line impedance; mitigated by installing 470 Ω series resistors or full Triple-Bypass mod.
Audio Path
edit1. YM3438 DAC (inside ASIC) → 2. passive RC low-pass → 3. 315-5684 mixer → 4. Sony CXA1645 (NTSC) or MB3514 (PAL) video encoder → 5. mini-DIN audio pins. No headphone amp, therefore no PWM scaling knob; DC bias ~1.3 V, so improper cable coupling causes “mono bleed” on some TVs.
Servicing & Maintenance
edit- Capacitors – fewer electrolytics; critical audio rails: C68 (47 µF/16 V), C41 (22 µF), C9 (10 µF 315-5684).
- DC jack – fragile centre pin; desolder & replace with Kycon KLDX-2.5A for loose-power issues.
- Fuse – surface-mount 1 A polyfuse PF1; easy to open-circuit after PSU spikes.
- AV port strain – mini-DIN anchors only by solder; re-flow cracked pads on units with heavy SCART cables.
Comprehensive procedures: see Sega Mega Drive / Genesis (Model 2) Maintenance Guide and Sega Mega Drive / Genesis (Model 2) Capacitor Replacement Guide.
Troubleshooting Cheatsheet
edit| Fault | Likely Cause | Remedy |
|---|---|---|
| No video / green LED | PSU MK-1631 failing (9 V sag) | Use 9 V DC 1 A centre-barrel replacement |
| Video ok, silence | 315-5684 audio caps dried | Replace C68, C69; check mini-DIN pin 2/4 continuity |
| Jail-bar vertical stripes | Poor VDP RGB drive | Install Triple-Bypass or series resistors R30-R32 = 470 Ω |
| 32X “error 04” | EXT port dirty | Alcohol scrub edge; check /RESET from CN-E |
Popular Mods
edit- Rapid-fire switch (JP2) – solder toggle to TH input via 74HC04.
- Region + 50/60 Hz – wire JP3/JP4 to 3-way slide; TMSS bios auto-selects RGB palette accordingly.
- RGB Bypass / Triple-Bypass – PCBs by RetroRGB, Voultar: taps raw VDP RGB & YM3438, yields crystal-clear output.
- Mega Amp v2 – dedicated discrete op-amp board recreates Model 1 headphone mix.
- MiSTer Analogue IO – Model 2 shell often re-purposed as FPGA console.
Technical Resources
edit- Mega Drive II Service Manual (Sega, 1993) – schematics VA1 & VA2.
PDF links available at Sega Service Manuals.
Trivia
edit- Early Model 2 NTSC boards left “GAMES = TOYS?” text in ROM v2.00; never displayed.
- Bundles: Sonic 2 (“Not For Resale”) cart in U.S.; The Lion King in PAL territories (1994).
- Brazilian licensee Tec Toy produced a turquoise “Mega Drive 3” reuse of Model 2 VA3 until 2007.
See Also
edit- Sega Mega Drive / Genesis (Model 1) – original “high-def” hardware
- Sega CD (Model 2) – side-loading optical add-on
- Sega 32X – 32-bit accelerator