Editing Sega Mega Drive / Genesis (Model 2)

{{Infobox computer
| name          = Sega&nbsp;Genesis&nbsp;Model&nbsp;2<br />(Mega&nbsp;Drive&nbsp;II)
| image         = [[File:Sega-Genesis-Mk2-6button.jpg|260px]]
| caption       = Sega Genesis MK2 with 6 Button Controller
| manufacturer  = Sega Enterprises&nbsp;Ltd.
| type          = 16-bit home video-game console
| release date  = {{Start date|1993|04|23}} (JP)<br />{{Start date|1993|10}} (US)<br />{{Start date|1993|10}} (EU)
| discontinued  = 1998 (superseded by '''Genesis 3''' & licensed clones)
| price         = JP&nbsp;¥12 800 • US&nbsp;US$99 • EU&nbsp;£99
| cpu           = '''Motorola MC68000''' @ 7.67 MHz (NTSC) / 7.61 MHz (PAL)
| coprocessor   = '''Zilog Z80''' @ 3.58 MHz (audio / SMS-compat)
| 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†
| graphics      = '''VDP ASIC 315-5660'''<br />320 × 224 / 256 × 224 active • 64/512 palette<br />80 sprites, H&V scroll, DMA, line-scroll
| sound         = '''ASIC YM3438-core''' 6-ch FM + DAC<br />'''TI SN76489 PSG'''<br />Left/right line-mix to AV multi-out
| dimensions    = 220 mm W × 210 mm D × 55 mm H
| weight        = ≈ 700 g (VA1 board + shielding)
| power         = 10 V-DC 0.85 A (AC Adaptor MK-1631), < 7 W
| os            = Minimal TMSS ROM v2.x (8 KB, inside ASIC)
| compatibility = Mega Drive/Genesis & Master System (via PBC) software • Sega CD (Model 2) & 32X
| predecessor   = [[Sega Mega Drive (Model 1)]]
| successor     = [[Sega Genesis (Model 3)]]
| codename      = ''Takatoku'' / “Mark VI” compact
| model         = HAA-N0 (JP) • MK-1631 (US) • MK-1631-50 (EU)
}}

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.

== Historical Context ==
* 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 ==
{| class="wikitable" style="width:70%; text-align:center;"
|+'''Integrated ICs (VA1 reference)'''
! 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? ===
* 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) ===
* 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) ===
{| class="wikitable" style="width:70%;"
! 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 ==
{| class="wikitable" style="width:70%;"
|+'''Mini-DIN 9 A/V (console female, front-view)'''
! 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 ==
{| class="wikitable" style="width:95%; text-align:center;"
|+'''Model 2 VA Boards'''
! 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 ==
1. 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 ==
* '''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 ==
{| class="wikitable" style="width:88%;"
! 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 ==
* '''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 ==
* Mega Drive II Service Manual (Sega, 1993) – schematics VA1 & VA2.
PDF links available at [[:Category:Sega Service Manuals|Sega Service Manuals]].

== Trivia ==
* 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 ==
* [[Sega Mega Drive (Model 1)|Sega Mega Drive / Genesis (Model 1)]] – original “high-def” hardware  
* [[Sega CD (Model 2)]] – side-loading optical add-on  
* [[Sega 32X]] – 32-bit accelerator

[[Category:Sega Systems]]