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Sega Mark III

The Master System's Japanese ancestor (1985): the third machine in the SG-1000 line, redesigned to move past the SG-1000's TMS9918 with an in-house VDP. Rebranded as the Master System in the West (1986). Takes cartridges and Sega Card (My Card), with an FM sound (YM2413) add-on.

Sega Mark III

Sega Mark III — photo by Sintegrity, CC BY-SA 4.0 (Wikimedia Commons).

Identification

Field Value
Manufacturer Sega
Released 20 October 1985 (Japan, ¥15,000)
Line SG-1000 → SG-1000 II → Mark III → Master System (West, 1986)
Media Cartridge (Mega Cartridge) + Sega Card / My Card (≤ 32 KiB)
Sound built-in PSG + FM Sound Unit (YM2413) add-on

Architecture (detail)

Block Chip Clock Role
CPU Zilog Z80A 3.58 MHz (rated 4 MHz, underclocked)
Video Sega in-house VDP (315-5124, TMS9918-derived) 256×192, 32 colours on-screen from 64
Sound Texas Instruments SN76489 (PSG, 3 square + noise) + YM2413 (FM) via add-on
Memory 8 KiB RAM · 16 KiB VRAM · 8 KiB ROM

The VDP derives from the TMS9918 (SG-1000) but borrows from Sega System 2 arcade hardware: more colours, better sprites. On sound, the FM Sound Unit (YM2413 chip) markedly enriches compatible games — it became standard on the Japanese Master System of 1987. In the West the Mark III becomes the Master System (new shell, same guts).

Repair (technician)

  • Cartridge & Sega Card contacts: the My Card slot and cartridge port oxidise → isopropyl clean, the No. 1 cause of "game won't start".
  • Capacitors: routine recap (period electrolytics) if picture/sound degrade or boot is flaky.
  • Video output: DIN; the Mark III outputs an RGB-capable signal (like the SMS) → a clean RGB mod is possible (see video mods).
  • FM Sound Unit: an external add-on (expansion connector) — check the connector and the YM2413 if FM is silent.
  • Compatibility: the Mark III reads SG-1000 carts; a Master System reads Mark III games (same ROMs), handy for testing.

Common faults

Symptom Likely cause Lead
Game won't start cartridge / My Card contacts Clean the connectors
Degraded picture/sound tired caps Targeted recap
FM silent FM add-on / YM2413 Check connector, test the chip
Nothing on screen PSU / joints / VDP Check PSU, reflow, VDP

Mods & preservation — what to buy, how

  • RGB mod 🟠: the 315-5124 VDP outputs RGB → same bypass procedure as the Master System (THS7314-style video buffer, sync extraction). See the detailed procedure in Master System § RGB — the Mark III shares the VDP.
  • Flashcart: the Master EverDrive X7 (Krikzz) — SMS/Mark III games on SD, plug-and-play 🟢 (the SG-1000 library runs too).
  • FM Sound Unit: period add-on that enriches the audio (~40 games 1987-89 drive the YM2413) — keep/connect it; the Japanese/Master System can take an internal FM mod (see Master System § FM).
  • Recap: few electrolytics → recap is secondary (check power/output).
  • ⚠️ Do not host ROMs — method only.

Photos / assets

(To add: Mark III, FM Sound Unit, My Card, motherboard.)

Advanced repair — cross-cutting methods

PCB fault-finding · Recap · Traces & vias.

Sources & attribution

Review & corrections

How this space is used

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