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Sega Master System (Mark III)

Sega's 8-bit console (1985, Mark III in Japan; Master System in export from 1986). A NES rival. Native RGB on the Model 1, FM sound on the Japanese models, and a cost-reduced Model 2 (composite only).

Identification

Field Value
Manufacturer Sega
Release 1985 (Mark III, JP) / 1986 (SMS, export)
Generation 3rd (8-bit)
Revisions Model 1 (RGB, card slot, ext.) · Model 2 (cost-reduced) · Mark III JP
Native video output RGB (Model 1, DIN socket); composite/RF (Model 2)
Region / lockout JP/export region + 50/60 Hz
Media format Cartridge + Sega Card (Model 1)

Architecture (detail)

Sega Master System Model 1 motherboard
Master System Model 1 board: Z80 (NEC D780C-1), Sega VDP 315-5124. Photo: Evan Amos (Vanamo Media), public domain.
Block Chip Clock Notes
CPU Zilog Z80 3.58 MHz
Video (VDP) Sega 315-5124 Derived from the TMS9918; Mode 4: 256×192, 32 colours out of 64, 64 sprites (8/line)
Sound SN76489 (PSG) 3 voices + noise; + YM2413 (FM, 9 voices) on Mark III / Japanese SMS
RAM 8 KB work + 16 KB VRAM
Media Cartridge + Sega Card (Model 1 slot) + Light Phaser, 3D glasses

Revisions

  • Model 1: native RGB (DIN), Sega Card slot, expansion port, LED, reset. The best for AV.
  • Model 2: cost-reduced — composite/RF only (RGB needs a mod), no card slot, no reset.
  • Mark III / Japanese Master System: FM sound (YM2413) stock.

Peripherals & built-in game

  • Light Phaser: an optical gun (CRT only).
  • SegaScope 3-D Glasses: active LCD-shutter glasses plugged into the card slot (Model 1) — stereoscopy on a few games (Space Harrier 3-D…).
  • Sports Pad (trackball), Paddle Control, Rapid Fire Unit.
  • ROM game (BIOS): many consoles have a built-in game (often Alex Kidd in Miracle World or Hang-On/Snail Maze) launched with no cartridge.
  • Sega Card / My Card: the thin, cheap card format of the earliest games (Model 1 slot only).

Video outputs & quality

  • Native RGB on the Model 1 (DIN) — excellent over SCART.
  • Model 2: RGB mod required to get RGB out.

The Brazilian case & preservation

  • Tectoy (Brazil) kept the Master System alive for decades (re-releases, exclusive games, consoles sold until recently) — the world's largest SMS territory, a major preservation resource.
  • Game Gear kinship: the Game Gear is a portable SMS (same Z80/VDP) — hence the Master Gear adapter to play SMS cartridges on it.
  • Flashcart: Master EverDrive (Krikzz); very mature emulation (Emulicious, MAME, Genesis Plus GX).

Common faults

Symptom Likely cause Lead
Cartridge not read Connector oxidation Contact cleaning
Yellowed plastic UV / ABS Retrobright
No image Power, VDP, multi-out Diagnosis
Incomplete sound PSG / (FM) Sound-stage diagnosis

(Few electrolytic capacitors: recap secondary.)

Original service manual ✅

Manufacturer manual archived locally. It typically carries the schematics, the block diagram, the expected voltages and waveforms, the parts list and the disassembly procedures — the reference when a symptom matches no known fault.

  • Master System — service manual (Sega) — PDF
  • Master System II — service manual (1991) — PDF

Status

A manufacturer document with no explicit licence, kept for preservation (machine long out of production) — the same position as for the cabinet and arcade manuals already archived here. Source: Internet Archive.

Source / board revision

Procedures transcribed from ConsoleMods (CC-BY-SA). The SMS has several boards (171-53xx / 55xx / 63xx): the points below are for the common VDP 315-5124; check your board number on the back. Soldering: 🟢 none · 🟡 · 🟠 · 🔴.

FM sound (the SMS export "signature" mod)

  • Only Japanese SMS / Mark III (+ FM Sound Unit) had the YM2413 (FM sound). Export units lack it — but the FM code stayed in ~40 games from 1987-89 (Wonder Boy, Phantasy Star, OutRun, Shinobi, Fantasy Zone II…) because Sega reused the same ROMs.

  • SMSFM (Tim Worthington board): adds a YM2413 FM chip (based on the FM Sound Unit) → FM sound on an export SMS, with a PSG/FM switch (some games break if they detect FM: Wanted, Walter Payton Football) and, from v2.0, an EN/JP region switch (needed for Wonder Boy III's FM). Chip variants: 1.x = through-hole YM2413, 2.x = YM2413B-F (SMD), 3.0+ (2025) = UM3567 (clone).

    • ⚠️ Install solder points are per-model in Tim Worthington's © illustrated manual — not copied/invented here (like the 2600RGB). Docs/purchase: timworthingtondesigns. Cartridge alternative: Neo Sega MKIII (mixes FM via audio jacks, see Audio Line Out below). → Audio: FM mod page.
Audio Line Out — clean separate audio out, exact points (transcribed from ConsoleMods, CC-BY-SA)

Takes audio directly from the A/V DIN port (to cut crosstalk from cheap SCART cables, or feed the Neo Sega MKIII which adds FM by mixing). The SMS is mono → 2 wires (audio + ground).

  • Tap on the DIN A/V: audio = left pin, ground = right pin.

  • Standard: drill the shell, fit a 3.5 mm female jack, wire audio/ground to the DIN. No-cut: run an RCA cable through a vent and solder to the same points on the board (audio left, ground right), Kapton underneath.

  • If using an RGB-SCART cable with no separate audio input: wire audio to both pins 2 AND 6 of the SCART, and audio ground to pin 4.

Video — RGB (exact procedure, solder points)

What to do depends on the model

  • SMS 1 (export): already outputs amplified RGB → no mod, just an RGB/SCART cable.
  • French SMS 1: unamplified RGB (the special "Adapteur R.V.B." cable) → a bypass is advised.
  • SMS II: no multi-out → you must add a jack and wire the RGB yourself.
Tapping RGBS from the VDP 315-5124 (transcribed from ConsoleMods, CC-BY-SA)

Stock video chip: Sony CXA1145 encoder (or Fujitsu MB3514 / Sony V7040, pin-compatible). The RGBS is tapped directly on the Sega 315-5124 VDP:

Signal 315-5124 VDP pin
Red (R) 26
Green (G) 27
Blue (B) 28
Composite sync (CSYNC) 29

Luma as sync (if CSYNC is troublesome — classic on SMS/Genesis): run a wire from pin 16 of the CXA1145 to the CSI input on the bypass board.

RGB Bypass Mod — replace the stock amp with a THS7314 (transcribed from ConsoleMods, CC-BY-SA)

The bypass replaces the stock amplified RGB path with a TI THS7314 amp (low-pass filter → less noise; greatly reduces — not fully — the jailbars). Irreversible mod, 🟠 (not for beginners). You need an RGB bypass board (THS7314), iron, desoldering braid/gun, Kapton, thin wire, multimeter.

  1. Isolate the stock output: if keeping the multi-out, cut the RGBs traces under the multi-out (confirm the break with a multimeter) — or cut/lift the CXA1145 pins (on most NTSC boards the CXA goes straight to the multi-out).

  2. Remove the RF modulator: fully disassemble (heat sink included), desolder the RF modulator (patience, braid/gun), clean the pads on top and bottom.

  3. Mount the bypass board where the modulator sat: Kapton underneath; ground through the 3 ground holes (thick wire, soldered both ends); +5 V from the modulator's 5 V mounting hole (confirm with a multimeter).

  4. Wire the RGBS from the 315-5124 VDP (pins 26/27/28/29) to the bypass board inputs; output CSO (+ R/G/B) to the multi-out.

  5. Picture too dark? The cable has 75 Ω resistors on the console side → remove the resistors on the bypass board and bridge them.

French SMS (unamplified RGB) (transcribed from ConsoleMods, CC-BY-SA)

French SMS units (Model 1 and II) only output unamplified/unbuffered RGB (hence the special cable). The bypass is easy (no video encoder to work around):

  • French Model 1: 3 bodge wires carry the VDP's RGB to the encoder output vias. Identify by the resistor footprint at the other end: R6 = Red, R7 = Green, R8 = Blue; composite sync where R19 meets the DIN connector. Desolder the bodge wires, inputs → bypass board, bypass outputs → encoder vias.

  • French SMS II: same but with jumpers: desolder JPR (red) / JPG (green) / JPB (blue), then wire the matching vias to the bypass board inputs/outputs.

  • Audio Line Out Mod: see the FM sound section above (exact procedure transcribed).

Region / speed — 50/60 Hz & PAL→NTSC

The SMS has little region lock (mostly 50/60 Hz + the encoder's PAL/NTSC video mode). Two levels: the simple 60 Hz switch, or a true PAL→NTSC conversion.

Simple 60 Hz (Model 1) + full PAL→NTSC conversion (transcribed from ConsoleMods, CC-BY-SA)

Set a Model 1 SMS to 60 Hz: the VDP's frequency-select pin goes to a pad, bridged by a JP1 trace to another pad (on PAL, to 5 V). Cut the JP1 trace between the centre pad (VDP pin 57) and the right pad (5 V), then wire the centre pad to the left pad (GND) → 60 Hz. (On the PAL SMS II the frequency pin is wired directly to 5 V → isolate it before grounding.)

True PAL→NTSC conversion (NTSC colour image, correct speed) — materials: 53.693 MHz oscillator (salvageable from a Mega Drive), 33 pF cap (early Model 1), wire.

  1. Oscillator: replace the PAL master oscillator 53.2034 MHz with the 53.693 MHz (mind the silkscreen orientation). The Z80 then runs at ~3.58 MHz = the NTSC colour subcarrier.
  2. Subcarrier: the resistor tied to pin 6 (subcarrier) of the CXA1145 comes from the subcarrier trace of the I/O chip 315-5237 (pin 42). Isolate the front end of that resistor (lift the lead or cut the trace — confirm no continuity with 315-5237 pin 42), then solder a short wire from Z80 pin 6 (CPU clock) to that resistor end.
  3. VDP 60 Hz: do the JP1 mod above (VDP pin 57 → GND).
  4. Encoder to NTSC mode: isolate encoder pin 7 from ground and tie it to 5 V (nearest 5 V = encoder pin 12).
  5. (Optional NTSC fine-tuning): replace L2 and L3 with 39 µH inductors and omit L1.

French SMS: these are essentially NTSC consoles with the encoder omitted, VDP set to 50 Hz by a jumper → conversion = just change that frequency jumper (they output RGB, Z80 clock already NTSC).

Library

  • Master EverDrive X7 (Krikzz) — SMS + Mark III games/homebrew on SD, plug-and-play 🟢.
  • Mega Drive backward-compat: SMS games run on the MD via the Power Base Converter (the MD's Z80). → Power Base Converter.

Recap

  • Cap kit per board/condition.

Photos / assets

(To add: Model 1 vs Model 2 board, RGB DIN socket, FM area.)

Advanced repair

Recap if picture/sound degrade; cartridge/card contacts = the No. 1 cause of "game won't start". Methods: Recap · PCB fault-finding · Traces & vias.

Sources & attribution

Image:

Review & corrections

How this space is used

Spotted a wrong value, an outdated procedure, a chip reference that does not match your board? Say so here, with what you observed (model, board revision, serial number, measurement). Every report is cross-checked against a source before anything changes — an unverifiable correction is published as “reported by …, not cross-checked” rather than silently applied.

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