Sega's 8-bit platform in the mid-80s: two Z80s (game + sound), tiles +
sprites, and above all many encrypted games (custom Z80 / MC-8123). Flicky,
Wonder Boy, Choplifter, Pitfall II… System 2 is an evolution (more sprites). A
MiSTer core exists → excellent preservation.
System 1 board. Photo uploaded by Nedlemouse to Sega Retro, a site under CC BY 4.0.
Sega custom Z80 (315-5xxx): the opcodes are scrambled; without the
(reverse-engineered) decryption table the program is unreadable.
MC-8123: a decryption module with a battery-backed key (like some System
16) → a dead battery can "suicide" those games. The keys have been extracted by
the community.
→ Per-game keys (FD1094/FD1089/MC-8123/8751):
System 16 — per-game keys (the table also
covers the MC-8123).
Video, power, control panel and upscalers depend on the cabinet rather than the board:
everything is centralised in Modernising a cabinet.
To run the board outside a cabinet (at home, on a display):
supergun.
Battery 🟢: a save battery is fitted (settings/counters/RTC). ⚠️ On this platform, a dead battery does NOT kill the game — it is a leaking battery that corrodes traces. Check and replace it before it leaks → save batteries.
Storage 🟡: the game lives in EPROM/mask ROM on the board. Changing or restoring a game means reprogramming the EPROMs (and, for a mask ROM, using a pinout adapter) → EPROM method.
Media preservation: replacing the original storage, multicarts, ODEs and modern options by family → PCB preservation hub.
Scan rate 📺: ~15.62 kHz / 60.1 Hz — standard resolution, a classic 15 kHz chassis is fine — the most common JAMMA case → tri-sync and upscalers.
(Computed from the MAME sources — sega/system1.cpp — pixel clock ÷ htotal.)
FPGA / emulation: see the Emulation & FPGA section below.
Values extracted from the MAME sources for this platform. On a scope or frequency counter, a clock that is missing or off-value points straight at the crystal, the divider or the chip at fault — the fastest starting point on a dead board.
Component
Expected clock
Role
I8751
8 MHz
protection MCU
SN76489A
2 MHz · 4 MHz
PSG
Z80
4 MHz
sound CPU (usually)
How to read this table
Several values for one component = several board variants in this family (or several instances of the chip clocked differently). These frequencies are those of the clock signal at the component's input, not of its original crystal — on many boards the clock is divided down from a single master crystal.
Manufacturer manuals for games running on this board. Beyond the instructions, they very often carry the full DIP settings, the connector pinout, the test procedure, the parts list and the expected voltages.
40 games on this platform, settings extracted from the MAME sources (INPUT_PORTS_START blocks). The Position column gives the switch's physical reference on the board (SW2:!3,!4 = bank SW2, levers 3 and 4) — which is what lets you set it up without the paper manual. Games are sorted by MAME set name.
Normal Game · Free Play · Test Mode · Endless Game
DIP settings — MAME set wmatch
Setting
Position
Options (default in bold)
Demo Sounds
SWB:2
Off · On
Time
SWB:3,4
Normal · Fast · Faster · Fastest
Difficulty
SWB:7
Easy · Hard
Check against your own board
These settings apply to the game and revision described by MAME. One title may have several DIP sets depending on region or revision, and some systems use soft-dips (settings in memory, via the test menu) rather than physical switches. In case of a discrepancy, the board's silkscreen and the test menu are authoritative.
The general method lives in
PCB diagnosis. This is its
application to this platform: the same order, but with the values
actually expected here, so you do not have to go back and forth.
A CRT holds its charge with the machine unplugged. →
CRT safety
1. Inspection, power off. Corrosion, cut tracks, oxidised EPROM legs, leaking
SMD capacitors, dubious earlier repairs.
On this platform the battery is the first suspect: its leakage
eats tracks under the holder, and the damage is often hidden by the
battery itself. Lift it to look.
2. The connector, before anything else. A large share of "dead boards" are a
contact problem, not a fault. Clean the contacts with isopropyl
alcohol and reseat — not with an abrasive eraser on a gold edge, which
takes the plating off and brings the problem back worse. It costs two minutes and settles the question.
3. Power, measured on the board. Aim for +5.0 V measured as close as
possible to where power reaches the board — not at the PSU output: the drop
is in the loom, and that is exactly what makes a board boot on the bench and
not in the cabinet. Where you probe depends on this board's connector — JAMMA
edge, JVS loom or a proprietary harness — plenty of makers kept one long
after JAMMA arrived — see Identification above. → Wiring
Check −5 V and +12 V too if the sound stage uses them. Suspected
short: bring it up current-limited and find the hot spot.
4. /RESET. It must release after power-up. Held low = the CPU never starts.
A 1–2 Hz loop is not the reset circuit, it is the watchdog: the CPU is
crashing at boot, so look at the bus, not at the reset.
5. Clocks — the values expected on this platform.
I8751 → 8 MHz(Protection MCU)
SN76489A → 2 MHz · 4 MHz(PSG)
Z80 → 4 MHz(Sound CPU (usually))
Several values for one component means several board variants in
this family, not several readings on the same one: identify yours before
calling a clock wrong. That is the mistake that replaces a healthy crystal.
6. Bus. Activity on the main CPU's address lines tells you whether it is
running or frozen. Frozen with good clocks and good reset → work RAM, bus
buffers, ROMs.
7. It boots but the picture or the sound is wrong — that is another job:
see Common faults on this page, then
signal tracing.
Procedure — replacing the battery without losing the key ✅¶
Not every board in this family is concerned
Check on this page whether your game is one of the encrypted
ones before touching anything. On the others the battery only holds
settings, and losing it costs nothing but a re-setup.
The order is the whole procedure
The decryption key of the MC-8123 lives in SRAM held up by
that battery. Cutting the old cell before the new one is soldered erases
it, instantly and for good: the board goes silent and only a de-suicide
brings the game back. So the new cell goes in first, in parallel.
Board powered off, old battery left in place.
Measure the old cell in circuit and note it (a CR2032 is 3 V nominal). This is
what tells you whether you are ahead of the failure or already behind it.
Solder the new cell in parallel with the old one — piggyback — watching
polarity. A reversed cell discharges the old one through itself and erases the
key just as surely.
Re-measure at the holder: the voltage must have come back up. Do not go on
before it has.
Z80 program in 8-32 KB ROMs: 2764 (8K) · 27128 (16K) · 27C256 (32K) depending
on the game's age (e.g. Choplifter/Shooting Master/WBML/UFO Senshi = 3× 27C256; Flicky/Wonder
Boy = small ROMs). ⚠ Many System 1 use an encrypted Z80 (317-xxxx): the program is
encrypted, you need the decrypted ROMs (MAME dumps) for a bare-Z80 board.
Precedes the System 16 (16-bit, 68000). See also the
System E (Master System hardware) and the Super Scaler
(sprite-scaling pseudo-3D) of the same period.
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.
Reading is open to everyone; posting requires signing in with Discord.
Reports from the wiki's declared reviewers are handled first; anyone else's are read
too, but go through a human before anything is changed.
