This troubleshooting guide provides a structured workflow, pin‑by‑pin test tables, and known failure modes for the Sega Master System II. Although simplified compared to the original Master System, the SMS II shares most core ICs—enabling diagnosis of over 90 % of “dead” units with basic tools and a flash cart.
- Isolated bench 9 V DC, ≥1 A supply with current limit (SMS II uses external 9 V adapter → 7805)
- DMM (200 mV, diode, continuity)
- Logic probe or 20 MHz oscilloscope
- ESR‑meter (for bulk caps: 220 µF, 470 µF)
- Solder station, hot‑air rework, flux & braid
- Diagnostic flash cart (EverDrive SMS or homemade 27C256 “Test ROM”)
| Custom IC |
Function |
Package |
Common Failures
|
| 315‑5216 |
Gate‑array: I/O, memory decode, banking |
100‑pin QFP |
Stuck RESET, no cartridge select
|
| 315‑5246 |
VDP (Video Display Processor) + PSG audio |
40‑pin DIP |
No video or mono screen, missing audio
|
| Z80A |
Main CPU @ 3.579545 MHz (NTSC) / 3.546894 MHz (PAL) |
40‑pin DIP |
Rare – ESD or over‑voltage
|
| TMS4164 × 2 |
8 KB VRAM (64 kbit × 2) |
18‑pin DIP × 2 |
Garbled tiles, vertical bars
|
| BIOS ROM |
256 KB mask ROM |
28‑pin DIP |
Blank until cart inserted
|
SMS II 5 V Rail – expected
| Test Point |
Nominal |
Notes
|
| 9 V DC adapter jack |
9–10 V DC |
Centre‑pin positive :contentReference[oaicite:7]{index=7}
|
| 7805 regulator output tab |
5.00 ± 0.15 V |
Ripple < 100 mV p‑p :contentReference[oaicite:8]{index=8}
|
| Z80 pin 11 (+5 V) |
same |
First CPU pin to probe
|
| VDP pin 13 (+5 V) |
same |
Loss = open LK3 or bad filter cap
|
No 5 V → blown fuse F1 (500 mA), oxidized power switch solder, or shorted electrolytic (C23 470 µF / C69 220 µF).
5 V low (< 4.6 V) under load → 7805 overheating; replace with NJM7805FA or SMPS module.
5 V high (> 5.3 V) = 7805 short to Vin – power off immediately to protect VDP & RAM.
- Reset (Z80 pin 26)
-
- Should be low ~30 ms on power‑up then high. Permanently low → bad 315‑5216, C38 10 µF or R44 10 kΩ
- Clock (Z80 pin 6)
-
- NTSC ≈ 3.579545 MHz; PAL ≈ 3.546894 MHz. Absent clock → open crystal, cracked Y1 solder or dead VDP oscillator
| Signal |
Pin |
Probe Result |
Meaning
|
| /MREQ |
19 |
0 ↔ 5 V activity |
CPU fetching ROM/RAM
|
| /IORQ |
20 |
Pulses ~ every 64 µs |
VBlank interrupt routine running
|
| /HALT |
18 |
HIGH |
OK – low means CPU halted (bad RAM/ROM)
|
If clock and reset are good yet all A/D lines static ⇒ CPU dead or memory decode fail (/MREQ stuck) ⇒ swap 315‑5216.
SMS II outputs via onboard composite; no RF cage on NTSC units.
- Check composite at JP3 pad – if sync present but TV “no signal” → dead CXA1145 encoder
- No sync at all → VDP not running (check HSync pin 35, VSync pin 36 ~5 kHz/60 Hz)
- HSync toggles but garbage → VRAM or address‑bus failure
| Pattern |
Cause |
Action
|
| Thin stripes |
One TMS4164 data bit |
Piggy‑back good chip; replace socket
|
| Checkerboard |
VDP VRAM latch inside 315‑5246 |
Swap VDP
|
| Flicker when warm |
Oxidized cartridge slot pins |
Clean with DeoxIT D5
|
PSG (SN76489 inside VDP)
| Issue |
Check |
Likely Cause
|
| No sound, video OK |
VDP pin 7 silent |
LM324 op‑amp or C68 1 µF coupling cap
|
| Buzz tied to brightness |
Ground loop in encoder |
Bypass RF box, use direct‑AV mod
|
| One channel only |
C27/C28 10 µF open |
Replace with 105 °C caps
|
- Both ports dead → 5 V missing on pin 5 (fuse F2 500 mA)
- One direction stuck → shorted 74HC157 or cracked trace under port socket
- RESET/PAUSE issue → faulty tact switch or broken via to 315‑5216 (/NMI pin)
- “Insert cartridge” loop → /CS (edge B1) must pulse low when address ≥ \$0000. Check R31–R34 (0 Ω buffers)
- Clean contacts with IPA; if still no read, isolate RAM by lifting U8/U9 pin 20
SMS II physical map (BIOS enabled)
| Range |
Size |
Device |
Notes / Entry
|
| \$0000–\$03FF |
1 KB |
BIOS vector table |
RESET \$0000, NMI \$0066
|
| \$0400–\$1FFF |
7 KB |
BIOS code & logo |
Calls cart at \$0150
|
| \$2000–\$3FFF |
8 KB |
BIOS bank 2 |
Unused in SMS II
|
| \$4000–\$BFFF |
32 KB |
Cartridge bank 0 |
Mapped when BIOS disabled
|
| \$C000–\$DFFF |
8 KB |
Work‑RAM |
IC3 GM76C28
|
| \$E000–\$FFFF |
8 KB |
Mirror of \$C000 |
Stack defaults here
|
- 7805 runs at ~80 °C in stock RF cage – add heatsink or drop‑in DC‑DC module
- Replace all 85 °C caps near hot ICs: C23, C69, C27/28
SMS II has only one board revision. Compared to Model 1, it removed the Sega Card slot, RESET switch, power LED, RF switch, and RGB port.
| Symptom |
Most Likely Fix
|
| Dead, no LED |
Replace F1 fuse & 7805
|
| LED on, black screen |
Verify clock; swap VDP; check /RESET
|
| Vertical stripes |
Replace bad TMS4164 DRAM
|
| Random resets |
Replace C38 10 µF reset cap; clean cart slot
|
| No sound |
Replace C68 1 µF & LM324; check VDP pin 7
|
| Controllers not detected |
Re‑flow port pins; replace 74HC157
|
