A first-generation Master System has only seven large custom ICs; with a meter, logic probe and a flash cartridge you can diagnose > 90 % of “dead” units at the component level. This page gives a structured workflow, pin-by-pin test tables and known failure modes.
- Isolated bench 9 V DC, 1 A supply with current-limit.
- DMM (200 mV, diode, continuity).
- Logic probe or 20 MHz oscilloscope.
- ESR-meter (for bulk 220 µF & 470 µF caps).
- Solder station, hot air, flux & braid.
- Diagnostic flash cart (EverDrive SMS, SD-Card GG PRO, or homemade 27C256 “Test ROM”).
- Service manual PDF & board-schematic (link on Sega Master System Service Manual (1986)).
| Custom IC |
Function |
Packages |
Common failures
|
| 315-5216 |
Gate-array: I/O, memory decode, banking |
100-pin QFP |
Stuck RESET, no controller read
|
| 315-5124 / -5246 |
VDP (Video Display Processor) + PSG audio |
40-pin DIP |
No video, single-colour screen, missing audio
|
| Z80A |
Main CPU @ ≈3.58 MHz (NTSC) / 3.55 MHz (PAL) |
40-pin DIP |
Rare – ESD or over-voltage
|
| 64 kbit ×2 DRAM |
8 KB VRAM (TMS4164, KM4164C) |
18-pin DIP × 2 |
Garbled tiles / vertical bars
|
| BIOS ROM |
256–512 KB mask ROM |
28-pin |
Blank screen until cart inserted
|
Master System VA-x expected rails
| Test-point |
Nominal |
Notes
|
| DC IN header |
9–10 V DC |
Center-pin positive
|
| Regulator tab (IC6 7805) |
5.00 ± 0.15 V |
< 100 mV p-p ripple
|
| 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 (1 A), oxidised power switch, shorted electrolytic (C69 220 µF or C23 470 µF).
- 5 V low (< 4.6 V) under load → 7805 overheating; replace with NJM7805FA or switch-mode “TRACO TSR 1-2450”.
- 5 V high (> 5.3 V) = 7805 short to Vin – power off immediately to save the VDP and RAM.
- Current draw > 700 mA at boot usually means shorted 315-5124 or reverse-polarity plug.
- Reset – Z80 pin 26
-
- Should sit low for ~30 ms on power-up, then high (5 V). Permanently low → bad 315-5216, C38 (10 µF) or R44 (10 kΩ).
- CPU clock – pin 6
-
- NTSC board ≈ 3.579545 MHz (10.738635 MHz crystal ÷ 3). PAL ≈ 3.546894 MHz. Absent clock = open XTAL, cracked Y1 solder or dead VDP.
| Signal (CPU) |
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 address/data lines are static ⇒ CPU dead or no memory decode (/MREQ stuck) ⇒ swap 315-5216.
- Confirm composite at JP3 pad – if composite has sync but TV shows “no-signal” your encoder (Sony CXA1145 / MC1377) may be dead.
- No sync at all → VDP not running (check pins 35 HSync, 36 VSync ≈ 5 kHz / 60 Hz).
- If HSync toggles but picture is garbage → VRAM or address-bus failure (see below).
| Pattern |
Probable part |
Action
|
| Thin 8-pixel vertical stripes repeating |
One 4164 DRAM data bit |
Piggy-back good 4164; replace socket
|
| Checkerboard, stable colour |
VDP VRAM address latch inside 315-5124 |
Swap VDP
|
| Random characters, flicker when warm |
Oxidised cartridge slot A18–A20 |
Clean with DeoxIT D5
|
Dead test ROMs (SMS Power “Hardware Test”) run from flash-cart and exercise VRAM; failures are printed on-screen.
- Common on VA3–VA6 due to 5 V ripple on VDP analog pins.
- Replace C23 470 µF and C9 100 µF; add 10 µF tant on VDP pin 1 (analog 5 V).
- Re-flow RF-box ground posts – cracked solder injects hum into composite.
PSG (SN76489-compatible inside VDP)
| Issue |
Check |
Likely cause
|
| No sound, video OK |
Pin 7 (YM PSG out) silence |
LM358 amp bad (IC11) or broken C68 1 µF
|
| Constant buzz, volume w/ screen brightness |
Ground loop inside RF modulator |
Bypass RF box, install direct-AV board
|
| Only one channel, clipping |
C27/C28 10 µF coupling caps open |
Replace caps, 105 °C type
|
Note: FM expansion board (Japan only) mixes at JP1; if jumpers removed audio will mute.
- Both pads dead → 5 V missing on pin 5 of both ports (fuse F2 500 mA open).
- One direction stuck → shorted 74HC157 (IC4, some VA boards) or cracked trace under port socket.
- PAUSE button dead but Reset works → defective tactile switch or broken via between SW1 and 315-5216 pin 71 (/NMI).
- “Insert cartridge” BIOS loop
- /CS pin (edge B1) must pulse low when address ≥ $0000. Check for 0 Ω resistors R31–R34 (buffer to 315-5216).
- Clean cart edge with IPA; if still no read, RAM /CE may be stuck pulling bus – lift pin 20 on each 4164 to isolate.
- My Card slot issues – Slot wiring is “straight-through” to cart bus; most failures are oxidised leaf contacts. Same troubleshooting as cartridge.
SMS physical address map (with BIOS enabled)
| Range |
Size |
Device |
Notes / entry points
|
| $0000–$03FF |
1 KB |
BIOS vector table |
RESET $0000, NMI $0066
|
| $0400–$1FFF |
7 KB |
BIOS code & logo |
Calls CART start at $0150
|
| $2000–$3FFF |
8 KB |
BIOS (bank 2) |
Game Gear mode stub
|
| $4000–$BFFF |
32 KB |
Cartridge bank 0 |
Mapped when BIOS disabled
|
| $C000–$DFFF |
8 KB |
Work-RAM (1 × 8 KB) |
IC3 GM76C28
|
| $E000–$FFFF |
8 KB mirror |
Mirror of $C000 |
Stack defaults here
|
If writes to $C000 do not read back → RAM bad. If writes to $4000 echo on bus → 315-5216 decode fail.
Download “SMS Hardware Test” from SMS Power; flash to EPROM/flash-cart.
- Tests Z80 RAM, VRAM, VDP registers, PSG tones, controller ports (loopback jig uses DB-9 diodes).
- 7805 runs at ≈80 °C in stock RF cage – install bolt-on heatsink or drop-in DC-DC module to reduce heat.
- Early VA0 boards use NEC uPC7805 with high dropout; replace proactively.
- Replace all 85 °C caps near heat sources: C23, C69, C27/28.
| PCB rev |
Distinguishing marks |
Known quirks / bugs |
Fix
|
| VA-0 |
Green mask, discrete 74LS logic |
RGB level too high (1.2 Vp-p) |
Add 75 Ω series R per colour
|
| VA-2 (NTSC) |
315-5124 DP VDP |
Jailbars common |
Recap + ground braid under VDP
|
| VA-3/VA-4 |
315-5246 VDP |
Poor RF audio balance |
Re-tune T4 pot or bypass RF
|
| VA-6 PAL-I |
315-5363 encoder |
Composite overly dark |
Change R46 from 75 Ω → 47 Ω
|
| Symptom |
Most likely fix
|
| Dead, no LED |
Replace fuse F1 & 7805
|
| LED on, black screen |
Verify clock; swap VDP; check /RESET
|
| Vertical stripes |
Replace bad 4164 DRAM (U8/U9)
|
| Game boots but resets randomly |
Replace C38 10 µF reset cap; clean cart slot
|
| No sound |
Replace C68 1 µF & LM358; check VDP pin 7
|
| Controllers not detected |
Re-flow JP1, replace 74HC157
|
