Sega Master System II Troubleshooting Guide

Sega's parts-location drawing for the PAL-G board (837-7367), with values marked against each position

Sega's April 1991 service manual for the Sega Master System II gives a test order, a fault list for each IC, and go/no-go figures for the supply, reset and clocks. The manual covers six regional boards; their assembly numbers are listed in the maintenance guide.

Sega's procedures were adapted from the original Master System manual and keep some of its text. The power fault list gives "LED doesn't light" as a symptom, but the Master System II has no power light, and the no-sound list names Q48, Q49, R46 and R47, which are not in either Master System II parts list. The original manual's no-sound list names Q48, R46 and R47 on the Model 1 board.

Before you start

The console runs from an external 9 V DC adaptor and has no mains wiring inside. The adaptor is a mains transformer unit; if you open one, unplug it and treat the primary side as live until you have measured it.

You will need a multimeter, an oscilloscope or frequency counter for the clock checks, a temperature-controlled soldering iron, a known-good cartridge and a known-good controller. See Recommended Tools.

Sega's test order

Sega's flowchart works from the supply outwards:

1. Power switch.
2. +5 V at the output of IC8.
3. Reset circuit.
4. 10.738 MHz output at IC4 pin 41.
5. 3.58 MHz clock at IC1 (Z80) pin 6.
6. Pulses at IC2 (ROM) pins 20 and 22.
7. Pulses at IC3 (work RAM) pins 1, 20, 22 and 27.
8. RGB, composite sync and audio signals at IC5 (VDP).
9. Pulses at IC6 and IC7 (video RAM) pins 20, 22 and 27.
10. RGB in at IC9 pins 1–4; composite video out at IC9 pins 19 and 20.
11. RF modulator input, and +5 V at the modulator.

Power and reset

Supply and reset checks

Test point	Sega's figure	Parts Sega names if it fails
IC8 (7805) input	DC 9 V	IC8, soldering, short circuit, cut track, power switch, heatsink attachment
IC8 output	DC 5 V (Vcc)	as above
Vcc at each IC	5 V	cut track, soldering
Test point JR	0 to 3.6 V, rising slowly	C18, R4, soldering

All four checks are from Sega's test procedures. On the PAL board C18 is a 10 µF 16 V electrolytic and R4 is 100 kΩ; with diode D1 they form the power-on reset. The board has no fuse; none appears in either parts list.

Sega supplied 9 V DC 0.5 A adaptors for Europe (400-5126), the UK (400-5129) and Australia (400-5130), and a 9 V DC 1 A adaptor (400-5048Y) with the French RGB version. Measure the voltage at IC8's input with the console switched on; a reading at the plug with nothing connected does not show whether the adaptor can supply the load.

Clocks

Clock checks

Point	Sega's figure	Level	Notes
IC4 pin 41	10.738635 MHz ±100 Hz	6 Vp-p	NTSC figure. Sega's fault list for this check names "OSC (53M)", the 53.203424 MHz oscillator fitted to PAL boards
IC5 pin 33	3.579545 MHz ±30 Hz	5 Vp-p	CPU clock. A PAL board gives 3.546893 MHz (53.2034 MHz ÷ 15)
IC4 pin 42	4.43361875 MHz ±30 Hz	5 Vp-p	PAL colour subcarrier (53.2034 MHz ÷ 12)

The figures are from Sega's test procedures. The manual's waveform drawings label the IC5 pin 33 trace "225.6 ns" and the IC4 pin 42 trace "279 ns". Those periods belong the other way round: 279 ns is one cycle at 3.58 MHz and 225.6 ns one cycle at 4.43 MHz.

For a fault in the oscillator circuit Sega lists the oscillator, soldering, a cut track, IC4 and a frequency outside tolerance, with the symptoms no picture or a monochrome picture. A missing or off-frequency 4.43 MHz subcarrier at IC4 pin 42 is the check to make for a picture without colour.

Symptoms by IC

Sega's table of symptoms for each IC, with the parts from the PAL parts list:

Master System II ICs and the symptoms Sega gives for each

IC	Part	Symptoms
IC1	Z80A CPU (315-0041)	System does not operate; no picture
IC2	OS mask ROM, 1 Mbit (MPR-12808)	No picture; sometimes a mosaic pattern
IC3	µPD4168C-20 work RAM (315-0298)	No picture; or a picture that appears but is easily distorted
IC4	Gate array 315-5237 (µPD65005C-114); 315-5216 on the RGB board	No picture; controller, pause or reset not responding; no 3.58 MHz clock if there is no oscillation
IC5	VDP 315-5246 (µPD9004G)	No picture; distorted characters; wrong colours; sound without picture; picture without sound
IC6, IC7	µPD4168C-15 video RAM (315-0298-15)	No picture; wrong colours; distorted characters; sluggish or jerky movement
IC8	MA7805UC regulator (313-0092)	No stable +5 V; system does not operate
IC9	CXA1145P RGB-to-composite encoder (313-5067)	No picture; abnormal colour

Sega's own descriptions: IC2 is "the program which operates the system", IC3 "temporarily records data", IC4 "emits several control signals", IC6 and IC7 store "color data", and IC9 "converts the RGB signal to composite". IC3, IC6 and IC7 are static RAM; Sega's specification gives 64 Kbit of RAM and 128 Kbit of video RAM. The RGB parts list has no IC9.

IC2 holds the built-in game. MPR-12808 is the Alex Kidd in Miracle World BIOS, a 128 KB ROM used in most Master System IIs; later European consoles have MPR-15155, a 256 KB ROM with Sonic the Hedgehog. Console5 lists MPR-15155 at IC10 on board 171-6395A.

No picture

Sega's checks, in order:

1. Is there video at pin 3 of the RF modulator?
2. Is composite video coming out of IC9 pins 19 and 20?
3. Are the RGB signals present at IC9 pins 1–4?

Sega's first listed cause is "C38, R29 is absent or faulty", then soldering, a cut track, the RF modulator (when there is a picture on RF), IC9 and the parts around it. On the PAL board C38 is a 100 µF 10 V electrolytic and R29 a 2.2 kΩ resistor; the schematic notes that R29 is 33 Ω on the PAL-B (VE) board.

If RGB reaches IC9's inputs and nothing leaves its outputs, suspect IC9. If there is no RGB at IC9, go back to IC5 and the video RAM.

For a dirty picture, no sound, heavy audio noise, or one channel working and not the other (CH3 but not CH4), Sega points at the RF modulator, its +5 V supply and IC9.

No sound

With a picture and no sound, Sega's check is the audio output at IC5 pin 15: up to 300 mVp-p, 20 Hz to 20 kHz. The listed causes are soldering, a cut track, IC5 and the RF modulator, plus the Q48, Q49, R46 and R47 entries carried over from the Model 1 manual. The sound generator is inside IC5; the PAL board has no ICs other than IC1–IC9.

Controls, pause and power switch

For controls that do not respond, Sega lists: CN2 or CN3 faulty or disconnected, soldering, a cut track, a faulty IC, a faulty EMI filter, a faulty 330 Ω resistor, and a continuity check from the CN2 and CN3 terminals to IC4. The 330 Ω resistors are R5–R8; the EMI filters are EM11–EM18 and EM21–EM28.

Sega lists these symptoms under a short circuit: the demonstration screen does not appear, a button stays pressed during a game, a direction stays held, or the game starts by itself. The causes are a short in CN2 or CN3, a solder bridge, a shorted track, or IC4.

Faults Sega lists for the power switch are a loose switch, soldering, a cut track and a short circuit, with symptoms including no power, no picture, spontaneous resets and characters suddenly becoming abnormal. For the pause switch it adds "blade is not parallel with circuit board". The pause button reaches the CPU as a non-maskable interrupt generated through the VDP.

Cartridge faults

For no picture, a monochrome picture or a mosaic pattern with a cartridge fitted, Sega lists a dirty or disconnected 50-pin connector, soldering, a short circuit, a missing or faulty capacitor, and an IC damaged by heat or static. Clean the cartridge contacts and CN1 first.

For characters that break up or resets in the middle of a game, Sega lists the 50-pin connector, soldering, a missing or faulty capacitor, "IC runaway overheating" and a faulty IC. For the overheating case it gives a fix: add a 100–180 pF capacitor at A13 (address line 13).

On the cartridge connector pins 1 and 33 are +5 V.

Not faults

• There is no power light, reset button, card slot or expansion port on the Master System II.
• With no cartridge fitted the console starts its built-in game after a static Sega logo, with no logo animation, music or "Snail Maze".
• The 7805 runs warm. It drops 9 V to 5 V and is fitted to a heatsink.
• A Brazilian "Master System II" is a revised Model 1 with a different board.

Quick reference

Where Sega points first

Symptom	Check
Dead	Adaptor; IC8 input and output; power switch; IC8 heatsink attachment
Black screen	5 V at the ICs; reset at JR (C18, R4); clock at IC1 pin 6
Mosaic pattern	IC2; cartridge connector
Picture appears but is easily distorted	IC3
Wrong colours, distorted characters, jerky movement	IC6, IC7
Sound but no picture, or picture but no sound	IC5
No picture on RF	C38 and R29; IC9; RF modulator
Monochrome picture	Oscillator circuit; 4.43361875 MHz at IC4 pin 42
CH3 but not CH4	RF modulator
Controls dead	CN2, CN3; EMI filters; 330 Ω resistors; continuity to IC4
Resets in the middle of a game	CN1; then 100–180 pF at A13

All entries are from Sega's repair procedures.

Related pages

• Sega Master System II
• Sega Master System II General Maintenance
• Sega Master System II Capacitor Replacement Guide
• Sega Master System II Service Manual (1991)
• Sega Master System Troubleshooting Guide