Sega Master System (Model 1) Controller Port Repair

Sega's controller-port circuit for the PAL board 171-5534: CN6 (player 2) and CN5 (player 1), each line through an EMI filter, with the 330 Ω resistors R114 to R117 in two lines of each port

The two control-pad sockets of the Sega Master System (Model 1) are nine-pin D connectors, CN5 for player 1 and CN6 for player 2. On Sega's PAL schematic each of the nine lines of a socket passes through an EMI filter on its way to the board (EM11 to EM19 for CN5, EM21 to EM29 for CN6). Two lines of each socket also pass through a 330 Ω resistor (R114 to R117), and the direction and button lines end at the custom gate array IC4. Pin 5 carries +5 V to the pad and pin 8 is ground.

Sega's troubleshooting guide lists the parts on this path as the causes of a pad that does not respond: the socket, soldering, a cut track, the EMI filter, the 330 Ω resistor and IC4, with a continuity check from the socket pins to IC4. The procedure below follows each line from the socket to IC4 and repairs the break or short.

Symptoms

Sega divides the faults into two groups:

Controller faults in Sega's guide

Symptom	Kind of fault	Where
Game does not start; a button does not respond; no response at all	An open circuit	Socket, joints, track, EMI filter, 330 Ω resistor, IC4
Demonstration screen does not appear; a button stays pressed; a direction stays held; the game starts by itself	A short circuit	Socket, solder bridge, track, IC4

Rule out the pad first. Sega's own list for a pad that does not work names a cut cable, a bad connection at the plug, a cut track, faulty conductive rubber and soldering; for a pad that moves by itself, a shorted cable, greasy solder and stuck conductive rubber. Try a pad that works on another console, and try the suspect pad in the other socket. A fault that follows the pad is in the pad; a fault that stays with one socket is in the console.

Pause and Reset are switches on the console, not on the pad; see the Sega Master System Troubleshooting Guide.

Tools and parts

• A known-good control pad.
• Multimeter with a continuity range.
• Phillips screwdriver, soldering iron, desoldering pump or braid.
• Replacement parts as needed: EMI filters (Sega 271-0007, 2200 pF, or 271-0006, 270 pF for EM17 and EM27), 330 Ω resistors, or a nine-pin D-sub socket (Sega 209-5017-01).

Controller socket pins (Sega PAL schematic)

Pin	Signal	Player 1 path	Player 1 at IC4
1	Up	CN5, EM11	Pin 25
2	Down	CN5, EM12	Pin 26
3	Left	CN5, EM13	Pin 27
4	Right	CN5, EM14	Pin 28
5	+5 V	CN5, EM15 to Vcc	–
6	TL (button 1)	CN5, EM16	Pin 29
7	TH	CN5, EM17	Pin 31
8	Ground	CN5, EM18 to ground	–
9	TR (button 2)	CN5, EM19	Pin 30

Pin functions and IC4 pins from Sega's schematic; the filter numbers follow the socket pin order on the drawing (EM21 to EM29 for player 2 on CN6). The 330 Ω resistors sit in two lines of each socket.

Procedure

Open the console

1. Unplug the adaptor and all leads.
2. Remove the screws in the bottom case and lift it off; on a West German console remove the bottom shield plate too.
3. Lift the board out with its top shield.
4. Find CN5, CN6, the two rows of EMI filters and R114 to R117 at the front edge of the board on Sega's parts-location drawing.

Sega's parts-location drawing. CN5 and CN6 are at the bottom edge, right of centre, with the EMI filters in rows above them and R114 to R117 beside them

Check for a break

With the console unplugged:

1. Check continuity from each pin of the faulty socket, through its EMI filter, to the IC4 pin in the table. Sega gives this continuity check from the socket terminals to IC4.
2. Where a line has no continuity, check each joint and track along it, and check the filter itself: continuity from its input to its output.
3. On the two lines with a 330 Ω resistor, measure the resistor out of circuit or with one end lifted: 330 Ω.
4. Check pin 5 for continuity to the 5 V rail and pin 8 to ground. A pad gets no power if pin 5 is open.

Check for a short

If a direction or button is held on by itself:

1. Measure between each signal pin of the socket and ground (pin 8), and between neighbouring pins. A line that reads short is the stuck one.
2. Look for solder bridges and damaged tracks between the socket pins and along the lines to IC4; Sega lists a shorted socket, a solder short and a shorted track.
3. Check the filter on that line for a short from its signal side to ground; replace it if it reads short.
4. If the line is clean all the way and still pulled low with power on, Sega's last cause is IC4.

Repair

1. Reflow cracked socket joints.
2. Repair a cut track with a wire link along the same path.
3. Replace a failed EMI filter with the same type and value as the original, 2200 pF for all positions except EM17 and EM27, which are 270 pF.
4. Replace an open 330 Ω resistor.
5. If the socket itself is cracked or its contacts are damaged, replace it with a nine-pin D-sub of the same footprint.

Reassemble

Refit the board, the shields and the bottom case in reverse order.

Testing

• With a known-good pad in each socket, every direction and both buttons respond in a game.
• The demonstration screen appears and the game does not start by itself.
• +5 V between pins 5 and 8 of each socket with the console on.

Revision notes

The circuit and the parts are from Sega's PAL documents for the 171-5534 board. The North American board 837-6067 in the photograph on Sega Master System (Model 1) carries the same designators, the filters EM11 to EM19 and EM21 to EM29 grouped above and beside the two sockets and R114 to R117 next to them, in a different layout; no schematic for that board is hosted here.

Related pages

• Sega Master System (Model 1)
• Sega Master System Troubleshooting Guide
• Sega Master System General Maintenance
• Sega Master System (Model 1) Power Regulator Replacement
• Sega Master System (Model 1) Video Output Repair
• Sega Master System Service Manual (1986)