Sega SG-1000 Joystick Repair

Enri's redrawn joystick circuit: the second joystick socket (JOY2) and the header for the fixed joystick (CN01), each line pulled up by 4.7 kΩ and read through the 74LS257 multiplexers IC18 and IC21

The first joystick of the Sega SG-1000, an SJ-200, is wired to the console, and the second plugs into a nine-pin socket on the side. Inside the console the SJ-200 is not soldered in: it plugs into a seven-pin header, so it can be unplugged once the case is open. On Enri's redrawn circuit, each line from the header (CN01) and from the side socket (JOY2) is pulled up to 5 V by a 4.7 kΩ resistor and read through two 74LS257 multiplexers, IC18 and IC21. A press connects the line to ground, which the console reads as 0.

The procedure below checks which side of the header the fault is on, tests the joystick and the board's inputs, and either repairs the joystick or replaces it with a socket for a Sega pad.

Symptoms

• The fixed joystick does nothing, or one direction or button does not respond.
• A direction or button acts as if held all the time.
• A pad in the side socket does nothing. The socket has no 5 V on pin 5, so a pad that needs power from the port cannot work there.
• Both joysticks are dead. Both are read through IC18 and IC21; see Sega SG-1000 Troubleshooting Guide.

The Hold (pause) button is on the console, not on the joystick; it is also covered in the troubleshooting guide.

Tools and parts

• Multimeter with a continuity range.
• Phillips screwdriver.
• A known-good Sega pad or joystick for the side socket.
• For a pad adapter: a Master System or Mega Drive extension cable, a header housing with at least seven positions at the same pitch as the board header, crimp pins and a crimp tool.

Joystick connections

Function	Fixed joystick header (CN01) pin	Side socket (JOY2) pin
Up	6	1
Down	5	2
Left	4	3
Right	3	4
Button 1	1	6
Button 2	2	9
Ground	7	8
Not connected	–	5

Header pin order as numbered on the board in D.Lo's console; socket pins and functions from Enri's drawing and port table, which also tie socket pin 7 to ground. The adapter below leaves pin 7 unused.

Procedure

Narrow it down from outside

1. Insert a cartridge before switching on; with no cartridge the console does nothing.
2. In a game, try every direction and both buttons of the fixed joystick.
3. Plug a known-good Sega pad into the side socket and try it in a game that reads the second joystick.
4. If the side socket works and the fixed joystick does not, the fault is in the joystick, its cable or plug, or the header's lines on the board. If both fail, check IC18, IC21 and /JOY SEL.

Open the console

An SG-1000 with its fixed SJ-200 joystick (photo: Evan-Amos, public domain)

1. Switch off, unplug the adapter and remove the cartridge.
2. Turn the console over and remove the four screws.
3. Unplug the joystick's seven-pin plug from the header, noting which way round it fits.

Test the fixed joystick

With the plug out and the meter on its continuity range, probe the pins of the joystick's plug:

1. Measure from pin 7 (ground) to each of pins 1 to 6 with nothing pressed. Each should read open.
2. Hold each direction and button in turn and check that its pin, from the table, reads closed to pin 7.
3. A pin that never closes has a broken wire or a contact that no longer makes; a pin that reads closed with nothing pressed has a short or a stuck contact.

Test the board's inputs

With the joystick unplugged, a cartridge in and the console switched on:

1. Measure each of the board header's pins 1 to 6 to pin 7. Each should be close to 5 V, held up by its 4.7 kΩ resistor.
2. Do the same at the side socket, pins 1 to 4, 6 and 9, to pin 8.
3. A pin near 0 V with nothing plugged in is shorted to ground on the board, or the input of IC18 or IC21 on that line has failed. Switch off, and look for solder bridges and damaged tracks between that pin and the chip.

Repair the joystick

1. Reseat the plug on the header and clean its pins.
2. Repair or replace a broken wire found in the test, and recheck it for continuity from the plug to the switch.
3. If the joystick cannot be repaired, fit a socket for a Sega pad in its place, as below.

Or fit a pad adapter

Sega sold the JC-100 adapter for this, so that SJ-150, SJ-151 and SJ-152 pads could replace the fixed joystick. D.Lo made an equivalent from an extension cable:

1. Cut the extension cable at the length you want, keeping the socket end that the pad plugs into. Strip the outer sheath and the ends of the wires.
2. Find which wire goes to each socket pin with the meter. The colours differ between cables, and did not match the original joystick's in D.Lo's console.
3. Crimp pins onto the wires for socket pins 1, 2, 3, 4, 6, 8 and 9, and fit them into the header housing in the order in the table: socket pin 6 to header position 1, 9 to 2, 4 to 3, 3 to 4, 2 to 5, 1 to 6 and 8 to 7.
4. Cut off the two unused wires, for socket pins 5 and 7.
5. Plug the housing onto the board header the same way round as the joystick's plug.

The header carries no 5 V, so only pads that need no power from the port will work through it. D.Lo found that a wireless six-button Mega Drive pad did not work. The change is reversible: the original joystick plugs back onto the same header.

Reassemble

Plug the joystick or adapter onto the header, refit the base and its four screws.

Testing

• With a cartridge in, every direction and both buttons respond, on the fixed joystick or the adapter, and on the side socket.
• Nothing acts as if held when the controls are released.

Revision notes

Two layouts of the SG-1000 main board have been photographed, with different designators. Enri's drawing, with IC18, IC21 and CN01, matches the later layout. D.Lo does not say which layout the console in the article had. Check the header's pin numbering on your own board before making an adapter. The Sega SG-1000 General Maintenance page shows how to tell the layouts apart.

Related pages

• Sega SG-1000
• Sega SG-1000 Troubleshooting Guide
• Sega SG-1000 General Maintenance
• Sega SG-1000 Power Regulator Replacement
• Sega SG-1000 Documentation