Sega SG-1000 Power Regulator Replacement
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The Sega SG-1000 runs from a 9 V DC adaptor, the SA-150, and makes its only supply rail, 5 V, with one 7805 regulator, IC15.[1][2] On Enri's redrawn circuit the 9 V comes in at the DC jack and passes the power switch SW2. After the switch, the power LED and its resistor R11 (560 Ω) are connected across the supply, and C22 (10 µF) sits on IC15's input. IC15's output has C23 (100 µF) and C24 (0.1 µF) and feeds the whole board.[3] Because the LED is lit from the switched 9 V and not from the 5 V rail, a lit LED shows that the adaptor and switch are working but says nothing about IC15.[3]
Sega's owner's manual has no circuit information.[4] The procedure below follows Enri's drawing from the jack to IC15 and the 5 V rail, finds where the supply stops, and replaces the failed part.
Symptoms
[edit | edit source]- LED off and nothing happens: no 9 V reaching the switch and LED. Check the adaptor, its plug, the DC jack and SW2.[3]
- LED on, but no picture and no sound: no 5 V from IC15, or the console is not running.[3]
With no cartridge fitted the SG-1000 does nothing, so always test with a cartridge in.[1][4] If 5 V is good and the screen stays blank, continue with the clock and reset checks in the Sega SG-1000 Troubleshooting Guide.
Safety
[edit | edit source]The console itself runs on low-voltage DC. The SA-150 is a mains adaptor rated for a 100 V input; on a higher mains voltage it needs a suitable step-down transformer.[2] If you open an adaptor, unplug it first and treat its primary side as live until you have measured it. A failed 7805 or a short on the 5 V rail can make IC15 and its heat sink hot, so let them cool before touching them.
Tools and parts
[edit | edit source]- Multimeter.
- Phillips screwdriver.[5]
- Soldering iron, desoldering pump or braid.
- A 7805 regulator in a TO-220 package. onsemi's MC7805 gives 4.8 to 5.2 V at 25 °C.[6]
- Thermal compound, if the original regulator had it on the heat sink.
- Optional: replacements for C22 and C23. Enri gives the capacitance only; read the voltage rating from the parts on your board, and see Sega SG-1000 Capacitor Replacement Guide.[3]
| Position | Part | Where it sits |
|---|---|---|
| SW2 | Power switch | After the DC jack |
| LED, R11 | Power LED, 560 Ω | Across the switched 9 V |
| C22 | Electrolytic, 10 µF | IC15 input |
| IC15 | 7805 regulator | Under the heat sink at the top right of the board |
| C23 | Electrolytic, 100 µF | IC15 output, 5 V |
| C24 | Ceramic, 0.1 µF | IC15 output, 5 V |
Parts from Enri's drawing; position of IC15 from the MSX Wiki's board photographs.[3][1]
Procedure
[edit | edit source]Check the adaptor
[edit | edit source]- Check that the adaptor is a 9 V DC unit with a centre-negative plug, as the SG-1000 needs.[1]
- Measure its output, then measure again with the console connected, switched on and running a cartridge.[2][4]
Open the console
[edit | edit source].jpg)
- Switch off, unplug the adaptor and remove the cartridge.[4]
- Turn the console over and remove the four screws.[5]
- Unplug the fixed joystick from its seven-pin header, noting which way round it fits, before lifting the board.[5]
- Find the power switch, the regulator heat sink and, on the later board, the pad marked 9V beside MAIN SW2.[1]
Follow the 9 V and 5 V
[edit | edit source]With a cartridge in, the adaptor connected and the switch on, measure to the board's ground:
- At the DC jack: the adaptor voltage. Nothing there means the adaptor, its plug, or the jack and its joints.[3]
- After the switch, at the pad marked 9V on the later board: the adaptor voltage. Voltage at the jack and none here means SW2 or its joints.[1][3]
- At IC15's input (pin 1): the same switched 9 V.[3][6]
- At IC15's output (pin 3): 5 V.[3][6]
- If the output is 5 V, measure 5 V at the supply pins of the chips.[3]
| IC15 input | IC15 output | LED | Likely fault |
|---|---|---|---|
| 0 V | 0 V | Off | Adaptor, plug polarity, DC jack or SW2 |
| About 9 V | 0 V or well below 5 V | On | IC15, or a short on the 5 V rail pulling it down |
| Well below 9 V | Low | On | A short on the 5 V rail loading the adaptor, or a weak adaptor |
| About 9 V | 5 V | On | Supply good; look at the clocks and reset |
To tell a dead IC15 from a short on the rail, switch off and measure the resistance from IC15's output to ground; a reading near zero means something on the 5 V rail, such as C23 or C24, is shorted.[3]
Replace IC15
[edit | edit source]- Switch off and unplug.
- Note how IC15 is fixed to its heat sink and which way it faces, and whether there is thermal compound between them.
- Release IC15 from the heat sink, desolder its three legs and lift it out.
- Test the new 7805 before fitting it: with 9 V on pin 1 and pin 2 to ground, pin 3 should give 4.8 to 5.2 V.[6]
- Fit the new regulator the same way round, pin 1 to the input and pin 3 to the output, and fix it to the heat sink as the old one was, with fresh compound if the original had any.[6]
- Solder the three legs.
- If C22 or C23 is bulged, leaking or reads short, replace it with the same capacitance and at least the voltage rating of the original, fitted the same way round.[3]
Reassemble
[edit | edit source]Plug the joystick back onto its header the same way round, refit the base and its four screws, insert a cartridge, then connect the adaptor and switch on.[5][4]
Testing
[edit | edit source]- The LED lights when the switch is on.[3]
- About 9 V at IC15's input and 5 V at its output with a cartridge running.[3]
- The game starts and runs with picture and sound.
Revision notes
[edit | edit source]Enri's drawing and its designators match the later board layout. The earlier layout numbers its parts differently and has a larger capacitor beside the regulator heat sink whose designator cannot be read in the photograph; trace the circuit from the DC jack and switch on that board, and see Sega SG-1000 Capacitor Replacement Guide.[1][3]
Related pages
[edit | edit source]- Sega SG-1000
- Sega SG-1000 Troubleshooting Guide
- Sega SG-1000 General Maintenance
- Sega SG-1000 Capacitor Replacement Guide
- Sega SG-1000 Joystick Repair
- Sega SG-1000 Documentation
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 MSX Wiki, "Sega SG-1000", retrieved 2026-10-25: power supply input DC +9 V, 850 mA, negative at the centre; an external SA-150 power supply; photographs of the earlier and later main boards, the later one with the regulator under its heat sink at the top right beside MAIN SW2, a pad marked 9V and test point TP3.
- ↑ 2.0 2.1 2.2 MSX Wiki, "Sega SA-150", retrieved 2026-10-25: input AC 100 V 50/60 Hz 14 VA; output DC 9 V, 850 mA (NTSC) or 1.1 A (PAL).
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 Enri, "SEGA SG-1000", Enri's Home Page (Japanese), retrieved 2026-10-25. Connector and power circuit (image sgcon.png, hosted as File:Sega SG-1000 power and connector circuit (Enri).png): DC jack, POWER SW2, LED with R11 560 Ω, IC15 7805, C22 10 µF on the input, C23 100 µF and C24 0.1 µF on the output.
- ↑ 4.0 4.1 4.2 4.3 4.4 Sega Enterprises, Ltd., SG-1000 owner's manual (Japanese, 1983), four pages: box contents, connection, use and the "these are not faults" list; insert the cartridge before connecting the adaptor and switching on; change cartridges only with the power switch off. Hosted as File:Sega SG-1000 Owner's Manual (1983, Japanese).pdf.
- ↑ 5.0 5.1 5.2 5.3 D.Lo, "Adding a Sega controller adapter to an original SG-1000 (home made JC-100)", Nintendo Sega Japan, 29 December 2015: the console opens with four screws; the fixed SJ-200 joystick plugs into a seven-pin header on the board.
- ↑ 6.0 6.1 6.2 6.3 6.4 onsemi, MC7800, MC7800A, MC7800AE, NCV7800: 1.0 A Positive Voltage Regulators data sheet: TO-220 pin 1 input, pin 2 ground, pin 3 output; the heatsink tab is connected to pin 2; the input must stay typically 2.0 V above the output; MC7805C output 4.8 to 5.2 V at 25 °C.