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Sega Master System (Model 1) Power Regulator Replacement

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Sega Master System (Model 1) PCB
Master System main board 837-6067 (M4 Powerbase/NTSC). The power jack is at the top left, the power switch at the bottom left, and the regulator IC8 and its capacitors between them along the left edge

The Sega Master System (Model 1) runs from a 9 V DC adaptor and makes its single 5 V rail with one 7805 regulator, IC8. The 9 V comes in at the power jack CON7, passes through a ferrite bead and the power switch to IC8, and IC8's 5 V output feeds every chip on the board and the power LED. There is no fuse.[1][2] When the 5 V rail is missing the console is completely dead: Sega's symptoms for a faulty IC8 are that the LED does not light and there is no picture.[3]

The procedure below follows the 9 V supply from the adaptor to IC8 and the 5 V rail out of it, finds where it stops, and replaces the faulty part. Because the LED is lit from the 5 V rail, a lit LED means IC8 is producing an output.[1]

Symptoms

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  • No power LED and no picture.[3]
  • Spontaneous resets, or characters suddenly breaking up, which Sega also lists against the power switch.[3]

If the LED lights but the screen stays black, the supply is working; go to the reset and clock checks in the Sega Master System Troubleshooting Guide. The same guide covers the Sega Master System II, which has its own power circuit.

Safety

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The console itself runs on low-voltage DC. The adaptor is a mains transformer unit; if you open it, 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 IC8 and its heat sink hot, so let the board cool before touching it.

Tools and parts

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  • Multimeter.
  • Phillips screwdriver.
  • Soldering iron, desoldering pump or braid.
  • A 7805 regulator in a TO-220 package (Sega part 313-0092).[2] onsemi's MC7805 gives 4.8 to 5.2 V at 25 °C.[4]
  • Thermal compound for the heat sink; Sega's schematic shows silicone grease between IC8 and its heat sink.[1]
  • Optional: replacements for C1, C2, C3 and C4 (see the table).
Power circuit parts (Sega PAL parts list and schematic)
Position Part Notes
CON7 Power jack, 212-5004 9 V DC in
JV, JG Bead inductors FBA04VA600AB, 271-0005 In the 9 V and ground lines from CON7
POWER SW Push switch, 509-5150
C1 Electrolytic, 10 µF 16 V in the parts list; 100 µF on the schematic IC8 input
C2 Ceramic, 0.1 µF 16 V IC8 input
IC8 7805 regulator, 313-0092 On a heat sink with silicone grease
C3 Ceramic, 0.1 µF 16 V IC8 output
C4 Electrolytic, 100 µF 10 V IC8 output
R5, LED 120 Ω; green LED SLP-280U-F50 LED lit from the 5 V rail

Parts from the parts list on pp. 19 to 21 and the schematic on p. 14 of Sega's manual; the two disagree on C1.[2][1]

Procedure

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The supply path

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Sega Master System (Model 1) power input schematic
Sega's power input circuit for the PAL board 171-5534: CON7, JV, the power switch, C1 and C2, IC8 (7805) on its heat sink, C3 and C4, and R5 with the LED

From the jack to the chips the path is: CON7, bead JV, power switch, IC8 input (with C1 and C2 to ground), IC8 output (with C3 and C4 to ground), then Vcc to the rest of the board, with R5 and the LED across it. The ground return passes through bead JG.[1] A break or a short anywhere along it stops the console.

Check the adaptor

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  1. Unplug the adaptor from the console and measure its output with no load. Sega's figure is 13 V ± 1 V; an adaptor that does not give that is replaced.[5]
  2. An adaptor that reads well above 9 V unloaded is normal; the console's own figure for IC8's input is 9 V with it running.[3]

Open the console

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Sega Master System (Model 1) exploded view (Sega)
Sega's exploded view of the Power Base: bottom case (A-2), bottom shield (A-5, West Germany only), IC board (A-3), top shield (A-4) and top case (A-1), with the screws A-18 to A-20
  1. Unplug the adaptor and all cables, and remove any cartridge or card.
  2. Turn the console over and remove the screws in the bottom case (A-2). Lift the bottom case off.[6]
  3. On a West German console, remove the bottom shield plate (A-5).[6]
  4. Remove the screws holding the IC board (A-3) and lift the board out with its top shield (A-4), noting how the power button (A-10) and its spring (A-11) sit against the switch.[6]

Follow the 9 V and 5 V

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Make these measurements with the board connected to the adaptor and switched on, probing to the board's ground.

  1. Measure at the power jack CON7. No voltage there means the adaptor, its plug or the jack's solder joints.[1]
  2. Measure on each side of the power switch, pressed on. Voltage on the jack side and none on the IC8 side means the switch or its joints; Sega lists a loose switch, soldering, a cut track and a short circuit.[3][1]
  3. Measure IC8's input, pin 1. Sega's figure is DC 9 V.[3][4]
  4. Measure IC8's output, pin 3. Sega's figure is DC 5 V.[3][4]
  5. If the output is 5 V, measure Vcc at the chips; Sega's next check is 5 V at each IC, with a cut track or a bad joint as the cause if it is missing.[3]

Read the results like this:

What the measurements mean
IC8 input IC8 output Likely fault
About 9 V 0 V or well below 5 V IC8, or a short on the 5 V rail pulling it down
Well below 9 V Low A short on the 5 V rail loading IC8 and the adaptor, or a weak adaptor
0 V 0 V Jack, bead JV, power switch or a cut track before IC8
About 9 V 5 V Supply good; look elsewhere

To tell a dead IC8 from a short on the rail, switch off and measure the resistance from IC8's output to ground. On SMS Power's forum, a Master System owner whose new 7805 gave only 3 V from 7 V was told that a faulty chip or capacitor was pulling the rail down. The advice was to unplug the control pads and the AV cable, feel for a chip that runs hot, look for bulging or leaking capacitors, and measure each electrolytic capacitor's resistance with the power off, a shorted one reading low. The same reply notes that about ten of the console's fifteen or so electrolytics sit on the supply rail.[7]

Replace IC8

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  1. Switch off and unplug.
  2. Note how IC8 sits on its heat sink, how it is fixed to it, and which way it faces.[1]
  3. Release IC8 from the heat sink, desolder its three legs and lift it out.
  4. Clean the old compound off the heat sink.
  5. 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. A forum member reported that some regulators bought online are bad.[4][7]
  6. Put a thin layer of thermal compound on the heat sink, fit the new regulator the same way round (pin 1 input, pin 2 ground, pin 3 output) and fix it to the heat sink as the old one was.[1][4]
  7. Solder the three legs.
  8. If C1 or C4 is bulged, leaking or measures short, replace it with a part of the same capacitance and at least the same voltage, observing polarity; see the Sega Master System Capacitor Replacement Guide.[2][7]

Replace the power switch

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If the switch was the break, desolder it and fit a replacement push switch (Sega part 509-5150), checking that the power button (A-10) and its spring (A-11) still operate it.[2][6]

Reassemble

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Refit the board and top shield, the bottom shield on West German consoles, and the bottom case, in the reverse order.[6]

Testing

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  • The LED lights when the switch is pressed.[1]
  • IC8 input about 9 V and output 5 V with the console running.[3]
  • 5 V at the supply pins of the chips.[3]
  • If the console now powers up but the screen stays black, continue with the reset circuit (test point JR, 0 to 3.6 V rising slowly) and the clocks in the Sega Master System Troubleshooting Guide.[3]

Revision notes

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Sega's manual covers the PAL board. Its schematic (171-5534) and its parts list disagree on C1, 100 µF on the drawing and 10 µF in the list.[1][2] The North American board in the lead photograph, 837-6067, has the same power jack, switch and regulator positions along its left edge, but no schematic or parts list for it is hosted here.

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References

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  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 Sega Enterprises, Ltd., Sega Master System Service Manual (Power Base), schematic "IC Board PB PAL VA1", 171-5534, sheet 2 (p. 14, PDF p. 15): DC 9 V in at CON7, bead JV, POWER SW, C1 and C2 to IC8 (7805) IN; IC8 OUT to C3, C4 and Vcc; R5 (120 Ω) and the LED from Vcc to ground; IC8 on a heat sink with silicone grease; bead JG in the ground return. Hosted on this wiki as File:Sega Master System (Model 1) Service Manual.pdf.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Sega Enterprises, Sega Master System Service Manual, PC Main M4 parts list, "Mark 4 Part List PAL Type", pp. 19 to 21 (PDF pp. 20 to 22): JV, JG and FB1 271-0005 bead inductors FBA04VA600AB; C1 10 µF 16 V; C2 and C3 0.1 µF 16 V; C4 100 µF 10 V; R5 120 Ω; LED 390-5128 SLP-280U-F50 green; IC8 313-0092 "REG 7805"; CON7 212-5004 power jack; POWER SW 509-5150 push switch; no fuse.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 Sega Enterprises, Sega Master System Service Manual, pp. 4 and 5, "Servicing the main unit (Power Base)": IC8 (7805) power check, in DC 9 V and out DC 5 V, with fault locations IC8, soldering, short circuit, cut track, power switch and heat sink attachment, and symptoms LED does not light and no picture; power button switch faults (loose switch, soldering, cut track, short circuit) with symptoms no power voltage, LED does not light, no picture, spontaneous resets and characters suddenly becoming abnormal.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 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.
  5. ↑ Sega Enterprises, Sega Master System Service Manual, p. 11, "AC adaptor": symptoms no picture, LED does not light, "13 V ± 1 V not produced (without load)"; remedy, replace.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Sega Enterprises, Sega Master System Service Manual, p. 17 (PDF p. 18), exploded view "Assy Power Base", and parts list p. 18 (PDF p. 19): A-1 top case, A-2 bottom case, A-3 IC board, A-4 top shield plate, A-5 bottom shield plate (West Germany only), A-10 power button, A-11 torsion spring, A-18 M3 × 6 screw with washer, A-19 tapping screw PH 3 × 16 black, A-20 tapping screw PH 3 × 12.
  7. ↑ 7.0 7.1 7.2 SMS Power! forums, "HELP Power-ON, No Display, weak voltage", January 2024: Master System with 7 V in and 3 V out on a new, tested 7805; replies by asynchronous: check that the adaptor gives more than 9 V unloaded; a faulty chip or capacitor pulling the rail to 3 V; feel for hot chips; look for bulging or leaking capacitors; unplug joypads and the AV cable; about 15 electrolytics, about 10 of them on the power rail, mostly 10 µF or 100 µF; measure each electrolytic's resistance unpowered. Reply by nuno14272: test a replacement 7805 off the board, as some bought online are bad.
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