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Sega Master System II Power Regulator Replacement

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Sega Master System II power circuit area
The power section of a Master System II PAL board: the DC jack at the top, the 7805 regulator IC8 on its heat sink at the left edge with its legs marked I, C and O, diode D2 and the capacitors C41, C42 and C43 beside it, and the slide power switch at the bottom left (photo: Evan-Amos, public domain)

The Sega Master System II runs from a 9 V DC adaptor and makes its 5 V supply with a single 7805 regulator, IC8, fixed to a heat sink at the left edge of the board. On Sega's PAL schematic the 9 V comes in at the DC jack CN4 and passes through bead JV, the power switch and the series diode D2 to IC8's input, which has C41 (100 µF 16 V) and CP4 to ground. IC8's output is the 5 V rail, Vcc, with CP5 and C42. A second bead, FB3, takes Vcc on to a filtered rail, VDD, with C43 and CP6; VDD supplies the RF modulator through bead FB4 and the 53.2 MHz oscillator through bead FB8. The ground return from CN4 passes through bead JG.[1][2] There is no fuse in Sega's parts lists, and the Master System II has no power LED.[2][3]

Sega's troubleshooting flowchart starts with the power switch and then IC8, which should have DC 9 V at its input and DC 5 V at its output. The causes Sega lists for a failed check are IC8 itself, soldering, a short circuit, a cut track, the power switch, and the heat sink attachment.[4] The procedure below follows the supply from the jack to IC8 and on to the two 5 V rails, finds where it stops, and replaces the faulty part, with the option of fitting a modern switching regulator in place of the 7805.

Symptoms

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  • No picture and no sound at all: no 5 V from IC8.[4]
  • Spontaneous resets, or characters suddenly breaking up: Sega lists these against the power switch.[4]

If 5 V is present at IC8's output and on the chips but the screen stays black, the supply is working; continue with the reset and clock checks in the Sega Master System II Troubleshooting Guide.[4]

Safety

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The console runs on low-voltage DC and has no mains wiring inside. The adaptor is a mains transformer unit; if you open one, 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 them cool before touching them.

Tools and parts

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  • Multimeter.
  • Phillips screwdriver.[3]
  • Soldering iron, desoldering pump or braid.
  • A replacement regulator, one of:
    • A 7805 in a TO-220 package (Sega part 313-0092, listed as an MA7805UC; the board in the photograph carries an MC7805CT), with thermal compound for the heat sink (Sega lists 601-0076).[2] onsemi's MC7805 gives 4.8 to 5.2 V at 25 °C.[5]
    • A Traco TSR 1-2450 switching regulator, which Wilson fitted in place of the 7805 on a 171-5922-A board.[3] Traco describes the TSR 1 series as drop-in replacements for 78xx linear regulators that fit the TO-220 footprint and need no heat sink; the TSR 1-2450 gives 5 V at up to 1 A from 6.5 to 36 V in.[6]
  • Optional: a DC jack, if the original is corroded,[3] and replacements for C41, C42 and C43 (see the table).
Power circuit parts (Sega PAL parts list and schematic)
Position Part Where it sits in the circuit
CN4 DC jack ("pin plug for DC"), 212-5004 9 V DC in
JV, JG Bead inductors FBA04, 271-0005-1 9 V and ground lines from CN4
POWER SW Slide switch HSW1699-01-010, 509-5240-01 Between JV and D2
D2 Diode 1SR35-100A, 481-5038-01 In series between the switch and IC8
C41 Electrolytic, 100 µF 16 V, 150-0059 IC8 input
CP4, CP5 Ceramic, 0.022 µF, 151-0170 IC8 input and output
IC8 7805 regulator, 313-0092 On a heat sink with thermal compound
C42 Electrolytic, 100 µF 10 V, 150-0047 IC8 output (Vcc)
FB3 Bead inductor FBA04 Vcc to VDD
C43, CP6 Electrolytic, 100 µF 10 V; ceramic 0.022 µF VDD

Parts from the parts list on pp. 19 to 21 and the schematic on p. 11 of Sega's manual.[2][1]

Procedure

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The supply path

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Sega Master System II power input schematic
Sega's power input circuit for the PAL board 171-5922A: CN4, JV, the power switch, D2, C41 and CP4, IC8, CP5 and C42 on Vcc, then FB3 to VDD with C43 and CP6, and JG in the ground return

From the jack to the chips the path is: CN4, bead JV, power switch, D2, IC8 input (with C41 and CP4 to ground), IC8 output (with CP5 and C42 to ground) as Vcc, then FB3 to VDD (with C43 and CP6). 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. Check that the adaptor is a 9 V DC unit. Sega supplied 9 V DC 0.5 A adaptors with the PAL consoles and a 9 V DC 1 A adaptor (400-5048Y) with the French RGB console.[2][7]
  2. Measure the supply with the adaptor plugged into the console and the console switched on, at IC8's input (below). Sega's figure there is DC 9 V.[4]

Open the console

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  1. Unplug the adaptor and the controllers, and remove any cartridge.
  2. Turn the console over and remove the five Phillips screws around the edge of the bottom case.[3]
  3. Lift the board assembly out. Wilson's board was held into the bottom case and the RF shielding by five Phillips screws around its edge, two on the voltage regulator, and two on each side of the cartridge port.[3] Sega lists a top shield (250-5185) and a bottom shield (250-5186).[2]
  4. Find CN4, IC8, D2, C41, C42, C43 and the power switch at the left of the board on Sega's parts-location drawing. IC8's three legs are marked I, C and O (input, common, output) on the drawing and on the board.[8]
Sega Master System II M4 Jr PAL-G parts location
Sega's parts-location drawing for the PAL-G board. CN4 is at the top left, IC8 (MA7805UC) at the left edge, with C41, D2, C42 and C43 beside it

Follow the 9 V and 5 V

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

  1. Measure at the DC jack CN4. No voltage there means the adaptor, its plug, or the jack and its solder joints.[1]
  2. Measure on each side of the power switch, switched 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.[4][1]
  3. Measure on each side of D2. Voltage on the switch side and none on the IC8 side means D2 is open or its joints are cracked.[1]
  4. Measure IC8's input, the leg marked I (pin 1). Sega's figure is DC 9 V.[4][8][5]
  5. Measure IC8's output, the leg marked O (pin 3). Sega's figure is DC 5 V.[4][8][5]
  6. If the output is 5 V, measure VDD on the far side of FB3, and 5 V at the supply pins of the chips. Sega's check is that Vcc 5 V is present at each IC, with a cut track or a bad joint as the cause if it is missing.[4][1]

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 CN4, bead JV, the power switch, D2 or a cut track before IC8
About 9 V 5 V, but no VDD FB3 or the track after it; the RF modulator and the oscillator lose their supply
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 was told that a faulty chip or capacitor was pulling the rail down, and to unplug the control pads, 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.[9]

Replace IC8 with a 7805

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  1. Switch off and unplug.
  2. Note which way IC8 faces and how it is screwed to the heat sink. The RGB parts list gives the screw as a PH M3 × 8 (029-0214).[7]
  3. Remove the screw, desolder the three legs and lift IC8 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.[5][9]
  6. Put a thin layer of thermal compound on the heat sink, fit the new 7805 the same way round, with pin 1 in the hole marked I and pin 3 in the hole marked O, and screw it to the heat sink.[2][8][5]
  7. Solder the three legs.

Sega lists a faulty heat sink attachment among the causes of a dead console, so refit the screw firmly.[4]

Or fit a TSR 1-2450

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  1. Remove IC8 as above.
  2. Fit the TSR 1-2450 in the same three holes: its pin 1 (+Vin) goes in the hole marked I, pin 2 (GND) in C and pin 3 (+Vout) in O, the same order as the 7805.[6][8]
  3. Solder the three legs. The TSR needs no heat sink and no thermal compound.[6][3]

Sega's 9 V input figure is inside the TSR 1-2450's 6.5 to 36 V input range.[4][6]

Replace the DC jack or the power switch

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  • If the jack is corroded or its plug is loose, desolder CN4 and fit a new jack that fits the board and the adaptor plug. Wilson fitted a new 2.5 mm DC jack to his board for this reason.[3]
  • If the switch was the break, desolder it and fit a replacement slide switch (Sega part 509-5240-01, HSW1699-01-010), checking that the power button still moves it.[2]

Capacitors

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If C41, C42 or C43 is bulged, leaking or reads short, replace it with a part of the same capacitance and at least the same voltage, fitted the same way round. Wilson used 100 µF 16 V parts for all three.[2][3] See the Sega Master System II Capacitor Replacement Guide.

Reassemble

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Refit the board in the bottom case with its screws and shielding, then the bottom case and its five screws.[3]

Testing

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  • IC8 input about 9 V and output 5 V with the console running.[4]
  • 5 V on VDD. Sega's RF modulator check asks whether +5 V is present; on the schematic the modulator's supply, VRF, is its pin 1.[4][1]
  • 5 V at the supply pins of the chips.[4]
  • A picture and sound on the television. 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) in the Sega Master System II Troubleshooting Guide.[4]

Revision notes

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The circuit is from Sega's documents for the PAL board 171-5922A, whose schematic draws IC8 as an LM7805 while the parts list gives an MA7805UC. The French RGB board, 171-5926A, lists the same DC jack, power switch, regulator and D2.[1][2][7] Later boards, such as 171-6395A with the Sonic the Hedgehog ROM, are not covered by the 1991 manual; see Sega Master System II for the board list, and check the positions on your own board. The board in the photograph carries an MC7805CT at IC8 and an RF modulator marked 200-5091-01; Sega's PAL parts list gives 200-5091-A as the PAL-I modulator.[2]

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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Sega Enterprises, Ltd., Sega Master System II Service Manual, April 1991, schematic diagram for the PAL version, "IC BD M4JR PAL" 171-5922A. Sheet 1 (p. 11, PDF p. 18): CN4, bead JV (FBA04), POWER SW (SPG-01N), D2 (1SR35-100A), C41 100 µF 16 V and CP4 0.022 µF on IC8 VI, IC8 drawn as an LM7805, CP5 0.022 µF and C42 100 µF on VO (Vcc, +5 V), bead FB3 (FBA04) to VDD with C43 100 µF and CP6 0.022 µF, bead JG (FBA04) from CN4 to ground. Sheet 2 (p. 12, PDF p. 19): VDD through FB8 to VDD2, the supply of the 53.203424 MHz oscillator. Sheet 3 (p. 13, PDF p. 20): VDD through FB4 to VRF, the RF modulator supply on its pin 1. Hosted on this wiki as File:Sega Master System II Service Manual April 1991.pdf.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 Sega Enterprises, Sega Master System II Service Manual, April 1991, parts list for the PAL version (pp. 19 to 21, PDF pp. 13 to 15): CN4 212-5004 "pin plug for DC"; POWER SW 509-5240-01 slide switch HSW1699-01-010; IC8 313-0092 MA7805UC; D2 481-5038-01 diode 1SR-35-100A; C41 150-0059 100 µF 16 V; C38, C42 and C43 150-0047 100 µF 10 V; CP1 to CP13 151-0170 0.022 µF; FB1 to FB6, FB8, JV and JG 271-0005-1 bead inductors FBA04; GRS 601-0076 thermal joint compound; AC adaptors 400-5126 (Europe), 400-5129 (UK) and 400-5130 (Australia), all 9 V DC 0.5 A; no fuse listed.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 Adam Wilson, "1992 SEGA Master System II Restoration & Modification", Adam's Vintage Computer Restorations, 13 February 2024: the Master System II dropped the card slot, reset button, power LED and expansion port; the bottom case is held by five Phillips screws around its edge (the sentence begins "The Game Gear is easy to dismantle", evidently a slip in an article about the Master System II); the main board is held into the bottom case and the RF shielding by five Phillips screws around its edge, two on the voltage regulator and two on each side of the cartridge port; on an early PAL board marked 171-5922-A the author replaced all the electrolytic capacitors (C41, C42 and C43 with 100 µF 16 V parts), fitted a new 2.5 mm DC jack because the original was corroded, and replaced the 7805 linear regulator with a TSR-1-2450, "which runs cool and doesn't require a heatsink".
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 4.14 Sega Enterprises, Sega Master System II Service Manual, April 1991, repair procedures, pp. 3 to 10 (PDF pp. 5 to 12). Flowchart (p. 3): power switch check, then IC8 OUT +5 V check, then the reset circuit. Main unit (p. 4): IC8 (LM7805) power check, input DC 9 V and output Vcc DC 5 V, faults IC8, soldering, short circuit, cut track, power switch and heat sink attachment; power check of ICs (Vcc 5 V present?), faults cut track and soldering, symptoms no picture and black screen; reset circuit, JR 0 to 3.6 V rising slowly, faults C18, soldering and R4. Button switches (p. 5): power switch loose, soldering, cut track or short circuit, symptoms no power voltage present, no picture, spontaneous reset and characters suddenly becoming abnormal. RF modulator (p. 5): is +5 V present? Symptoms by fault (p. 6): a faulty IC8 means no uniform +5 V, so the system does not operate and no picture appears.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 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.
  6. ↑ 6.0 6.1 6.2 6.3 Traco Power, TSR 1 Series, 1 A, Non-Isolated DC/DC Converter data sheet: drop-in replacement for 78xx linear regulators, pin compatible with LMxx linear regulators, SIP package fits an existing TO-220 footprint, no heat sink required; TSR 1-2450 input 6.5 to 36 V DC, output 5 V DC, 1 A; pinout pin 1 +Vin, pin 2 GND, pin 3 +Vout.
  7. ↑ 7.0 7.1 7.2 Sega Enterprises, Sega Master System II Service Manual, April 1991, parts list for the RGB version (pp. 25 and 26, PDF pp. 16 and 17), PCB 171-5926A: AC adaptor 400-5048Y AC 220 V / DC 9 V 1 A; CN4 DC jack; POWER SW 509-5240-01; IC8 313-0092 MA7805UC; D2 1SR-35-100A; R5 to R8 330 Ω; EM11 to EM18 and EM21 to EM28 EMI filters; GRS 601-0076 thermal joint compound; 7805-HOLDER 029-0214 screw PH M3 × 8.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 Sega Enterprises, Sega Master System II Service Manual, April 1991, parts location drawing "IC BD M4JR PAL-G" 837-7367 (p. 15, PDF p. 22), with sister drawings for the other regional boards on PDF pp. 21 to 26. Hosted on this wiki as File:Sega Master System II M4 Jr PAL-G parts location.png.
  9. ↑ 9.0 9.1 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: 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; 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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