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Sega CD (Model 1) Power Fuse Replacement

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Sega CD Model 1 sub board power input
The power input corner of the export sub board 837-8953 in a Model 1 Sega CD: the DC jack CN2, fuse F1 (bottom left), diode D1, transistors TR3 and TR4, and the choke L2 (photo: Evan-Amos, public domain, cropped)

The Sega CD (Model 1) (Mega-CD) has its own AC adaptor, and the DC input, fuse, protection diode and 5 V regulator are all on the sub board along the back of the unit.[1] When the fuse F1 opens, the unit is dead: no power LED and no CD boot screen. Two owners brought dead units back by replacing F1; one had found his unit with a 17 V adaptor attached, and the other suspected a centre-positive adaptor.[2][3] The steps below follow the supply from the jack to the regulator before the fuse is replaced. They cover the export boards in Sega's MEGA-CD Maintenance Manual; Sega's schematic in the same manual is drawn for the earlier sub board 837-8046-01, which uses the same designators for the parts in the power path.[4][1]

Symptoms

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  • No power LED on the front of the unit with its adaptor plugged in and switched on.[2] The two front LEDs, LD1 (red) and LD2 (green), are on the main board.[5]
  • The Sega CD boot screen never appears.

Sega's repair flow chart puts a unit that will not power on down to the AC adaptor or the sub board.[6] With the power LED lit and no picture, check the connection between the two units first; cooltrainer.org gives that as the first check.[2] A unit whose LEDs light but which shows no CD-ROM screen has a different fault: see the Sega CD (Model 1) Troubleshooting Guide.

Safety

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The unit runs from an external adaptor rated 10 V DC 1.2 A, so there is no mains voltage inside it.[7] Unplug the adaptor before opening the case or soldering. The CD drive has a laser; keep it powered down while the drive is out of the case.

The supply circuit

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Sega Mega CD sub board power input schematic
The power input on Sega's sub board schematic (drawn for 837-8046-01): DC jack CN2 with its centre pin on the ground return through FB3, FB2, fuse F1, D1 across the supply, TR4 with TR3, C27, and the regulator IC6 (µPC2405HF), whose output is VCC2. L1 takes VCC2 to the analogue supply AVCC.

On Sega's schematic the DC jack CN2 has its centre pin on the ground return, which passes through the ferrite bead FB3; the outer contact feeds the supply line through FB2 and the fuse F1.[4] The adaptor is therefore centre negative, as one owner found.[2] After F1, diode D1 (1SR35-100A) is drawn from the supply line to ground with its cathode on the supply side, so it conducts only if the input polarity is reversed, and the current then flows through F1. Transistor TR4 (2SB963-Z) sits in series between F1 and the regulator, with TR3 (2SC1623) and R20, R21, R37 and R42 around it. C27 (100 µF 16 V on this drawing) is at the regulator input. IC6, a µPC2405HF, produces VCC2, which L1 (100 µH) passes on to the analogue supply AVCC.[4] Sega's pin data for the sub board's PCM D/A converter, IC2, gives its supply as +5 V.[8]

Power path parts on the export sub board 837-8953[1]
Position Part Sega part number
CN2 DC jack (pin plug for DC) 212-5004
F1 Fuse, Pico II 2.5 A, radial 514-5044
D1 Diode 1SR-35-100A, radial 481-5038-01
TR4 Transistor 2SB963-Z 482-5146-01
TR3 Transistor 2SC1623, chip 482-5126
IC6 µPC2405HF, on heat sink 17PB23L25 (530-5017) with thermal compound (601-0076) and an M3 × 8 screw (000-0308) 313-5193
L2 Choke coil PLT09H-2003R (Murata) 180-5093
µPC2405HF pins, front view[9]
Pin Function
1 Input
2 Ground
3 Output

Tools and parts

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  • A multimeter with continuity and diode ranges, a temperature-controlled soldering iron, desoldering braid or a pump, and a cross-head screwdriver.
  • A replacement fuse. Sega's part is a 2.5 A Pico II radial fuse, 514-5044.[1] One owner fitted an in-line automotive fuse holder with a 1.5 A fuse instead, soldered into the F1 holes, so that a later failure would only need a new fuse.[2]
  • The correct adaptor: 10 V DC at 1.2 A. Sega's European list gives 400-5122A and 400-5122B for 220 V mains and 400-5127 and 400-5127A for 240 V.[7]

Procedure

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Check the adaptor first

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  1. Measure the adaptor's output before opening the unit. Sega rates it at 10 V DC, 1.2 A.[7]
  2. Check that it is centre negative, matching CN2.[4][2] A unit found with a 17 V adaptor attached had a blown fuse, and one owner suspected his had been killed by a centre-positive adaptor.[3][2]

Open the unit

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Sega Mega CD CD drive unit and sub board assembly
Sega's drawing of assembly 1006: the sub board (102) and CD drive unit (401) in the bottom case, with the 201 (3 × 10 mm) and 202 (3 × 8 mm black) screws

Sega's screws are 201 (tapping, 3 × 10 mm), 202 (tapping, 3 × 8 mm, black), 203 (tapping, 4 × 10 mm) and 204 (tapping, 3 × 12 mm).[10]

  1. Turn the unit over and remove the six screws (201) from the bottom case.[11]
  2. Remove the two screws (202) and lift the top case off the bottom case.[11] F1 is now in view on the sub board, beside the DC jack CN2 (see the photograph at the top of the page).[2]
  3. With the adaptor unplugged, test F1 for continuity across its two leads. If it is open, carry on to remove the sub board.[2]
  4. Remove the side knob assembly, then the two screws (203), and lift off the side cover assembly.[11]
  5. Remove the four screws (204) from the side cover assembly to free the side cover shield and the connect board.[11]
  6. Remove the three screws (201) and lift out the CD-ROM unit shield.[11]
  7. Remove three more screws (201) and lift off the main board shield top. Release the 21-pin flat cable from the main board, then the 25-pin cable from its connector. Lift out the main board, then the main board shield bottom.[11]
  8. Remove the four screws (202) and two screws (201), and lift out the CD drive unit and the sub board.[11] In Sega's drawing the two 201 screws and one 202 screw go through the sub board, and the other three 202 screws hold the drive.[12]
  9. Release the 21-pin flat cable from the sub board.[13]

Find the fault

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  1. Inspect the sub board around CN2, F1, D1, TR3 and TR4 for burnt parts, cracked joints and leaked electrolyte.
  2. Test D1 with the meter's diode range. A healthy diode conducts one way only; a diode that reads as a short both ways will blow a new fuse as soon as power is applied, because it sits across the supply after F1.[4]
  3. Measure the resistance to ground from the supply line after F1 and from IC6 pins 1 and 3. A dead short on any of them has to be found before a new fuse is fitted.[4][9]

Replace the fuse

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  1. Desolder F1's two leads and lift it out.[2]
  2. Fit the new 2.5 A fuse in the same holes and solder it.[1] For a fuse holder, as cooltrainer.org fitted, thin its wires so they pass through the holes and keep the holder clear of the case.[2]
  3. Check the new fuse for continuity before reassembly.

Reassemble

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Sega's assembly order:[13]

  1. Set the sub board in the bottom case with each hole over its boss, and connect the 21-pin flat cable firmly to the sub board.
  2. Set the CD drive unit in the bottom case and fit the two screws (201) and four screws (202).
  3. Fit the main board shield bottom, connect the 21-pin and 25-pin flat cables firmly to the main board, set the main board on its bosses, fit the main board shield top and the three screws (201).
  4. Set the CD-ROM unit shield on the CD drive unit, lining its holes up with the bosses.
  5. Fit the connect board and side cover shield to the side cover with the four screws (204).
  6. Fit the side knob assembly, connect the side cover assembly to the main board and fit the two screws (203).
  7. Before fitting the top case, connect the Mega Drive and the adaptor and switch on; owners test this way with the top case off.[3] Measure IC6: pin 1 should carry the adaptor supply after F1 and TR4, and pin 3 the VCC2 supply, +5 V, each measured to pin 2.[9][8] If pin 1 is missing, work back through TR4, F1 and the jack; if pin 1 is present and pin 3 is not, suspect IC6 or a short on VCC2.[4]
  8. Switch off and unplug. Fit the top case and its two screws (202), turn the unit over and fit the six screws (201).[13]

Testing

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Checks after replacing F1
Check Where Expected
Adaptor output Adaptor plug 10 V DC, centre negative[7][4]
Fuse Across F1, unplugged Continuity[2]
Regulator input IC6 pin 1 to pin 2 The adaptor supply, after F1 and TR4[9][4]
Regulator output (VCC2) IC6 pin 3 to pin 2 +5 V[9][8]
Power LED Front of the unit Lit[2]

Then attach the Mega Drive, switch on and check that the Sega CD boot screen appears. On one repaired unit the boot screen came up and the tray opened and closed, but discs were spat out; that is a separate drive fault, covered in the Sega CD (Model 1) Troubleshooting Guide.[3] Sega's check cartridge and check CD exercise the rest of the unit.[14]

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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 Sega Enterprises, Ltd., MEGA-CD Maintenance Manual (Export), August 1992, Rev. A, section 8-2, parts list IC BD MEGA-CD SUB BD EXPORT, 837-8953 (PDF pp. 97-98): IC6 µPC2405HF (313-5193) with heat sink 17PB23L25 (530-5017), thermal compound (601-0076) and M3 × 8 screw (000-0308); TR3 2SC1623; TR4 2SB963-Z; L2 choke PLT09H-2003R; D1 1SR-35-100A; F1 fuse Pico II 2.5 A radial (514-5044); CN2 DC jack (212-5004). See Sega CD Service Manual.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 "Reviving an unresponsive Mega CD Model 1", cooltrainer.org, 27 April 2012: a Mega CD Model 1 with no power LED, centre-negative supply, fuse F1 tested for continuity, desoldered and replaced with an automotive fuse holder and 1.5 A fuse. Personal repair write-up.
  3. ↑ 3.0 3.1 3.2 3.3 "Model 1 Sega CD repair help", Racketboy forum, Technical Help, March to May 2013: a unit found with a 17 V adaptor attached; a replaced fuse brought back the boot screen and tray, after which discs were rejected (posts by CamaroCam and Hobie-wan). Community discussion.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 MEGA-CD Maintenance Manual (Export), August 1992, section 9, schematic "PC BD MEGA-CD SUB BD", 837-8046-01, drawing 171-6115C rev. C (PDF p. 105): CN2, FB2, FB3, F1, D1, TR3, TR4, R20, R21, R37, R42, C27, IC6, VCC2, L1 and AVCC.
  5. ↑ MEGA-CD Maintenance Manual (Export), August 1992, section 8-1, parts list IC BD MEGA-CD MAIN BD EXPORT, 837-8952 (PDF pp. 94-96): LD1 red LED GL5PR44, LD2 green LED GL5KG44.
  6. ↑ MEGA-CD Maintenance Manual (Export), August 1992, section 5, PCB repair flow chart: power not on, AC adaptor or sub board (PDF p. 30).
  7. ↑ 7.0 7.1 7.2 7.3 MEGA-CD Maintenance Manual (Export), August 1992, section 4, accessories list for Europe: AC adaptors 400-5122A and 400-5122B (AC 220 V, DC 10 V 1.2 A) and 400-5127 and 400-5127A (AC 240 V, DC 10 V 1.2 A) (PDF p. 28).
  8. ↑ 8.0 8.1 8.2 MEGA-CD Maintenance Manual (Export), August 1992, section 7, parts specification, sub board IC2 (LC7881M) pin data: VDD +5 V (PDF p. 91).
  9. ↑ 9.0 9.1 9.2 9.3 9.4 Sega Enterprises, Ltd., Service Manual, Sega CD II / Mega CD II, No. 002, August 1993, section 12, parts specifications: IC301 µPC2405HF three-terminal voltage regulator, front view, pin 1 input, pin 2 GND, pin 3 output (p. 27). The Model 2 uses the same regulator part. See Sega CD 2 / Mega CD 2 - Service Manual.
  10. ↑ MEGA-CD Maintenance Manual (Export), August 1992, section 2-1, general reference number list: screws 201 to 204 (PDF p. 7).
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 MEGA-CD Maintenance Manual (Export), August 1992, section 2-3, procedure of disassembly, processes 1 to 7 (PDF p. 23).
  12. ↑ MEGA-CD Maintenance Manual (Export), August 1992, section 2-2, assembly 1006 "CD drive unit & sub board assembly": parts list and drawing (PDF pp. 19-20).
  13. ↑ 13.0 13.1 13.2 MEGA-CD Maintenance Manual (Export), August 1992, section 2-3, procedure of assembly, processes 1 to 7 (PDF p. 24).
  14. ↑ MEGA-CD Maintenance Manual (Export), August 1992, section 6, soft and hard check manual (PDF pp. 31-40).
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