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Sega CD (Model 1) Maintenance Guide

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Sega CD (Model 1) PCB
Sega CD Model 1 export main board (IC BD MEGA-CD MAIN BD EXPORT), with the back-up cell at top right

The front-loading Sega CD Model 1 (Mega-CD) was first released in December 1991.[1] The faults that stop it now are mostly age faults: leaking electrolytic capacitors, a flat or leaking back-up cell, hardened grease in the drive, and dirty contacts. Sega's own maintenance manual for the export machine is hosted as Sega CD Service Manual.

Safety

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  • The Model 1 runs from an external AC adaptor, 10 V DC at 1.2 A, so there is no mains voltage inside the unit.[2] Unplug the adaptor before opening it.
  • The drive has a laser. Sega's Mega CD II manual warns that it is dangerous to look straight at the laser beam and not to bring your eyes close to it.[3]
  • The gate array at IC2 (315-5548) and the PCM chip at IC3 (315-5476A) are Sega custom parts.[4] Use an anti-static strap.

Tools

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The case and shields are held by Sega's self-tapping cross-head screws, reference numbers 201 (3 × 10 mm), 202 (3 × 8 mm, black), 203 (4 × 10 mm) and 204 (3 × 12 mm).[5] Beyond a driver that fits them, you need isopropyl alcohol, lint-free swabs, a temperature-controlled iron, desoldering braid, and hot air or a second iron for the surface-mount capacitors on the main board. See Recommended Tools.

What is inside

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Boards in the export machine[4]
Board Assembly Main parts Electrolytics
Main 837-8952 (PCB 171-6326A) IC1 68HC000; IC2 315-5548 gate array; IC3 RF5C164A PCM; IC4 boot ROM in a 40-pin socket; IC5 µPD424270 program RAM; IC6 MB3790; IC7 and IC8 TC511664 word RAM; IC10 and IC11 PCM RAM; IC13 LC8951 CD-ROM decoder; IC14 and IC15 MB8464A buffer RAM; IC16 MB8464A back-up RAM; 50 MHz oscillator; LD1 red and LD2 green LEDs; back-up cell BT 12
Sub 837-8953 (PCB 171-6327A) IC1 LC7883KM and IC2 LC7881M-C D/A converters; IC3 to IC5 µPC844 op-amps; IC6 µPC2405HF 5 V regulator on a heat sink; fuse F1; diode D1; DC jack CN2, stereo mini jack CN3 and phono jacks CN4 18
Connect 839-0534 (PCB 171-6328A) 60-pin edge connector to the console, resistor packs, eighteen 150 pF ceramic chips None
Drive 610-5270 (Sony KSK-1200A) or 610-5270-10 (JVC ELM-135) Complete mechanism; Sega does not itemise its parts See Sega CD (Model 1) Capacitor Replacement Guide

Two flat cables connect the boards: a 25-pin cable at main board CN1 to the drive, and a 21-pin cable from main board CN3 to sub board CN1.[4][6]

Strip-down

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Sega's order:[5]

  1. Turn the unit over and remove six screws (201) from the bottom case.
  2. Remove two screws (202) and lift off the top case.
  3. Remove the side knob assembly, then two screws (203) and the side cover assembly.
  4. Remove four screws (204) to release the side cover shield and the connect board.
  5. Remove three screws (201) and the CD-ROM unit shield.
  6. Remove three screws (201) and 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.
  7. Remove four screws (202) and two screws (201), then lift out the CD drive unit and the sub board.

Reassemble in reverse. Sega's assembly steps say to seat the flat cables firmly in their connectors and to line up the holes in each board with the bosses in the bottom case.[5]

Cleaning

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  • Lens: a cotton swab lightly dampened with isopropyl alcohol, wiped gently. Let it dry before closing the tray.
  • Sled rails and worm gear: remove old, hardened grease before applying fresh lubricant.
  • Edge connector: clean the 60-pin connector on the connect board and the console's expansion port with isopropyl alcohol and let them dry.
  • Boards: wash off any leaked electrolyte, which keeps corroding the board if left. See Battery Explosion, Capacitor or Corrosion Damage.

The back-up cell

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Saves are kept in an 8K × 8 SRAM (IC16) on the main board, held up by a cell at position BT.[7][4] Sega's parts list gives two alternatives for BT: 401-0036, an AL2032-HC1, and 401-0037, an ML2016-HS1.[4] The ML2016 is a rechargeable lithium manganese dioxide cell,[8] and Console5 describes the MB3790 at IC6 as the battery back-up IC.[6] Fit the same type. Sega specifies a rechargeable cell for this position, so do not fit a non-rechargeable CR2032 without first confirming from the schematic that the circuit does not charge it. Check the board around the cell for corrosion. Battery Refurbishment covers cell replacement in general.

The drive

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The manual gives no drive adjustment procedure for the Model 1 and no laser or RF figures. Its remedy for a tray that will not open or shut is to change the CD drive unit, which Sega supplied only as a complete assembly.[9][5] For mechanical spares or a replacement pickup, search on the mechanism's own part numbers: KSK-1200A with a KSS-240A pickup, or the JVC unit (PCB EMW10258 / ENN-335A-1) with an Optima-5S.[6]

Recapping

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Every electrolytic in the unit, by board and designator, is on the Sega CD (Model 1) Capacitor Replacement Guide. On an export machine there are ten surface-mount 10 µF 16 V and two radial 100 µF 6.3 V capacitors on the main board, fourteen 10 µF 16 V and four 100 µF capacitors on the sub board, and none on the connect board, plus those on the drive PCB.[4][6]

After service

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Functional checks
Test What it exercises
Power on with no disc Adaptor, fuse F1, the regulator IC6 on the sub board
Play an audio CD Drive, the LC7883 D/A converter and the analogue stages on the sub board[7]
Load a Mega-CD game Boot ROM, sub CPU, program RAM, word RAM, gate array, CD-ROM decoder; the game's PCM sound tests the RF5C164 and the LC7881[7]
Save and reload in a game that uses back-up RAM The cell, IC6 and the back-up SRAM
Play a cartridge with the unit attached The 60-pin edge connector and the connect board

Sega's check cartridge and check CD procedure, with its error message list, is in section 6 of the manual.[10]

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References

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  1. ↑ Sega CD—link(accessed 2026-10-01)
  2. ↑ Sega Enterprises, Ltd., MEGA-CD Maintenance Manual (Export), August 1992, Rev. A, section 4, accessories list for Europe (PDF p. 28). See Sega CD Service Manual.
  3. ↑ Sega Enterprises, Ltd., Service Manual, Sega CD II / Mega CD II, No. 002, August 1993, section 5, p. 9. See Sega CD 2 / Mega CD 2 - Service Manual.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Sega Enterprises, MEGA-CD Maintenance Manual (Export), August 1992, Rev. A, section 8, parts lists for main board 837-8952 (PDF pp. 94-96), sub board 837-8953 (pp. 97-98) and connect board 839-0534 (p. 99). See Sega CD Service Manual.
  5. ↑ 5.0 5.1 5.2 5.3 Sega Enterprises, MEGA-CD Maintenance Manual (Export), August 1992, Rev. A, section 2-1 general reference number list (PDF p. 7), section 2-2 assembly lists (pp. 8-22) and section 2-3 disassembly and assembly procedure (pp. 23-24). See Sega CD Service Manual.
  6. ↑ 6.0 6.1 6.2 6.3 Sega CD v1—link(accessed 2026-10-01) Secondary source.
  7. ↑ 7.0 7.1 7.2 Sega Enterprises, MEGA-CD Maintenance Manual (Export), August 1992, Rev. A, section 1, block diagram (PDF p. 5). See Sega CD Service Manual.
  8. ↑ Coin Type Lithium Manganese Dioxide Rechargeable Batteries (ML), Maxell—link(accessed 2026-10-01)
  9. ↑ Sega Enterprises, MEGA-CD Maintenance Manual (Export), August 1992, Rev. A, section 5, PCB repair flow chart (PDF p. 30). See Sega CD Service Manual.
  10. ↑ Sega Enterprises, MEGA-CD Maintenance Manual (Export), August 1992, Rev. A, section 6, soft and hard check manual (PDF pp. 31-63). See Sega CD Service Manual.
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