Sega CD (Model 1): Difference between revisions
Infobox photo; correct audio path (LC7883 for CD audio, LC7881 for PCM), mixing-cable direction and Sega's flow chart branches from the manual; dates and prices per Wikipedia (Sega Retro unreachable); page-specific manual refs; copyedit |
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Sega's block diagram, dated 2 July 1991, shows the following blocks.<ref name="mcd1bd" /> Board positions are from the parts list for the export main board, 837-8952.<ref name="mcd1pl" /> | Sega's block diagram, dated 2 July 1991, shows the following blocks.<ref name="mcd1bd" /> Board positions are from the parts list for the export main board, 837-8952.<ref name="mcd1pl" /> | ||
{| class="wikitable styled-table" style="width: | {| class="wikitable styled-table" style="width:70%; text-align:left;" | ||
|+'''Main functional blocks''' | |+'''Main functional blocks''' | ||
! Block !! Device (export main board) !! Notes | ! Block !! Device (export main board) !! Notes | ||
Latest revision as of 20:30, 1 October 2026
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| A Model 1 Sega CD with the steel joining plate fitted | |
| Specifications | |
|---|---|
| Manufacturer | Sega Enterprises, Ltd. |
| Type | CD-ROM add-on for the Mega Drive / Genesis |
| Released | 12 December 1991 (Japan) 15 October 1992 (North America) 2 April 1993 (UK)[1] |
| Intro price | ¥49,800 (Japan), US$299 (North America), £269.99 (UK)[1] |
| CPU | 68HC000 at 12.5 MHz (sub CPU), fitted as MC68HC000FN12, HD68HC000CP-12 or TMP68HC000T-12[2] |
| Memory | 4 Mbit program RAM, 2 × 1 Mbit word RAM, 64K × 8 PCM RAM, 16K × 8 CD-ROM buffer, 8K × 8 back-up RAM[3] |
| Storage | Single-speed CD-ROM drive unit, Sony KSK-1200A or JVC ELM-135[4] |
| Display | None; video is produced by the Mega Drive |
| Sound | Ricoh RF5C164A (Sega 315-5476A) 8-channel PCM, with LC7881 and LC7883 D/A converters[3][2] |
| Power | External AC adaptor, 10 V DC 1.2 A[5] |
| OS / Firmware | 1 Mbit boot ROM, EPR-15045 or MPR-15045[3][2] |
| Successor | Sega CD (Model 2) |
The Sega CD Model 1, sold as the Mega-CD outside North America, is Sega's original front-loading CD-ROM add-on for the Mega Drive and Genesis. It sits under the console, connects through the console's expansion port and adds a second 68000 CPU, more RAM, a custom graphics chip with scaling and rotation, eight-channel PCM sound and a CD drive with a motorised tray.[1] It produces no video of its own, and it has its own mains adaptor.
History
[edit | edit source]Sega released the Mega-CD in Japan on 12 December 1991 at ¥49,800. It reached North America as the Sega CD on 15 October 1992 at US$299, and the UK on 2 April 1993 at £269.99. Sega followed it with the smaller, top-loading Model 2, released in Japan on 23 April 1993 to bring down manufacturing costs.[1]
Architecture
[edit | edit source]Sega's block diagram, dated 2 July 1991, shows the following blocks.[3] Board positions are from the parts list for the export main board, 837-8952.[2]
| Block | Device (export main board) | Notes |
|---|---|---|
| Sub CPU | IC1, 68HC000 at 12.5 MHz | Runs from the boot ROM and program RAM. The Mega Drive's own 68000 remains the main CPU |
| Gate array | IC2, Sega 315-5548 "custom chip MCE2" (Fujitsu MB634120) | Labelled MEGA-CD ENGINE on the block diagram. Connects both CPUs, the word RAM, the CD-ROM decoder and the PCM chip |
| Boot ROM | IC4, EPR-15045 or MPR-15045, 1 Mbit | In a 40-pin socket |
| Program RAM | IC5, µPD424270, 4 Mbit | |
| Word RAM | IC7 and IC8, TC511664, 1 Mbit each | WORD-RAM 0 and WORD-RAM 1 |
| PCM sound | IC3, Ricoh RF5C164A (Sega 315-5476A), with IC10 and IC11 (64K × 8 wave RAM) | Eight channels |
| CD-ROM decoder | IC13, Sanyo LC8951, with IC14 and IC15 (16K × 8 buffer) | Error correction for CD-ROM data |
| Back-up RAM | IC16, MB8464A, 8K × 8 | Held up by the cell BT on the main board |
| Audio | Sub board: LC7883 and LC7881 D/A converters, µPC844 op-amps | CD audio from the drive goes through the LC7883; PCM goes through the LC7881 and a low-pass filter. The two are mixed, filtered at 20 kHz and sent to the LINE OUT jacks |
| Clock | 50 MHz crystal oscillator |
Boards
[edit | edit source]The unit has three boards and a drive unit. The export manual gives parts lists for the export boards and carries Sega's schematics and mount diagrams, which are drawn for the earlier main board 837-8015-01 and sub board 837-8046-01.[2][6]
| Board | Export assembly | Earlier assembly | What it carries |
|---|---|---|---|
| Main | 837-8952 (PCB 171-6326A) | 837-8015-01 | Sub CPU, gate array, all RAM, boot ROM, CD-ROM decoder, PCM chip, back-up cell, LEDs |
| Sub | 837-8953 (PCB 171-6327A) | 837-8046-01 | DC input, fuse, 5 V regulator, audio D/A converters and output stage, MIX and LINE OUT jacks |
| Connect | 839-0534 (PCB 171-6328A) | 837-8326 or 837-8327-01 | The 60-pin edge connector to the console |
| Drive unit | 610-5270 (Sony KSK-1200A) or 610-5270-10 (JVC ELM-135) | Complete CD mechanism, servo and pickup |
Console5 records the Sony KSK-1200A with a KSS-240A pickup and the JVC drive (PCB EMW10258 / ENN-335A-1) with an Optima-5S pickup. The two mechanisms share no parts.[6]
Power
[edit | edit source]The Model 1 has its own external adaptor. Sega's accessories list for Europe gives 400-5122A and 400-5122B for 220 V mains and 400-5127 and 400-5127A for 240 V, all 10 V DC at 1.2 A.[5] On the sub board the DC input goes through fuse F1, a Pico II 2.5 A radial part, and diode D1 to IC6, a µPC2405HF 5 V regulator on a heat sink.[2][6]
Servicing
[edit | edit source]Sega's PCB repair flow chart works at module level.[7] With no picture and no power, it blames the AC adaptor or the sub board. With a picture, it sends a missing CD-ROM screen to the main board or the Mega Drive, a tray that will not open or shut to the CD drive unit, no sound to the FFC cable between the main and sub boards, and a mixing fault to the plug cord or the Mega Drive. The drive unit is a complete assembly with a single Sega part number, and the manual gives no adjustment procedure for it.[4]
The electrolytic capacitors are the usual age fault: twelve on the export main board, ten of them surface-mount, and eighteen on the sub board.[2] See the Sega CD (Model 1) Capacitor Replacement Guide for the list, the Sega CD (Model 1) Troubleshooting Guide for fault finding and the Sega CD (Model 1) Maintenance Guide for strip-down and cleaning.
Things that are not faults
[edit | edit source]- No picture from the Mega-CD on its own. It has no video output.
- No CD sound through the television with a Model 1 Mega Drive. The CD unit's sound comes out of its own LINE OUT jacks; to hear the console and the CD together, the Mega Drive's headphone output is cabled into the input jack on the back of the CD unit and the mixed sound is taken from the CD unit. A Model 2 console does not need the cable.[1]
- Two mains adaptors. The add-on has its own by design.
Documentation
[edit | edit source]- Sega CD Service Manual: Sega's MEGA-CD Maintenance Manual (Export), August 1992, Rev. A, with parts lists, schematics, mount diagrams, the repair flow chart and the check-cartridge error list.
- Mega CD / Sega CD Schematics: larger A3 scans of the block diagram of the 315-5548, the schematics and the main board mount diagrams.
- Sega IC Specifications (Genesis VA7, Mega Drive VA6.5, Mega CD): Sega's IC pin data for this family.
Gallery
[edit | edit source]-
Sega's mount diagram for the main board 837-8015-01 -
Sega's mount diagram for the sub board 837-8046-01
Related pages
[edit | edit source]- Sega CD (Model 1) Maintenance Guide
- Sega CD (Model 1) Capacitor Replacement Guide
- Sega CD (Model 1) Troubleshooting Guide
- Sega CD (Model 2)
- Capacitor Failure Symptoms
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 Sega CD—link(accessed 2026-10-01) Secondary source; the dates and prices are cited there to Sega's own Mega-CD page, Retro Gamer issue 153 and Computer and Video Games issue 138, p. 8.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Sega Enterprises, Ltd., 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.
- ↑ 3.0 3.1 3.2 3.3 Sega Enterprises, MEGA-CD Maintenance Manual (Export), August 1992, Rev. A, section 1, block diagram (PDF p. 5, "MEGA-CD HARD MANUAL", Ver. 0.5, 2 July 1991). See Sega CD Service Manual.
- ↑ 4.0 4.1 Sega Enterprises, MEGA-CD Maintenance Manual (Export), August 1992, Rev. A, section 2-2 assembly lists (assembly 1006, PDF pp. 19-20), section 2-3 disassembly and assembly procedure (pp. 23-24) and section 3 spare parts list (p. 26). See Sega CD Service Manual.
- ↑ 5.0 5.1 Sega Enterprises, MEGA-CD Maintenance Manual (Export), August 1992, Rev. A, section 4, accessories list for Europe (PDF p. 28). See Sega CD Service Manual.
- ↑ 6.0 6.1 6.2 Sega CD v1—link(accessed 2026-10-01) Secondary source: IC lists and capacitor lists read off real boards, including the earlier Japanese boards and both drive PCBs.
- ↑ 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.