Sega CD (Model 1)
 | |
| Sega's mount diagram for the Mega-CD main board | |
| Specifications | |
|---|---|
| Manufacturer | Sega Enterprises, Ltd. |
| Type | CD-ROM add-on for the Mega Drive / Genesis |
| Released | 12 December 1991 (JP) 11 November 1992 (US) 2 April 1993 (UK)[1] |
| Intro price | ¥49,800 (JP) • US$299.95 (US) • £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 • 64 K × 8 PCM RAM • 16 K × 8 CD decoder SRAM • 8 K × 8 battery-backed RAM[2] |
| Storage | Single-speed CD-ROM drive unit (Sony KSK-1200A or JVC ELM-135)[2] |
| Display | — |
| Sound | Ricoh RF5C164A (315-5476A) 8-channel PCM, LC7883KM digital filter and LC7881M DAC[2] |
| Power | External AC adaptor, 10 V DC at 1.2 A[2] |
| OS / Firmware | Boot ROM, 1 Mbit (EPR-15045 / MPR-15045)[2] |
| Predecessor | — |
| Successor | Sega CD (Model 2) |
| Model no. | HAA-2910 (JP) • 1690 (US) • 1690-18 (UK)[1] |
The Sega CD Model 1 — sold in Japan and Europe as the Mega-CD — is the original front-loading compact-disc add-on for the Mega Drive / Genesis. It bolts to the right-hand expansion port, sits under the console, and adds a second 68000, more RAM, hardware scaling and rotation, eight channels of PCM audio and a CD-ROM drive.[1]
It is not a stand-alone machine and it does not generate video: everything reaches the television through the Mega Drive. It does, however, need its own mains adaptor, so a Mega Drive plus Mega-CD occupies two sockets.[1]
Architecture
Sega's block diagram for the machine (reproduced in the maintenance manual, dated 1991) shows the following.[2]
| Block | Device | Notes |
|---|---|---|
| Sub CPU | 68HC000 at 12.5 MHz | Second-source part: Motorola, Hitachi or Toshiba. Runs from the boot ROM and the program RAM; the Mega Drive's own 68000 remains the main CPU |
| Gate array | Sega 315-5548, "custom chip MCE2" (Fujitsu MB634120) | Labelled MEGA-CD ENGINE on the block diagram. Arbitrates the two CPUs, drives both word-RAM banks, and carries the scaling and rotation hardware |
| Boot ROM | EPR-15045 / MPR-15045, 1 Mbit | CD game BIOS, CD player software and CD+G support |
| Program RAM | 4 Mbit | Sub-CPU workspace |
| Word RAM | 2 × 1 Mbit (WORD-RAM 0, WORD-RAM 1) | Shared between the two CPUs by the gate array |
| PCM sound | Ricoh RF5C164A (Sega 315-5476A) with 64 K × 8 wave RAM | Eight channels |
| CD decoder | Sanyo LC8951 with 16 K × 8 SRAM | Error correction and CD-ROM data handling |
| Backup RAM | 8 K × 8, battery-backed | Saves; the cell is fitted on the main board |
| CD audio path | LC7883KM digital filter → LC7881M DAC → µPC844 op-amps → 20 kHz low-pass and 4 kHz pre-emphasis → LINE OUT | All on the sub board |
| Clock | 50 MHz crystal oscillator | On the main board |
The CD drive itself is a complete purchased assembly and Sega's parts lists do not break it down into components.[2]
Boards
The machine is built from three boards plus the drive unit. Sega's export manual carries parts lists for the export boards and mount diagrams for the earlier Japanese boards, so both sets are documented.[2]
| Board | Export assembly | Earlier Japanese assembly | Function |
|---|---|---|---|
| Main | 837-8952 (PCB 171-6326A) | 837-8015-01 | Sub CPU, gate array, all RAM, boot ROM, CD decoder, PCM, backup cell |
| Sub | 837-8953 (PCB 171-6327A) | 837-8046-01 | DC input, fuse, 5 V regulator, CD audio DAC and output stage, MIX IN and LINE OUT jacks |
| Connect | 839-0534 (PCB 171-6328A) | 837-8326 / 837-8327-01[3] | Carries 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 |
The two drive units are genuinely different mechanisms. The Sony KSK-1200A carries a KSS-240A pickup; the JVC deck (PCB EMW10258 / ENN-335A-1) carries an Optima-5S.[3] Identify which one you have before ordering a pickup or a capacitor kit.
The connect board is worth noting: the export 839-0534 board carries eighteen 150 pF ceramic chip capacitors and no electrolytics at all, while the earlier Japanese connect boards do carry electrolytics.[2][3]
Power
The Model 1 is powered by its own external adaptor, not from the console. Sega's accessories list for Europe specifies 10 V DC at 1.2 A: parts 400-5122A and 400-5122B for 220 V mains and 400-5127 and 400-5127A for 240 V mains.[2] The Model 2 uses an adaptor of the same rating.[4]
Inside, the sub board takes the DC in through fuse F1 — a Pico II 2.5 A radial part — and a series diode, and regulates it to 5 V with a µPC2405 switching regulator at IC6.[2][3] The main board carries an MB3790 supply monitor at IC6, which is also the backup-cell controller.[2]
A great many "dead Sega CD" reports are the adaptor rather than the machine. Sega's own repair flow chart begins there: if the unit will not power on, the branches it offers are a faulty AC adaptor, a faulty plug cord, a faulty Mega Drive or a faulty sub board — the main board is not on that branch at all.[2]
Servicing
Sega's PCB repair flow chart divides faults by what you see on screen.[2]
- Picture appears, but no CD-ROM command displayed — main board or Mega Drive.
- Tray will not open or shut — CD drive unit; the remedy given is to change the drive unit.
- No sound — CD unit, the Mega-CD audio side, or the FFC cable between the main and sub boards.
- Sound present but mixing faulty — the plug cord, or the Mega Drive.
- No power — AC adaptor, plug cord, Mega Drive or sub board.
Two mechanical details follow from the construction. Everything inside the drive unit — belt, gears, sled, tray motor, spindle — is supplied only as part of the complete assembly, so none of it has a published Sega part number; and because the assembly is an ordinary Sony or JVC CD deck, those are the part numbers to search on when sourcing mechanical spares.[2]
Age faults on this machine are dominated by its aluminium electrolytics. The ten on the export main board are surface-mount chip parts and the fourteen on the sub board are radial; leakage from the surface-mount parts, and the board damage that follows it, is the reason console5 files this machine under bad capacitors.[3] Full lists, per board, are on Sega CD (Model 1) Capacitor Replacement Guide; symptom-by-symptom diagnosis is on Sega CD (Model 1) Troubleshooting Guide; strip-down and cleaning are on Sega CD (Model 1) Maintenance Guide.
Things that are not faults
- No picture of its own. The Model 1 outputs no video. A blank screen with the Mega-CD attached is a Mega Drive, cable or television fault until proved otherwise.
- Needing the mixing cable. CD audio leaves the Mega-CD on its own LINE OUT phono jacks and is fed back into the Mega Drive; on a Model 1 Mega Drive that route is the console's headphone socket.[1] Without that cable you get cartridge sound and no CD sound, and nothing is broken.
- Two mains sockets. The add-on has its own adaptor by design.[1]
Documentation
- Sega CD Service Manual — Mega-CD Maintenance Manual (Export / Europe), August 1992, Rev. A.
- Mega CD / Sega CD Schematics — A3 schematic sheets.
- Sega IC Specifications (Genesis VA7, Mega Drive VA6.5, Mega CD) — Sega's IC data for this family.
Related pages
- 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
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Sega Retro, Sega Mega-CD, https://segaretro.org/Mega-CD — release dates, prices and Sega model codes, each footnoted to a contemporary source on that page. Secondary source.
- ↑ 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 2.13 2.14 2.15 Sega Enterprises, Ltd., Mega-CD Maintenance Manual (Export / Europe), August 1992, Rev. A — hosted on this wiki as File:Sega CD Service Manual.pdf (see Sega CD Service Manual). Section 2-3 is the disassembly procedure, section 3 the spare parts list, section 4 the accessories list, section 5 Sega’s own PCB repair flow chart, section 8 the parts lists for the 837-8952 main board, 837-8953 sub board and 839-0534 connect board, and sections 9–10 the schematics and mount diagrams.
- ↑ 3.0 3.1 3.2 3.3 3.4 console5 TechWiki, Sega CD v1, https://wiki.console5.com/wiki/Sega_CD_v1 . Secondary source — a community capacitor map read off real boards. Its lists for the 837-8952 main board and the 837-8953 sub board agree position for position with Sega’s own parts list, and it adds the Japanese-revision boards and the two CD-ROM PCBs, which Sega’s export manual does not itemise.
- ↑ Sega Enterprises, Ltd., Sega CD II / Mega CD II Service Manual, No. 002, August 1993 — hosted on this wiki as File:Sega cd 2 - mega cd 2 - service manual.pdf (see Sega CD 2 / Mega CD 2 - Service Manual). Section 1 gives the ratings, section 3 the disassembly, section 4 the CD drive adjustments, section 5 the CD interface, section 7 the TOC read-in flow chart, section 8 the focus, tracking, disc and feed servicing flow charts, and section 13-3 the electrical parts list.