README.md

Sega Mega CD Main BD Reproductions

This repository contains recreations of main board PCBs for Sega's Mega CD/Sega CD add-on for the Mega Drive/Genesis console. These boards are often subject to damage caused by leaking capacitors and/or batteries. The recreations are based on the available schematics and reverse engineering using scanned images of the original boards' copper layers, so should be reasonably accurate reproductions.

NOTE: In order to use these PCBs sucessfully you will need to be proficient in:

• Soldering fine-pitched surface mount components.
• In most cases, desoldering said components to obtain these from a donor board.
• Troubleshooting any issues that may arise.

The authors are not obliged to provide support, nor shall they be held responsible for botched attempts at using these boards.

Should you however find a legitimate flaw in the board's design or have suggestions for improvements, please feel free to submit an issue. Please note that the current scope of the project is to provide a working replica; we are not at this moment entertaining feature requests for significant modifications.

Revisions

Current research suggests that there were 3 PCB part numbers for the main board. One of these was used with two different MCE chips (the large 208-pin custom chip) resulting in four known revisions. Aside from the markings on the board, the revisions are easily identified using the table below. Obviously, be sure to select the correct replacement board for your purposes.

Board	Program RAM	MCE Chip	MCE Logic	Region	PCM Chip	PCM Logic
837-8015	Zip	315-5477 (MCE1)	JK-FF (IC19)	Japan	315-5476	None
837-8015-01	Zip	315-5548 (MCE2)	D-FF (IC19)	Japan	315-5476A	XOR (IC16)
837-8952	SOJ	315-5548 (MCE2)	D-FF (IC17)	Export	315-5476A	XOR (IC12)
837-8952	SOJ	315-5632 (MCE3)	None (IC17)	Export	315-5476A	XOR (IC12)

Of these revisions, only the schematics for 837-8015-01 are easy to find online. Luckily there are only a few minor differences between the revisions, all of which are now documented in the schematics of this project.

Different revisions of the MCE require different support logic chips. On the 837-8015 this is partially implemented in the "tree house" daughter board (on which the oscillator is mounted) and the "connect board" which connects to the console (see this project if you require a replacement). Later versions have no daughter board and have connect boards with only passive components. Note that if you are mixing and matching parts, there are also variations of the passive connect board with and without particular capacitors, etc. installed. An incorrect combination will usually manifest itself as graphical glitches on the BIOS "intro" screen.

For the 837-8952 be doubly sure to check which MCE revision is used and therefore whether IC17 should be populated or not.

Rumour has it that the MCE3 chip (315-5632) can be used on the 837-8015 board if you also use the passive style connect board. We have not yet confirmed this to be the case but it is plausible judging by the schematics.

If you have a revision or combination not listed above, we'd be interested to hear about it.

PCB Production

Minimum track widths, clearances and via sizes are within the standard offering of modern PCB fabricators. Development was done using JLCPCB and the boards therefore have their order number placeholder on the silkscreen, which you may wish to remove if using another service.

We recommend using at least 4 layers for all boards. For optimal EMI performance you may opt to use 6 layers by replicating the internal ground and power planes (e.g. top, ground, power, power, ground, bottom) but this is likely to be significantly more expensive.

• 837-8015: the original boards were produced using a process that added a partial layer of copper on top of the signal layers, separated by some sort of dielectric coating. During testing, 2-layer boards without this copper layer suffered from noise issues so these have now been updated with internal power and ground planes. Despite these additions, this particular board can still present challenges even to seasoned builders.

• 837-8015-01: this revision has improved routing and 2-layer boards have been shown to work. The design does however have 4 layers and we recommend using these.

• 837-8952: the original "Export" board was 4 layers so this is also the only option due to reliance on internal ground and power planes.

Bill of Materials

Most parts are marked on the board and it is expected that these will be reused from a donor board. It is completely possible that your particular board uses different (but compatible) parts so it's advisable to take photos before starting. The zip RAM is not marked on the PCB but their position is obvious from the number of pins. Take care to mount the SMD RAM in the correct positions as some parts have very similar part numbers.

Below is therefore a partial bill of materials for parts whose values are not marked on the boards and/or look otherwise alike. For the 837-8952 "Export" boards, almost all references were renumbered so be sure to use the correct column on the left to identify the parts. I would seriously recommend printing the list and covering/removing/otherwise obscuring the irrelevant column to avoid confusion.

Values for the capacitors unique to the 837-8952 "Export" board are based on measurements of components on an original example and therefore potentially inaccurate. The values are however also likely not very critical, e.g. 100pF will probably be fine (maybe even closer to the original specification) where 133pF is stated. If in doubt, you may like to reuse the components from your donor board.

Capacitor voltages may exceed those given, within reason. E.g. you may substitute 10V for a 6.3V part. Do not, however, use a lower voltage than specified. No voltage is specified for the ceramic capacitors; any 0805 part is expected to exceed the required voltage anyway. For SMD electrolytics a diameter of 3-4mm is specified. The originals are about 3mm but you can just about get away with 4.3mm.

The right-most columns indicate whether (and where) the part is populated on that revision. T = Top of board, B = bottom of board and a "-" = unpopulated. As a rule THT and electrolytic capacitors are on top, ceramic capacitors and resistors on the bottom. The exception is the two LEDs (L1 and L2) which mount, bent at right angles, to the bottom of the board. For best results, mount these facing outwards, towards the front of the machine.

Reference (Japan)	Reference (Export)	Value	Voltage	Footprint	837-8015	837-8015-01	837-8952
C1	C1	1uF		0805	B	B	B
C2	C2	22nF		0805	B	B	B
C3	C3	22nF		0805	B	B	B
C4	C6	22nF		0805	B	B	B
C5	C5	22nF		0805	B	B	B
C6	-	22nF		0805	B	B	-
C7	-	22nF		0805	B	B	-
C8	-	22nF		0805	B	B	-
C9	C41	10uF	16V	SMD 3-4mm	T	T	T
C10	-	10uF	16V	SMD 3-4mm	T	T	-
C11	C42	10uF	16V	SMD 3-4mm	T	T	T
C12	-	10uF	16V	SMD 3-4mm	T	T	-
C13	-	10uF	16V	SMD 3-4mm	T	T	-
C14	C4	22nF		0805	B	B	B
C15	C32	n/u	-	-	-	-	-
C16	C22	n/u	-	-	-	-	-
C17	-	n/u	-	-	-	-	-
C18	-	n/u	-	-	-	-	-
C19	-	n/u	-	-	-	-	-
C20	-	n/u	-	-	-	-	-
C21	-	n/u	-	-	-	-	-
C22	-	n/u	-	-	-	-	-
C23	C43	10uF	16V	SMD 3-4mm	T	T	T
C24	C7	22nF		0805	B	B	B
C25	C11	22nF		0805	B	B	B
C26	C51	100uF	6.3V	THT 5mm pitch	T	T	T
C27	C9	22nF		0805	B	B	B
C28	C45	10uF	16V	SMD 3-4mm	T	T	T
C29	C23	100pF		0805	B	B	-
C30	C10	22nF		0805	B	B	B
C31	C46	10uF	16V	SMD 3-4mm	T	T	T
C32	C26	100pF		0805	B	B	-
C33	C27	100pF		0805	-	-	B
C34	C28	100pF		0805	B	B	B
C35	C24	100pF		0805	-	-	B
C36	C25	100pF		0805	B	B	B
C37	C47	10uF	16V	SMD 3-4mm	T	T	T
C38	C13	22nF		0805	B	B	B
C39	C14	22nF		0805	B	B	B
C40	C15	22nF		0805	B	B	B
C41	C48	10uF	16V	SMD 3-4mm	T	T	T
C42	C49	10uF	16V	SMD 3-4mm	T	T	T
C43	C16	22nF		0805	B	B	B
C44	C18	22nF		0805	B	B	B
C45	C17	22nF		0805	B	B	B
C46	C50	10uF	16V	SMD 3-4mm	T	T	T
C47	C36	200pF		0805	-	B	-
C48	C52	100uF	6.3V	THT 5mm pitch	T	T	T
C49	C19	22nF		0805	B	B	B
C50	C20	22nF		0805	B	B	B
C51	C12	22nF		0805	B	B	B
C52	C8	22uF		0805	-	B	B
C53	C44	10uF	16V	SMD 3-4mm	-	T	T
C54	C21	100pF		0805	-	-	B
C55	C33	33pF		0805	-	-	B
C56	C34	82pF		0805	-	-	B
C57	C30	n/u		0805	-	-	-
C58	C53	n/u		0805	-	-	-
-	C29	50pF		0805	-	-	B
-	C31	133pF		0805	-	-	B
-	C35	60pF		0805	-	-	B
-	C37	200pF		0805	-	-	B
-	C38	133pF		0805	-	-	B
-	C39	100pF		0805	-	-	B
-	C40	200pF		0805	-	-	B
-	C54	n/u		0805	-	-	-
-	C55	n/u		0805	-	-	-
-	C56	n/u		0805	-	-	-
-	C57	n/u		0805	-	-	-
-	C58	n/u		0805	-	-	-
CX1	-	51pF		THT axial*	B	-	-
R1	R1	4.7K		0805	B	B	B
R2	R2	4.7K		0805	B	B	B
R3	R5	2.2K		0805	B	B	B
R4	R7	360		0805	B	B	B
R5	R6	160		0805	B	B	B
R6	R3	4.7K		0805	B	B	B
R7	R4	4.7K		0805	B	B	B
-	R8-12	0		0805	-	-	See note**

* CX1 (to give it a name) was mounted on the bottom of 837-8015 boards from the factory, between pins 2 and 38 of IC4. You may like to replicate this the same way, or you may use the extra 0805 footprint provided for this purpose. You may even want to live dangerously and leave it off.

** These 0-ohm resistors are used to configure the board depending on the MCE revision, etc.:

• R8 and R9: these have the pads shorted on the board so no resistor need be fitted.
• R10: Fitted only for MCE3, in which case IC17 is unpopulated.
• R11 and R12: these affect how /CLWE is passed to the PCM chip. With R11 installed, this is passed directly like on the 837-8015 board with the 315-5476 (no "A" suffix) PCM chip. With R12 installed the signal is passed through an XOR gate like on the 837-8015-01 board with the 315-5476A (with "A" suffix) installed. This may be useful if you are mixing and matching parts from different boards but the best course of action is to note which of these resistors/jumpers is installed on the original board you are replacing. In any case, DO NOT install both R11 and R12 as this will likely damage IC12.

Modifications

Whilst the PCB layouts are meant to be close replicas of the original, some minor practical modifications have been made.

837-8015 (Japanese "Rev 0")

This revision came from the factory with a capacitor installed directly to the underside of IC4 (the ROM). For your convenience a dedicated SMD footprint has been added so this can be mounted more neatly.

Two bare copper areas have been left near the edge of the board for fitting the additional ground wires originally connected on the underside to pin 5 of IC6 and IC8. These wires were originally required as these chips and IC7 are otherwise grounded only by means of one thin trace. As the reproduction board has an internal ground plane, these wires should no longer be required.

Zip-to-SOJ DRAM Adapter Boards

Due to concerns regarding the availability of the smaller zip DRAM chips, two adapter boards have been created to allow the use of the SOJ part found on the 837-8952 with earlier revisions. The footprints are tailored to the reproduction boards but should also be usable on originals.

Installation is reasonably self-explanatory; fit the SOJ RAM to the adapter board and use header pins or similar to install this in place of the original zip chips. Capacitor C2 may benefit from bending the legs 90 degrees so it sits to the side of the board, depending on its physical size. On both boards, it may also be useful/necessary to desolder FR5 and orient this parallel to the PCB to improve clearance. It should be fine to remove/leave off the decoupling capacitors for the zip RAM (C10, C11, C12, C13) from the main board too if this helps.

Many pins are common to all the zip chips so not all pins need to be connected to the adapter. The required pins are circled on the board. It should not harm anything to connect the others (e.g. to improve structural stability) but this may introduce some additional noise.

Additional Resources

If you require the SEGA font for producing a logo on the board, you may consider this one. Note that the author specifies that this is allowed for personal, non-commercial use only and may not be redistributed.

Thanks

• Simon "Aergan" Lock (@Aergan) for your insights, advice, support and expert testing of the first boards.

• Chrissy (@chris-jh) for the connect board reproduction project, support and testing.

• Aaron Lavery (@ayyyyyyyron) for building/testing the Export/MCE2 combination.

• Adrian Soh (@klin1309) for verifying the 837-8015/MCE1 combination.

• Zaxour, @PointerFunction, Leo Oliveira and the rest of the Board Folk for their support and general coolness.

• 240p Test Suite by @ArtemioUrbina was indispensible for testing and troubleshooting the boards.

Legal

As the product of this project is a replica of a proprietary product, the the author makes no claim of copyright to the schematics nor PCB layouts and releases these into the public domain, solely for the purposes of study and historical preservation.

You are free to produce PCBs based on this project's designs at your own risk and without limitation, for your own use or for sale and/or repair at a reasonable price. Attribution is appreciated. The authors are not obliged to provide support of any kind.

Under no circumstances will the authors be held responsible or liable in any way for losses, damages or costs resulting from the use of the information and/or resources of this project.

The resources are provided "as-is" without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose.