Neptune 32X/MD2

The Sega Neptune was an unreleased console that would have been a stand-alone 32X. This project provides a PCB that combines the 32X and Mega Drive/Genesis 2 that fits in an original Mega Drive/Genesis 2 case or the reimagined Neptune case designed by DVIZIX.

NOTE: In order to use these PCBs successfully 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.

Development and testing of these PCBs has shown that successfully assembling a working board is challenging even for skilled, experienced builders. Please read the build notes below carefully before starting. If you are in any doubt, it is highly recommended to enlist the help of someone with a known track record.

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.

Features and Limitations

• Fits original Mega Drive 2 (PAL), Genesis 2 or DVIZIX Neptune cases. Separate power/reset switch locations are provided for each (see the stl directory for files with which to 3D print spacers for the buttons if using the DVIZIX case).
• NOT compatible with the Japanese Mega Drive 2 case due to this using a sliding power switch with a different footprint to that of the push button style.
• Modern power circuit by Zaxour (requires 9V DC PSU rated for at least 1A).
• Integrated switchless region mod using PIC16F630 or compatible programmed with the switchless_xxxx.hex files in this repo (source here). Use the file that matches your LED's pinout.
• Integrated "Triple Bypass" mod for improved RGB and audio.
• Provisional support for Sega Master System games via integrated mod by DogP.*
• Compatible with the original Sega CD/Mega CD.**
• Not compatible with flash cart-based Sega CD/Mega CD implementations such as the Mega EverDrive Pro.
• As the 32X hardware is integrated into the design, there is NO way to disable this.
• No mono audio output.

* Master System compatibility is not extensively tested. Original cartridges have been shown to work when using simple pass-through adapters. Using Mega Drive flash carts to run Master System games may or may not work. Some such cartridges check for the presence of a 32X and, if detected, will refuse to boot in Master System mode, as this would usually cause a hardware conflict. In some cases this check can be disabled in the flash cart's menu. While we believe that this is safe, you do so at your own risk.

** The model 2 Sega CD/Mega CD uses the +9V pin on the expansion connector to power itself on along with the attached console. This board does not however provide 9V to this pin. Testing has shown that the CD hardware will also power on with only 5V on this pin. We're not aware of any add-ons that require 9V on the expansion connector, so what would usually be +9V is connected to +5V by a trace between the pads near B26 and B28. If you do however really need it, there is another +9V pad near the power jack. This is however not switched along with the console and will remain powered as long as there is power to the jack. You could rig up something with a transistor or MOSFET to make 9V switch on along with the console. Of course, making any such modification will require the trace between +5V and +9V be cut.

PCB Production

Minimum track widths, clearances and via sizes are within the standard offering of modern PCB fabricators. Gerber files are generated with the relevant options for JLCPCB. Placeholders for the order number are provided so remember to select the "Specify location" option for this when ordering. If using another service you may wish to remove or replace this placeholder.

The design is verified to work as a 4-layer PCB with 1 oz outer and 1/2 oz inner layers. The intended layer order is Front, In1, In2, Back. A leaded HASL finish is adequate and ensures compatibility with the original solder that will inevitably be left behind on salvaged parts. If you plan to use a Mega CD/Sega CD or anything that uses the edge connector you might want to opt for an ENIG or other gold finish for this. Chamfering of the expansion edge connector is recommended.

For independent testing of the Mega Drive side, you might like to try Chrissy's experimental bypass boards for IC504 and IC505 in the helpers directory (more info below in the build notes).

Bill of Materials

For a complete BOM, consult the generated Interactive BOM or the KiCad project itself. Parts are numbered according to the following scheme:

Number	Description
< 500	Original to Mega Drive/Genesis 2
500-599	Original to 32X (subtract 500 for original number)
>= 600	Additional/replacement parts

It is assumed that most parts will be obtained from donor Mega Drive/Genesis and 32X units. The project was initially based on a VA1 PAL Mega Drive 2 and VA1 PAL 32X. Other Mega Drive/Genesis revisions may be usable if these also use a 315-5660 or compatible ASIC. Be aware that the 315-5660-10 variant found on some Japanese revisions are known not to support 50 Hz operation which will render the integrated switchless region mod superfluous. All 32X revisions should be usable but this has not yet been confirmed. Note the differences in parts between MD and 32X revisions/regions as documented in the schematics. It is probably not feasible to list the differences between all possible revisions but should you work out a solution for a particular donor board, please let us know so we can include it in this documentation.

Parts marked "PAL" on the board are supposedly only used on PAL Mega Drives. Parts marked "VA0" and "VA1" are for those revisions of 32X only. However, should your donor hardware have any of these parts it would seem wise to transfer them to the new board regardless of the original region/revision.

For 32X main boards with the SCA chip, pay particular attention to D699. If your donor 32X has a THT diode for /VRES on the main board you will need to install this or an SMD equivalent at this location. The footprint is however bridged, as on other revisions there is no diode and this is simply a direct connection. If installing the diode you will therefore need to cut the trace between the pads. The accompanying capacitor may be installed at location C695.

All SMD passives on the board have 0805 or larger footprints for easier soldering. Inductor footprints are 1210 but you can of course use narrower such as 1206. If reusing MD2 passives, these should therefore fit well. Many 32X passives are smaller but should still fit. If you plan to reuse them, do note that particularly the smaller capacitors are prone to losing their metal end caps and, being so small, this is not always immediately apparent. If you see solder balling up instead of flowing nicely, this may well be the problem.

Build Notes

General

• Before disassembling the donor consoles, test these thoroughly using the "Mars Checker" diagnostic ROM. Please note that running this from some flash carts (particular EverDrive models or Mega SD) may yield unreliable results. Even on known-good original hardware, you may be presented with a red screen when booting or test runs may/may not succeed seemingly at random. For this reason, it's recommended to run the tests several times. If you are unable to boot the diagnostics using your particular flash cart, consider a simpler single-image (i.e. no menu) one or something EPROM-based.

• The ICs on at least one side of the 32X main board (the "cartridge" part) are glued on making removal tricky. The glue can be softened using isopropyl alcohol, so you may want to give the board soak in this before starting to avoid having to use too much heat. Low-melt solder (e.g. ChipQuik) is very much recommended. Avoid extended use of hot air to avoid damaging these potentially fragile chips.

• It may sound obvious, but pay close attention to orientation when fitting ICs. The 32X SDRAM (IC503) and some of the QFP packages have particularly subtle markings. It helps to mark (e.g. with tape or a groove in the package casing) pin one before removing the ICs from the donor boards. The legend silkscreen on the board should match that on the IC but this may not always be a reliable indicator of orientation.

• Clean all boards thoroughly with e.g. isopropyl alcohol prior to starting. Especially HASL boards from JLC have been known to arrive with some contaminants present which prevent reliable solder joints. You may even like to tin the SMD pads and clean up with braid/wick to ensure that solder will adhere properly.

• The Triple Bypass mod features a low pass filter that can be enabled or disabled by fitting R966 or R965 respectively. As the LPF has both advantages and disadvantages, the choice is left to the builder. When using the provided PCB assembly files, the LPF is enabled by default. Otherwise, you must fit one of these resistors; the enable pin may not be left floating. Enabling the LPF may improve Mega Drive image quality at the expense of artefacts in 32X graphics. Disabling it may be preferable if you are using a video scaler. YMMV, so do what works for your particular setup.

Order of Operations

It is possible to break down parts the build into separate steps to allow for easier testing and troubleshooting. The following is a suggested order of operations.

• Build the power circuit (all parts in the X800 range) and test this separately. It should provide a steady 5V supply even without any load.

• Add the 3.3V regulator (IC510) for the 32X SDRAM and its associated capacitors. Check for 3.3V on the regulator output as labeled on the board.

• You may wish to install the PIC for the switchless region mod at this point as it should operate independently of the rest of the system. Install also the LED, passives in the X700 range and a temporary ~10K pull-up on /WRES (pin 11). R702 limits current through the LED and therefore determines its brightness. The value is based on the original MD2 LED, so you may prefer to use a higher value when using a modern LED, which tend to be brighter for the same current. If your LED has protruding "lugs" instead of just straight pins, it's recommended to trim these off if possible before installation to avoid damaging the board. Hold down the reset button to cycle through regions as indicated on the RGB LED. If the LED changes constantly, check pull-up R701 is installed. The PAL/NTSC and JP/ENG outputs should change as appropriate. Verify that a short press of the button generates a /WRES pulse.

• If not using the switchless mod, or to bypass it for testing/troubleshooting, link the indicated pins and set the desired PAL/NTSC and JP/ENG values using the +5V and GND pads on the IC701 footprint. Connect the LED pin of your choice to +5V and a single colour LED for LED701.

• At this point it's time to strap in and install all the MD2 components and the audio and video parts shared between MD and 32X. This cannot be independently tested without bypassing the 32X parts that usually connect to the cartridge slot. To this end, you may like to use the bypass flexes for IC504 and IC505 (see the helpers directory). These boards are designed to fit in place of the 32X ICs and simply pass the required signals directly to the Mega Drive components. If using the bypass boards, you will also need to temporarily connect the /YS and /AS signals to the cartridge slot. /YS has a labeled test point for this near pin B12 on the underside of the slot. /AS can be connected by bridging pins 8 and 12 on the footprint for IC516. Remove these connections before installing the remaining 32X hardware.

• When testing a newly-built Neptune, trying different kinds of cartridges and peripherals is useful for troubleshooting issues. Original Mega Drive cartridges are a good place to start as these should run even if the 32X hardware isn't fully operational. Running Master System cartridges via an adapter also hits different functionality in the MD VDP/ASIC, Z80, memory, etc. If no cartridges work, you may be able to boot to the BIOS screen of a Mega CD/Sega CD, which would point to issues around the cartridge slot and/or 32X bus multiplexing. Once the basic functionality is verified, diagnostics such as those in the 240p Test Suite and Mars Checker can be used to troubleshoot any remaining issues.

• For a fully assembled board WITHOUT any cables connected there should be around 350 Ohms between VCC and ground. If you measure something wildly different this may indicate a short, an incorrectly installed IC or similar. With the video cable connected the resistance will drop to around 150 Ohms.

Thanks/Credits

• Zaxour for the power circuit design, invaluable support and hardware review.

• Chrissy (@chris_jh) for testing, support and work on making pre-assembled PCBs possible.

• Leo Oliveira, Simon "Aergan" Lock, Ian (@grandoldian) and Dennis (@PointerFunction) for their insights, support and assistance during testing.

• The rest of the Board Folk for their support and general coolness.

Legal

Copyright ©2024

You are free to produce PCBs using this project's designs at your own risk, for your own use or for sale and/or repair at a reasonable price. Attribution is appreciated. Publishing files for PCB production (such as, but not limited to, Gerber files) for financial compensation (sales commission, credit, etc.) on for example "shared project"-style websites or similar is prohibited.

Derivative works must be released under similar Open Source terms such that all sources are publicly available. The original authors' names and copyright statements may not be removed.

The authors are not obliged to provide support of any kind. Under no circumstances shall 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.