Genesis:Motherboard Differences: Difference between revisions

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[[Category:Genesis]]
There were numerous motherboard revisions of the standalone Genesis and Mega Drive consoles. This page serves to document these board revisions and the technical information for each.
'''Some page content & researching taken from [https://io55.net/wiki/eop/video_game_consoles-home/4th_generation/sega_genesis_plus_add-ons io55.net's Sega Genesis page] with permission, in collaboration.'''


There were numerous models and variations of the Genesis/Mega Drive hardware produced over the years, ranging from standalone consoles to handhelds and even an unusual boombox/game console hybrid. This page serves to document these models and all variations (if any) found within each.
== Model 1 ==


== Models and Revisions ==
=== 837-6656 - "VA0" (1988, Japan) ===
[[Image:MDVA0.png|none|thumb|350px]]
This is the earliest revision of the model 1 and is only found in Japanese Mega Drive consoles. These contain a small daughterboard (part number 839-0231) which has the master oscillator crystal, a 74LS74 D-type flip flop and two 74161 (one 74LS161 and one 74F161) 4-bit counter chips. During manufacturing, Sega discovered that the EDCLK (external dot clock) signal did not work properly in the H40 video mode. Using the master clock frequency and the horizontal sync (HSYNC) signal, the daughterboard generates a proper EDCLK signal which is fed back into the board. Sega also included a discrete 74LS00 logic gate on the motherboard to fix RAM compatibility issues with Master System games.<ref>http://gendev.spritesmind.net/forum/viewtopic.php?t=2922</ref>


''Models are arranged chronologically. Revisions are underneath their respective model.''
Although the board is not designated as such, it is commonly referred to as the "VA0" in keeping with Sega's revision naming practices and the fact that it was the first board revision produced.


=== Model 1 (1988-1993) ===
The VA0 revision also has a significant flaw with its stereo audio output. While the balance between frequencies is excellent, the preamp is far too high, resulting in distortion in games that are too loud. However, this can be fixed [[Genesis:Preamp_Fix_(Model_1)|by substituting several capacitor and resistor values on the underside of the motherboard]]. Video quality is also compromised as composite video is afflicted with "rainbow banding" and the RGB output has noticeable jailbars. These would be carried over into later model 1 revisions.


The model 1 Genesis/Mega Drive is easily distinguished from the model 2, as it is rectangular, with a large and raised circular area on top containing the cartridge slot, "16-Bit" text and the console logo in the case of the Genesis. Audio output is known as being vastly superior on most of these models, although stereo must be routed through the headphone jack instead of the usual AV port, which was inherited from the [[Master System: Master System Mods Wiki|Master System]]. One downside is that most revisions have a minor "vertical bar" issue in the video output, while revisions with lessened vertical bars come with the TMSS lockout program. Earlier revisions of the Model 1 have an EXT port on the back, similar in pin structure to a 9-pin RS-232 serial port. One specific late revision of this model has horrible audio quality. Production largely stayed in Japan though most Western models were manufactured in Taiwan, China and Thailand.
Sega only produced the VA0 for a fairly short period, as it was quickly replaced by the VA1 (detailed below) in order to cut production costs and presumably to reduce the amount of manual labor involved during assembly, since the daughterboard and its connecting wires had to be soldered in by hand.


==== "VA0" (1988) ====
=== VA1 (1989, Japan) ===
This is the earliest revision of the model 1 and is only found in Japanese Mega Drive consoles. These are distinguished by a small daughterboard which contains the master oscillator crystal and a few support chips. During manufacturing, Sega discovered a flaw with the EDCLK signal on the motherboard as designed, and this daughterboard generates a correct EDCLK signal which is fed back into the VDP. Although the board is not designated as such, it is commonly referred to as the "VA0" in keeping with Sega's revision naming practices and the fact that it was the first board revision produced.
[[Image:JP MDVA1 Recapped2.jpg|none|thumb|350px]]
Introduced very early in 1989, the VA1 incorporated the EDCLK daughterboard and discrete 74LS00 logic gate into an additional gate array chip, the 315-5339. Like the "VA0" before it, this board is only found in Japanese Mega Drives. It also has the same issue with the overdriven preamp over the headphone output.


The VA0 revision also has a significant flaw with its stereo audio output. While the balance between frequencies is excellent, the preamp is far too high, resulting in horrible distortion in games that are too loud. However, this can be fixed by substituting several capacitor and resistor values on the underside of the motherboard.
=== VA2 (1989, Japan and North America) ===
{|style="margin: 0 auto;"
| [[Image:JPVA2Board.jpg|none|thumb|350px|A VA2 board from a Japanese Mega Drive.]]
| [[Image:Model1GenesisVA2.jpg|thumb|upright|390px|A VA2 board from a model 1 Genesis. Note how the audio and video circuitry have been shifted down to make room for the RF modulator.]]
|}


==== VA1 (1989) ====
This is the first board revision for the Genesis and is also found in Japanese Mega Drive consoles. On this revision, the 315-5339 EDCLK generator chip was replaced with the 315-5345, which added an additional bus acknowledgment signal (/BGACK) going to the VDP. It uses this signal to prevent potential glitches related to DMA (direct memory access) refresh timing.<ref>https://twitter.com/nukeykt/status/1718625118031839563#m</ref> No other major changes were made.
Introduced very early in 1989, the VA1 incorporates the EDCLK fix into the motherboard. Like the "VA0" before it, this board is only found in Japanese Mega Drives and also has the same issue with the overdriven preamp.


==== VA2 (1989) ====
Sega made some changes to the board layout for North American models in order to accommodate an internal RF modulator and RF shielding, neither of which the Japanese Mega Drive ever had. The retention of the EXT port also meant that the power input jack had to be moved onto a separate PCB, connected to the motherboard by two wires. These layout differences between Japanese and Western consoles would be carried on into all later model 1 revisions until the VA7.
This is the first board revision for the Genesis and is also found in Japanese Mega Drive consoles. There are no appreciable differences between this revision and the VA1, and the overdriven preamp issue was also carried over.


==== VA3/VA4 (1989-1991) ====
=== VA3 (1989-1991, Brazil and North America) ===
The audio situation has been changed from previous board revisions, lowering the preamp, but somewhat altering the balance between frequencies. This results in no more distortion in loud games, but a slightly poorer frequency response curve, leading to a minor muffling. The 315-5308 and 315-5345 bus arbiter chips in previous board revisions were also consolidated into a single IC in the VA3, known as the 315-5364. VA3 boards are only found in US Genesis consoles, while the near identical VA4 board is only found in both Japanese and PAL Mega Drive consoles (being the first board revision for the latter).  
[[Image:USModel1VA3Board.jpg|none|thumb|350px|A VA3 board from a model 1 Genesis.]]


PAL Mega Drive consoles with a VA4 board can be distinguished by the presence of the EXT port at the rear and the silkscreened text "High Definition Graphics - Stereo Sound" above the cartridge port.
Introduced in late 1989, the VA3 revision was only used in North American and Brazilian consoles (being the first revision for the latter). The 315-5308 bus arbiter and 315-5345 EDCLK generator were consolidated into a single IC, the 315-5364, likely to reduce production cost. A 4.7k ohm pull-up resistor array was also added on the Z80's data bus pins, though it was soldered directly onto the Z80 itself and not placed on the board. The audio circuitry was changed as the overdriven preamp issue from previous revisions was fixed and some low pass filtering was added to the output stage, which slightly muffled the audio. Despite this, the audio quality is generally regarded as being excellent and this variant is often used as a baseline for audio quality in Genesis consoles. Its audio profile is also the basis for the audio portion of the [[Genesis:Triple Bypass Mod|Triple Bypass]] modification board.


==== VA5/VA6 (1991-1992) ====
For Brazilian VA3s, some changes were made in order for them to output PAL-M video. A 53.634 MHz master oscillator was fitted and pin 7 of the video encoder was directly connected to GND to force it into PAL mode; this pin is normally controlled by JP3 and JP4, which also set the VDP's video frequency.<ref>https://www.youtube.com/watch?v=NQeTLohYzTg</ref>
Of the changes on this revision, the crucial difference is the inclusion of the “TMSS” lockout screen (Produced by or under license etc.) on the VA6 that appears for a bit before playing any game. These are MOSTLY identifiable by the FCC ID, which has been shortened to “FJ8USASEGA”, although some VA6es do NOT have this FCC ID. Furthermore, the video output is moderately improved, as the “jailbars” or “rainbow banding” issue has been fixed on these. Otherwise, all is identical to the VA3 and VA4 (EXT, great audio). The VA5 board is identical but has a different I/O chip which does not have TMSS. VA5 boards are only found in Asian and Japanese Mega Drive models.


''For the remaining model 1 revisions, the "High Definition Graphics" slogan at the top is removed. The "AV Intelligent Terminal" text on Asian/Japanese models was also removed at around the same time.''
=== VA4 (1989-1992, Asia, Japan, Korea and PAL) ===
[[Image:PALVA4BoardModded.jpg|none|thumb|350px|A VA4 board from a PAL Mega Drive. Note that this board has been slightly modified but is largely identical to a stock configuration.]]


==== VA6.5 (1992-1993) ====
The VA4 is largely identical to the VA3, and was first introduced in late 1989 in Japanese Mega Drive consoles. It was also the first board revision for Asian, PAL and Korean consoles. Asian boards use the same board layout as the PAL models, and have an internal RF modulator. The video encoder was flipped around with the front of the chip pointing towards the front of the console. In addition, the resistor array on the Z80's data bus was given a footprint on the board and was not soldered directly onto the Z80 as before. These are the only significant differences between the VA3 and VA4.
These are easily identified by the aforementioned slogan removal, alongside viewing the ports at the back. The EXT port is no longer available on these consoles, but the other ports have not changed position compared to prior revisions. This presentation will result in the four remaining ports being justified to the right of the I/O area, leaving a reasonable gap to the left where the EXT port was. Outside of this change, this revision is identical to other VA6 boards. The "VA6.5" revision name is found in Sega service documentation but not the actual motherboard, since it is simply a VA6 with the EXT port and its supporting circuitry unpopulated.


It is worth noting that the EXT port was ''not'' removed for Japanese models as the Mega Modem attachment, which made use of this port, was still being actively supported. As a result, VA6.5 boards are only found in US Genesis and PAL Mega Drive consoles; most model 1 PAL Mega Drive consoles are of this revision.
PAL VA4 boards made in 1992 (such as the one pictured above) use a [https://en.wikipedia.org/wiki/Die_shrink die-shrunk]<ref>https://github.com/emu-russia/SEGAChips/tree/main/VDP</ref> version of the VDP, the 315-5313'''A''' (Yamaha FC1001).


==== VA6.8 (1992) ====
==== SPC-200/201R Rev. A (1992-1993, Korea) ====
[[Image:Spc200rreva.jpg|none|thumb|350px|The VA4 variant from a Samsung Super Aladdin Boy.]]


Only found in European Mega Drives, the VA6.8 revision is largely identical to the VA6 but has a surface-mount Fujitsu MB3514 video encoder in place of the DIP Sony CXA1145 found in all other models.
By 1992, Samsung began manufacturing their own motherboards for the Super Gam-Boy/Super Aladdin Boy, which was the version of the Mega Drive sold in Korea. This board is heavily based on the VA4 and uses the same layout, but does not have the EXT port populated.


==== VA7 (1992-1993) ====
=== VA5 (1990-1991, Japan) ===
[[Image:JPVA5Board.jpg|none|thumb|350px|A VA5 board from a Japanese Mega Drive.]]
Introduced in late 1990, this board design saw further consolidation by combining the 315-5309 I/O chip and 315-5364 bus arbiter into a single ASIC, the 315-5402. Composite video output was moderately improved with the reduction of the "rainbow banding" common on earlier revisions (though it is still present), and the excellent audio quality of the VA3 and VA4 boards was carried over.  This particular board revision was only used in Japanese Mega Drives.  The region and video frequency jumpers were relocated from the lower center to a new position closer to the expansion port.


Introduced in late 1992, the VA7 was the final and most drastic revision of the model 1 released. It is the first standalone Genesis/Mega Drive to use the FC1004 ASIC found in most model 2 consoles, which combines the VDP, I/O and a YM3438 audio chip into a single chip. All VA7s use a surface-mount version of the Sony CXA1145 encoder as well as a QFP version of the Motorola 68000 CPU.
One interesting quirk with this board revision is the presence of three jumper wires which are sometimes mistaken as an aftermarket modification. These jumper wires connect the CE (Chip Enable) and OE (Output Enable) signals of the two 68000 RAM chips to the main system bus; JP5 connects the CE signal of both chips, JP6 the OE signal of IC2, and JP7 the OE signal of IC3. It is unknown why these signals were connected using jumper wires and not properly routed on the board itself as on previous revisions. The jumper wires are also present on the VA6 and its variants (detailed below).


The ports on the back have been shifted significantly to the left, having the AV out occupy where the EXT port used to be (except on Japanese VA7s which still have the EXT port present), which leaves a large gap to the right. In addition, the sound output is notoriously bad when compared to other revisions; it is muffled, filled with hiss, and possesses a flat frequency range with excessive distortion. This is due to Sega using an inadequate amplifier circuit and can be corrected using a modification board such as the Triple Bypass or Mega Amp. This revision is only found in US Genesis and Japanese Mega Drive systems.
Two different versions of the 315-5402 I/O chip exist, one which completely lacks TMSS, and another which will display the TMSS screen ''only'' if the system's region setting is changed from Japanese to English/export, which is accomplished by cutting the trace on jumper JP1 and connecting JP2 in its place.


=== TeraDrive (1991) ===
This was the last board revision where Sega used 68000 CPUs made by Signetics, who had been Sega's sole supplier of this component for the Mega Drive since its launch. During 1991, Sega began sourcing CPUs from Motorola and Hitachi instead, and VA5s manufactured in 1991 (denoted with a "1" as the second digit of the serial number) have Motorola CPUs.
Introduced in May 1991 as a collaboration between IBM and Sega, the '''TeraDrive''' is a combination of a Mega Drive and an IBM PC.  The system is a slim desktop PC with the Mega Drive cartridge slot, 3.5" floppy drive(s), headphone jack with volume control, Mega Drive controller ports and two PS/2 ports at the front panel. At the rear are a 9-pin RS232 serial port, DB25 parallel port, RGB out, composite video out with stereo audio, a Mega Drive EXT port and space for a single ISA expansion card. The TeraDrive was available in three configurations; the Model 1 featured 640 KB of RAM and a single floppy drive, the Model 2 with 1 MB RAM and two floppy drives, and the Model 3 with 2.5 MB RAM along with a single floppy drive and a 30 MB hard drive. Because of the TeraDrive's large form factor and lack of a Mega Drive expansion port, it is not compatible with a Mega CD unit.  


However, both the Mega Drive and PC-compatibles were not popular in Japan during this time, not helped by the fact that the TeraDrive had a fairly weak 286 CPU while most PCs had moved on to 386 or 486 processors. As a result, the TeraDrive sold very poorly and it has since become a very rare collector's item.
=== VA6 (1991-1992, Asia, Brazil, Japan and North America) ===
{|style="margin: 0 auto;"
| [[Image:USVA6.jpg|none|thumb|350px|A VA6 board as found in a Genesis.]]
| [[Image:JPMegaDriveVA6.jpg|thumb|upright|290px|A VA6 board from a Japanese Mega Drive. It is completely identical to the VA5 board above but with a different I/O controller.]]
|}


=== Wondermega RG-M1 (1992) ===
Completely identical to the VA5 but substitutes the I/O chip with the 315-5433, which adds the [https://segaretro.org/TradeMark_Security_System '''T'''rade'''M'''ark '''S'''ecurity '''S'''ystem] (TMSS) lockout screen ("Produced by or Under License by Sega Enterprises, Ltd") that appears for several seconds before playing any game. TMSS checks for the word "SEGA" at two locations in the game code, and if it is not at either one of these memory locations, the game will not run.  
Perhaps the most exorbitant of all Mega Drive consoles, the '''Wondermega''' (model number RG-M1) is the first console to combine both a Mega Drive and a Mega CD in a single unit. A Japan-only release, the Wondermega was released in April 1992 under both the Sega and Victor (JVC) brands, with the Victor model coming first by a few weeks. In addition to playing both Mega Drive and Mega CD games, the Wondermega is loaded with features such as S-video out, MIDI out, two microphone inputs for karaoke, and a DSP for audio effects. To add to the opulence, the top-loading disc door is also motorized and can automatically open or close.  


Both the Sega and Victor models are identical with only very minor cosmetic differences. The Sega model also retailed for a slightly lower price (79,800 yen vs. 82,800 yen) than the Victor model. Due to their very high prices, both Wondermega models sold in limited numbers and are now highly sought after.
In addition, the VA6 was the last board revision in most markets to have the EXT port. Sega had planned to release a modem which used this port, but it ultimately only saw a Japanese release (known there as the Mega Modem), and Sega abandoned their plans to release it elsewhere.


=== Model 2 (1993-1998) ===
The VA6 first appeared in US Genesis consoles that were sold beginning in September 1991<ref>https://law.justia.com/cases/federal/district-courts/FSupp/785/1392/2144879/</ref>, with other regions following later that year. VA6 boards found in later Japanese Mega Drives used the revised 315-5313A VDP. The VA6 did not appear in PAL regions as Sega transitioned directly from the VA4 to the VA6.5 (detailed below) in those markets.


These models introduced a redesigned smaller case, removing the raised circular area and changing the console’s shape to a square. The headphone jack has been removed, although the A/V output now supports stereo. However, the EXT port is completely removed, and audio output on some revisions is noticeably worse compared to the model 1. The RF port and channel switch have also been removed on Western models, as all models now make use of an external RF modulator.
=== VA6.5 (1992-1993, Asia, North America and PAL) ===
[[Image:PAL MD VA6.5.jpg|none|thumb|350px|A VA6.5 board from a PAL Mega Drive.]]
The VA6.5 is simply a VA6 with the EXT port and its line filters removed, since the EXT port was unused in most markets. No other changes were made, though consoles with this board have a large blank space on the rear where the EXT port once was. The "VA6.5" name originates from Sega service documentation and is not denoted on the board itself. Many of these used the 315-5313A or 315-5313A-01 VDP.


''All model 2 revisions can be distinguished by viewing the motherboard through the cartridge slot. See the "Sega-16" guide linked below for details.''
=== VA6.8 (1993, PAL) ===
[[Image:Model1VA6.8.jpg|none|thumb|350px|A VA6.8 motherboard. Note the video encoder area, located directly below the RF modulator.]]


==== VA0/1/1.8 full-size motherboard (1993-1995) ====
Only found in European Mega Drives, the VA6.8 revision is largely identical to the VA6 but has a surface-mount Fujitsu MB3514 video encoder in place of the DIP Sony CXA1145P found in all other models. No other changes were made.


These are for the most part, very similar to the VA7 model 1. The sound quality still suffers though is not as bad as the VA7, leaving only the incessant hissing. Alas, the lack of a volume slider (since the headphone jack was removed) eliminates any ability to change the volume of the hiss. At times, the video output may also be very poor if the console uses the Samsung KA2195D video encoder; this is something that can only be determined by opening the console.
=== VA7 (1992-1993, Brazil, Japan, North America and Taiwan) ===
{|style="margin: 0 auto;"
| [[Image:USVA7Board.jpg|none|thumb|350px|A VA7 board as found in a Genesis.]]
| [[Image:Taiwaneseva7.jpeg|thumb|upright|350px|A VA7 board from a Mega Drive sold in Taiwan. It uses the same board layout as the Japanese VA7, but the EXT port is not populated.]]
|}


The VA1 differs from the VA0 mainly in that it consolidates the two VRAM chips and two 68K RAM chips with one chip each - and there are major trace routing changes to accommodate this. However, there are also some resistor values changed in the audio circuit that likely reduce distortion in these later revisions: R42 through R45 are increased from 4.7 kohm to 7.5 kohm, while R81 is decreased from 7.5k to 6.8k.
Introduced in mid-1992, the VA7 was the most drastic revision of the model 1 released. It is the first standalone Genesis/Mega Drive to use the Yamaha FC1004 ASIC found in most model 2 consoles, which combines the 315-5313 VDP, 315-5433 I/O controller, and a modified Yamaha YM3438 audio chip into a single chip. Specifically, the VA7 uses the 315-5487 variant, which first debuted in the Japan-only Wondermega console earlier that year. All VA7s use a surface-mount version of the Sony CXA1145 encoder, a PLCC version of the Motorola 68000 CPU, and only have a single 7805 voltage regulator as opposed to two like on previous board revisions. Both the main RAM and Z80 RAM were also changed to SOP type surface mount chips. These changes also allowed for all of the SMD passive components (resistors, capacitors, etc.) to be moved to the top side of the board.


It is unclear what exactly is different between the VA1 and VA1.8, as the boards appear mostly identical with the exact same components and no documentation of this difference has been found.
On non-Japanese consoles, the ports on the back have been shifted significantly towards the center, having the AV out occupy where the EXT port used to be (except on Japanese VA7s which still have the EXT port present), which leaves a large gap to the right. This was also the first Western revision to have the power jack on the motherboard itself, as the revamped board layout allowed for having both the RF modulator and power jack on the same PCB. VA7s sold in Taiwan use the same board layout as the Japanese VA7, but the EXT port and its line filters are unpopulated, leaving a large blank spot on the rear panel similar to the VA6.5. Taiwan seems to have been the only Asian country outside of Japan where the VA7 was sold.


All three of these mainboard revisions come with or without HAL (Hot-Air Solder Leveling), although the "HAL" text is not always present on the boards which have it.
The sound output is notoriously bad when compared to other revisions; it is muffled, filled with hiss, and possesses a flat frequency range with excessive distortion. This is due to Sega using an inadequate amplifier circuit and can be corrected using a modification board such as the Triple Bypass or Mega Amp. Some of the components in the audio circuitry [[Genesis:Audio_Circuit_Mod_(Model_2)|can also be swapped with different ones to improve the audio quality]] without these boards. Despite this, video quality is slightly better than previous revisions, with only some minor jailbars on the blue output and the near elimination of the "rainbow banding" on composite.


All three of these mainboard revisions also come with or without plated through-holes, and the layout is slightly different between these variations. The non-plated variations have extra vias. Additionally, the non-plated variations have jumper links in place of some power traces.
Although it is possible to region mod these as with any other Genesis/Mega Drive console, switching a VA7 to 50 Hz mode will cause all audio to stop working for many games, and some games will even crash. This is likely due to the use of the 315-5487 ASIC, which was only used in NTSC consoles and has a broken 50 Hz mode. Later versions of this ASIC such as the 315-5660 fixed this problem, but by that time, Sega had discontinued the model 1 in favor of the cost-reduced model 2.


In Japan, the Mega Drive 2 only features either the VA0 or VA1 revision; later board revisions did not appear in Japan due to the Mega Drive's unpopularity there.
== Model 2 ==


[[File:MD2 VA0 USA.jpg|thumb|left|Model 2 VA0 Motherbaord]]
=== VA0 (1993, All Regions) ===
<br clear=all>
{|style="margin: 0 auto;"
| [[Image:JFT-MD2-VA0USA-Top-20240121.png|thumb|upright|350px|US VA0 board revision.]]
| [[Image:JPMegaDrive2VA0.jpg|thumb|upright|350px|JP VA0 board revision.]]
|}


==== VA2/2.3 full-size motherboard (1994-1995) ====
The first board revision for the model 2, a smaller and greatly cost-reduced redesign of the Genesis/Mega Drive. It is very similar to the VA7 model 1 with many of the same components, though the Z80 CPU was changed to a much smaller QFP (quad flat pack) package in order to save space.


Sound-wise, these are noticeably better than the 7/0/1/1.8 revisions, but it is still not perfect. Hissing is removed, but volume balancing, PSG sound, Master System, CD, and 32X sound are all faulty in some way. However, the likelihood of getting the bad Samsung video encoder chip is higher, although some of these will have a Sony CXA1645 instead, which is bright, sharp, and vivid. These are also the only revisions of the model 2 which have a discrete YM2612 like most model 1s, as the ASIC used in these revisions does not have a YM3438 core.
Early NTSC VA0 boards made use of the 315-5487 ASIC from the VA7, which has a broken 50 Hz mode and so this particular chip was not used in the PAL Mega Drive II. Sega would replace it with the 315-5660 early in the model 2's life, and this is the ASIC found in the vast majority of these consoles. The 315-5708 variant was also used, which was a Fujitsu-manufactured version of the 5660. For some unknown reason, Japanese and Asian Mega Drive 2 consoles use a slide switch for power like the model 1 instead of a push button switch, and lack a power LED.


This board revision is only found in US Genesis consoles.
In terms of sound quality, the VA0 is similar to the VA7, though the PSG volume is reduced and so does not distort as much. The FM output is still very distorted and has some hissing.  


''For the remaining Model 2 revisions, the motherboard’s size has been reduced by a quarter lengthwise. To determine the difference, look at the bottom of a model 2. If the metal plates visible through the exhaust vents end a significant distance from the edge of the console, it is a ¾ motherboard. If the metal plates practically touch the edge of the console, it is a full size.''
RGB output is also similar to the VA7, again with only some minor jailbars on the blue output. However, composite video can differ based on what video encoder is used. Most North American consoles with this revision used the Samsung KA2195D, which has fairly blurry video output with noticeable jailbars. Most Japanese and some North American systems used the Sony CXA1145M, which was previously used in the VA7 and has less artifacts than the KA2195D. All PAL Mega Drive IIs with this revision and the later VA1 and 1.8 boards had the Fujitsu MB3514 encoder, which is similar in output to the CXA1145. A few Japanese Mega Drive 2s also used this particular encoder.


==== VA3 ¾ motherboard (1994-1996) ====
All VA0s come with or without HAL (Hot-Air Solder Leveling), although the "HAL" text is not always present on the boards which have it. In addition, it was also manufactured with or without plated through-holes, and the layout is slightly different between these variations. The non-plated variations have extra vias and also have jumper links in place of some power traces.


These are about as hit and miss as the full-size motherboards. Identifying these can be done by looking for square cutouts on the metal plates (thru the vents). If the console has no square cutouts, it is of this revision. Depending on the encoder chip, video quality of these can range from utterly poor to acceptable. If this revision has a Sony CXA1645 or a Samsung KA2195D, the video output will likely be mediocre to horrible (although the Samsung isn’t the worst of the two). Luckily, sound is universally improved on all ¾ revisions, resulting in sound quality close to the first Model 1s (although with minor PSG difficulties). These will have compatibility issues with the MK-84000 model of the 32X.
This and the following two board revisions are among the most difficult to region mod as the two pins on the ASIC responsible for region and video frequency are hardwired to either 5V or GND depending on the console's region. Region modding one of these will involve either ''very'' careful trace cutting or lifting these delicate ASIC pins.


Like the VA2 and VA2.3 consoles, this board revision is only found in US Genesis consoles.
Early Brazilian Mega Drive III consoles have a VA0 board but with a small RF modulator board crudely soldered onto the side of the board, with the RF output jack sticking out of the side of the console. In later versions, TecToy used a board revision of their own design, which was heavily based on the VA1/VA1.8. This revision integrated the RF out on the motherboard (with the output jack now at the rear) and featured an internal power supply with switchable input voltage.


==== VA4 ¾ motherboard (1996-1998) ====
=== VA1 (1993-1994, All Regions) ===
{|style="margin: 0 auto;"
| [[Image:USModel2VA1.jpg|thumb|upright|350px|US VA1 board revision.]]
| [[Image:JP Mega Drive 2 VA1 Board.jpg|thumb|upright|350px|JP VA1 board revision.]]
|}


Appears to be the best model 2 revision for output quality. All video issues have been eliminated, as the Sony CXA1645 chip in these is back to being crisp, colorful, and vivid. Furthermore, the sound has been improved a little more, eliminating further DAC noise than on the VA3. To identify one of these, look for two square cutouts on the metal plates (through the vents). These will also have compatibility issues with the MK-84000 model of the 32X.
The VA1 differs from the VA0 mainly in that it consolidates the two VRAM chips and two 68K RAM chips with one chip each - and there are major trace routing changes to accommodate this. Some resistor values were also changed in the audio circuit in an attempt to reduce distortion: R42 through R45 are increased from 4.7 kohm to 7.5 kohm, while R81 is decreased from 7.5k to 6.8k. Like the VA0, some VA1s had plated through-holes while others did not. Video encoders were also the same as before.


Later VA4s in North America were sold under license by Majesco in 1998, who marketed the Genesis as a budget console. The build quality is largely identical to the earlier Sega-built examples. This was also the final board revision for the PAL Mega Drive II. PAL VA4 boards also feature a ROHM BH7236AF encoder, which is pin-compatible with the CXA1645.
This was also the final board revision for Japan.


=== Amstrad Mega PC (1993) ===
=== VA1.8 (1994-1996, Asia, North America and PAL regions) ===
Much like the TeraDrive before it, the '''Amstrad Mega PC''' is a combination between an IBM-compatible PC and a Mega Drive. It was only released in Europe and Australia during 1993 and also shipped with a dual 15/31 kHz monitor. Unlike the TeraDrive, the Mega PC featured a 386 processor and had the Mega Drive hardware on a separate ISA card (this was integrated in the TeraDrive's motherboard). Also unlike the TeraDrive, the Mega Drive portion could not interact in any way with the PC side, and a sliding door on the front covers the Mega Drive cartridge slot or 3.5" floppy drive depending on which section is used.
[[Image:JFT-MD2-VA1.8USA(CMK-NPTH)-Top-20240121.png|thumb|none|350px|US VA1.8 board revision.]]


Although it saw somewhat greater success than the TeraDrive, the Mega PC still sold in fairly small quantities, possibly due to its high price and the fact that it used a 386 while most PCs were using 486 CPUs. Amstrad announced a revised model with a 486 called the "Mega PC Plus" to rectify this, but it was never released.
It is unclear what exactly is different between the VA1 and VA1.8, as the boards appear mostly identical with the exact same components and no documentation of this difference has been found. Some VA1.8 boards have two wires between the positive and negative pins of the power jack and two solder pads on the underside which are not present on earlier boards. In addition, some US VA1.8s had the Fujitsu MB3514 video encoder, making them the only standalone US Genesis consoles to use this particular encoder.


=== Wondermega RG-M2/X'Eye (1993-1994) ===
=== VA2 (1994, North America) ===
In 1993, Victor/JVC introduced a completely redesigned Wondermega, known as the '''Wondermega M2''' (model number RG-M2). This new Wondermega featured a smaller, sleeker case and came packaged with a wireless infrared 6-button controller which could also double as a basic TV remote. This controller could also turn on the console remotely and had a controller port for a second player, not unlike 3DO controllers. However, a number of cost-cutting changes were made, including the removal of the DSP audio effects, the motorized disc door and MIDI out. The Wondermega M2 could also still use wired controllers, although the controller ports were moved to the rear of the console to leave room for the infrared receiver at the front.  
[[Image:Model2VA2.jpg|thumb|none|350px|A VA2 motherboard. Note the bodge wires and extra PCB.]]


Much like its closest competitor, the PC Engine Duo, the Wondermega M2 also suffers from leaky surface-mount capacitors and any M2 unit should be fully recapped in order to keep it functioning.
Introduced during 1994, the VA2 is the first of two model 2 revisions which use a discrete YM2612 FM chip like most model 1s, as the Toshiba-manufactured ASIC (Sega 315-5786) used in this model does not have a YM3438 core. Sound-wise, it is somewhat better than the previous model 2 revisions, but it is still not perfect. The hissing was removed, but the volume balance was altered with the FM channels being considerably louder than the PSG and external audio from both the cartridge and expansion ports. Sega used the 315-5684 audio amplifier chip as opposed to the dual LM324-based setup used in previous model 2 revisions. Some consoles make use of the Samsung KA2195D video encoder while others have the Sony CXA1645, an improved version of the CXA1145 used in previous consoles. Largely due to the board layout, the RGB output has very noticeable jailbars which cannot be totally eliminated by a Triple Bypass mod.


During the spring of 1994, JVC brought over the Wondermega M2 to North America as the '''JVC X'Eye''', which retailed for US$499.95 and came bundled with a variety of CD software, including ''Compton's Interactive Encyclopedia'', ''Prize Fighter'', and a karaoke CD. Further cost reductions were made with the X'Eye, with both the S-video output and infrared receiver being removed, as well as one of the microphone inputs and the volume control for the headphone out. The controller ports were relocated to the front of the console to compensate for the removed IR receiver, and the power button was also changed from the momentary type used in the M2 to a push button switch like in a model 2 Genesis. A JVC-branded version of the standard Genesis 3-button controller was also bundled with the console.
The VA2 can also be identified by the presence of a small "bodge" PCB with a 74HC14 [https://en.wikipedia.org/wiki/Schmitt_trigger Schmitt trigger]. This fixes an issue with the /ASEL signal on pin B26 of the cartridge port, used by ''Virtua Racing'' and the 32X. In addition, there is a shielded wire which routes the clock signal from the ASIC to the CPU, tacked down by either tape (VA2s made in China) or hot glue (VA2s made in Indonesia). These bodges are also visible with the cartridge flaps opened, making this easy to tell apart from other revisions.


Most X'Eyes also feature the same 9-pin mini-DIN connector found in the model 2 Genesis, meaning that a 32X can be connected to these X'Eyes. However, the 32X blocks the disc door, so it must be removed every time the disc door is opened. For 32X owners, JVC offered a service [https://www.sega-16.com/forum/showthread.php?26616-JVC-X-Eye-model-variation where the console was given a modified disc door], allowing it to fully open with a connected 32X. This service was [https://web-archive-org.translate.goog/web/20141104171705/http://www5.plala.or.jp/star-route13/sub8.html?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en also made available to Wondermega M2 owners in Japan] for a 5,000 yen fee until 2013.
Region modding one of these boards is as easy as a model 1, as Sega added jumper traces for both region and video frequency instead of directly wiring the corresponding ASIC pins to 5V.


=== Mega Jet (1993-1994) ===
This revision was only used in North American Genesis consoles.


Perhaps one of the most novel versions of the Genesis/Mega Drive hardware, the Mega Jet is a semi-portable console initially made available in 1993 on certain Japan Air Lines flights and on March 10, 1994 to the general public in Japan only. It is essentially a VA3 revision model 2 condensed into a handheld form with a built-in 6-button controller, although it lacks a built-in screen or the ability to accept batteries, basically making it akin to a "plug & play" controller with a cartridge slot. Curiously, the Mega Jet has a fully wired cartridge port, allowing it to support Master System games and passthrough audio from a flash cart such as a Mega Everdrive Pro or Mega SD.
=== VA2.3 (1995-1996, North America) ===
[[Image:Model2VA2.3.JPG|thumb|none|350px|A VA2.3 motherboard with the 315-5786 ASIC.]]


The Mega Jet's hardware would later form the basis for the truly portable Genesis Nomad.
During 1995, Sega introduced a revised version of the VA2 known as the VA2.3. Much of the board layout was changed on this revision, with many major components being shuffled around and the Schmitt trigger circuit now being integrated into the board. As a result of the changed layout, the RGB output from this board is much cleaner than the VA2 and is similar to previous model 2 revisions. The different layout also eliminated the separate clock signal wire for the 68000 since it was positioned much closer to the main ASIC.


=== CDX/Multi-Mega (1994) ===
Sega used two different ASICs made by Toshiba in the VA2.3, the 315-5685 (TC6158AF) and 315-5786 (T9N13BF), with the older 5685 oddly being used in later-built VA2.3s. The 5685 has a flawed VDP core with a broken shadow and highlights mode and glitchy raster effects. These issues were fixed in the 5786, which had been used in all VA2s and is found in earlier VA2.3s. More information regarding the issues on the 315-5685 can be found on [[Genesis:315-5685 Information|this page]].


Released in 1994, the '''Sega Genesis CDX''' is a small console combining the Sega CD and Genesis/Mega Drive together, while also functioning as a portable CD player. Only some thousands of these were made, resulting in significant rarity. Video quality of these systems is quite good, but the sound is a bit mixed (too quiet, weak bass without headphones). FMV-heavy Sega CD games may freeze up on this system. It is compatible with the 32X and Power Base Converter despite its different physical form compared to standalone Genesis/Mega Drive consoles, and also features the same 9-pin mini-DIN video out as the model 2 with RGB output.
VA2.3s manufactured in China (serial numbers beginning with "162" or "752") had the CXA1645 encoder while those manufactured in Malaysia (serial numbers beginning with "452") or Thailand (serial numbers beginning with "252") had the KA2195D.


Unlike the JVC/Victor combination consoles, the CDX was also released in PAL regions, Asia and Brazil, where it was known as the '''Multi-Mega'''. Curiously, the CDX was not released in Japan by Sega themselves, but by the global language training company Linguaphone, who sold it as an educational tool and branded it as the '''Education Gear''' .
As before, the VA2.3 is only found in North American Genesis consoles. Most VA2.3 consoles can be identified by their different model number, MK-1631'''A'''; consoles with this model number also have the 315-5685 chip.


=== Aiwa Mega CD (1994) ===
=== VA3 (1994-1996, North America) ===
One of the most unusual console variants ever released for any system, the Aiwa Mega CD (model number CSD-G1M) is a combination of a Mega Drive, Mega CD and boombox complete with CD player, cassette deck and radio tuner. Both the boombox and Mega Drive/Mega CD are separate components, presumably so that the boombox can be taken on the go to play music while the game hardware remains behind at home; the two are connected via a DB25 cable. The front of the "dock" has the cartridge port at the center with the two controller ports on each side of it. The rear features a DB25 connector for the docking cable and the same 9-pin mini-DIN found on the Mega Drive 2. Power is provided via the boombox's power supply. Although not designed for them, the CSD-G1M is compatible with the 32X and even Master System adapters, although care must be taken due to the cartridge port being mounted at the front.
[[File:Model2VA3.jpg|thumb|none|350px|A VA3 motherboard as found in a model 2 Genesis.]]


The Aiwa Mega CD did not sell very well and remains a sought-after collector's item, mostly due to its rarity but also for the sheer novelty of having a Mega Drive with a boombox.
The first of the short board revisions. It uses a variant of the FC1004 ASIC as used in the VA0 through 1.8, but has a simplified audio section based on the 315-5684 amplifier chip instead of two separate LM324 op-amp chips. This in addition to some layout changes allowed Sega to reduce the size of the motherboard, cutting down production costs in the process.


=== Genesis Nomad (1995) ===
Identifying these can be done by looking at the RF shielding through the bottom vents. If the shielding doesn't extend all the way across the width of the console, and does not have rectangular holes, it will be this revision. Sega used three different video encoders on the VA3, the CXA1145M, CXA1645 and the KA2195D. The encoder used can be roughly determined by the serial number; if it starts with "163" or "253", it is almost guaranteed to have a Samsung encoder. Serials starting with "743", "753" or "R43" are extremely likely to have a Sony encoder, with the latter two being more likely to have a CXA1645. Sound is universally improved on the VA3, but the PSG volume is reduced and the frequency response is lower at higher frequencies.
A natural evolution of the Mega Jet, the '''Genesis Nomad''' (though usually referred to as simply the "Nomad") was released in late 1995 as the first handheld game system since the NEC TurboExpress to play the same games as a home console. Designed by both Sega of Japan and Sega of America, the Nomad is compatible with the vast majority of the Genesis library, although it does not support SMS, 32X or Sega CD games due to not having an expansion port nor the required signals present on the cartridge port. The Nomad is a very bulky system, made even bulkier by the fact that it does not have an internal battery compartment; batteries are inserted into a separate pack which clips onto the back of the Nomad, making it very unwieldy to carry around.  


Typical for color handhelds of the time, the Nomad's battery life is fairly poor as its six AA batteries can be depleted in just 2-4 hours. Screen modifications exist which replace the original fluorescent-backlit screen with a more modern panel, greatly improving both battery life and visual clarity. Like the Mega Jet, the Nomad can be connected to a TV, featuring the same mini-DIN connector as the model 2 Genesis and a controller port for a second player. However, the built-in screen is still active so use of an AC adapter is recommended in this scenario. Modifications can also be performed to restore SMS backwards compatibility and external audio for flash carts such as the Mega Everdrive Pro and Mega SD, allowing the Nomad to play Sega CD games. 32X support is greatly hampered by the fact that the 32X blocks the AV out.
Most VA3s use the 315-5660 ASIC, but a few made use of the 315-5700 (Yamaha FF1004) which was also used in the Nomad handheld. As with the VA0 through VA1.8 boards, the ASIC pins for selecting region and video frequency are hardwired to 5V. However, region modding a VA3 is somewhat easier as Sega added an exposed pad for each of these pins, meaning a simple trace cut is all that is needed to isolate these pins.


All Nomad systems were built in Taiwan and do not have any revisions unlike most Sega hardware.
Like the VA2 and VA2.3 consoles, this board revision is only found in US Genesis consoles.


Although it received some praise from critics, the Nomad was a slow seller due to its high price and the fact that Sega did not do much to market the system, as they were transitioning from the Genesis to the new 32-bit Saturn.
=== VA4 (1996-1998, Asia, Brazil, Korea, North America and PAL) ===
{|style="margin: 0 auto;"
| [[Image:JFT-MD2-VA4USA-Top-20240121.png|thumb|upright|350px|US VA4 board revision.]]
| [[Image:JFT-MD2-VA4USA-Bottom-20240121.png|thumb|upright|350px|Underside of a US VA4.]]
|}


=== Genesis 3 (1998-1999) ===
This was the final board revision for the model 2. It is especially notable in that it is the first Genesis console to use a GOAC (Genesis On A Chip) ASIC, specifically the Sega 315-5960 (Yamaha FJ3002). This incorporates all of the major components of the console (68000, Z80, VDP, YM3438, I/O controller, Z80 RAM) into a single chip, save for the main RAM, video RAM, video encoder and audio amplifier. Due to this change, the VA4 also consumes far less power than any other model 2. This is reflected by the position of the 7805 voltage regulator, which is now mounted horizontally on the motherboard and partially uses the top RF shield as a heat sink.


Only released in North America. Less commonly seen than the prior two versions, this variant was actually released by a licensee, Majesco, instead of Sega themselves (who designed it). These are easily identified by their much smaller size, and of course the large “3” on the top. Many features have been removed, such as the headphone port, EXT port, expansion port (meaning no Sega CD compatibility, power LED, and even stereo sound. Furthermore, certain games such as Gargoyles have compatibility issues with these consoles (revision dependent) due to the removal of a memory addressing bug. Other games as well as the 32X and Power Base Converter are incompatible due to the cartridge port not being fully connected as with earlier consoles.
To identify one of these, look for two square cutouts on the shielding visible through the bottom vents. These will also have compatibility issues with both revisions of the 32X, which can be bypassed by loading 32X games from a flash cart instead of using original cartridges.


==== VA1 (1998) ====
All US VA4s use the Sony CXA1645 encoder, which provides slightly improved composite video with less artifacts than its implementation on the VA3. RGB output has some noticeable jailbars. Most Asian and PAL VA4 boards instead feature a ROHM BH7236AF encoder, which is pin-compatible with the CXA1645. Curiously, Sega added the footprints for an internal RF modulator and its support circuitry, though no VA4 ever had these populated.


Essentially a highly condensed VA4 model 2 without an expansion port. All compatibility issues (except Sega CD and TAS instruction issues) can be fixed on this variant. Virtua Racing will boot on this one, while it will stay at a black screen on the VA2. Without a copy of Virtua Racing, it can be hard to determine the revision; the cartridge slot must be opened, and the traces need to be inspected. It appears that the VA1 has a vertical trace on the far left, which cannot be found on the VA2. Lastly, the video output is known to be extremely high quality and still has RGB output. The sound is also high quality, although it is monophonic with a slight hiss.
Both region and video frequency are set by 0 ohm jumpers for all VA4s. Region is selected by R31 (JP) or R32 (EN), and video frequency is controlled by R33 (50 Hz) or R34 (60 Hz).  


==== VA2 (1998-1999) ====
Later VA4s in North America were sold under license by Majesco in 1998, who marketed the Genesis as a budget console. The build quality is largely identical to the earlier Sega-built examples.


Generally inferior to the VA1, as almost all of the compatibility issues cannot be fixed due to the VA2 using a different ASIC. In particular, 32X and Master System compatibility cannot be restored on this revision; only the Game Genie and Virtua Racing problems are fixable on the VA2. Video output quality was not changed, but the audio is a little better in low frequencies, and does not hiss.
== Genesis 3 ==


== References ==
=== VA1 (1998) ===
[[File:Genesis3MotherboardVA1.jpg|thumb|none|350px|VA1 motherboard]]


https://segaretro.org/Sega_Multi-Mega (CDX discontinue date info)
Essentially a highly condensed VA4 model 2 without an expansion port. Although it is not compatible with the 32X or Master System games, it can be modified for these due to it using the same 315-5960 GOAC as the VA4 model 2. One other major change is that stereo sound was omitted, and all audio goes through a single LM324 op-amp. The Sony CXA1645 video encoder was also carried over, providing decent quality composite video output and RGB.


https://en.wikipedia.org/wiki/Sega_Genesis
=== VA2 (1998-1999) ===
[[File:Genesis3MotherboardRevisions.jpg|thumb|none|350px|VA2 motherboard]]


https://www.retrorgb.com/genesisversions.html
The VA2 uses a different GOAC, the 315-6123 (Yamaha FQ8002). This board also combined both the main RAM and video RAM into a single 128 KB SDRAM chip, leaving it, the video encoder and audio amplifier as the only remaining discrete ICs. 32X and Master System compatibility cannot be restored on this revision; only Game Genie and Virtua Racing compatibility issues are fixable on the VA2. The TAS instruction in the 68000 CPU was also fixed, which breaks compatibility with the game ''Gargoyles''. Video output quality was not changed, but the audio is a little better in low frequencies, and does not hiss.


https://gametrog.com/sega-cdx-information-specs/
== References ==


https://segaretro.org/Sega_Mega_Drive/Hardware_revisions (board pics)
[[Category:Genesis]]
 
https://gametrog.com/sega-genesis-1-2-3-information-specs/
 
https://retroconsoles.fandom.com/wiki/Sega_Mega_Drive/Genesis
 
https://www.sega-16.com/forum/showthread.php?27397-VA2-Preamp-fix
 
https://www.sega-16.com/forum/showthread.php?22892-Sega-Genesis-Mega-Drive-Manufacture-Date-Registry (other manuf dates)
 
https://www.sega-16.com/forum/showthread.php?7796-GUIDE-Telling-apart-good-Genesis-1s-and-Genesis-2s-from-bad-ones (invaluable info)
 
https://medium.com/rgb-inside/comparing-native-rgb-video-quality-of-different-models-of-mega-drive-and-sega-genesis-english-e3d3703270ba (video output info + other variants)
 
https://www.reddit.com/r/SEGAGENESIS/comments/j6qg7k/sega_genesis_models_1_2_3_sound_question/
 
https://www.reddit.com/r/SEGAGENESIS/comments/biueke/gensis_va2mega_drive_va02_preamp_fix/

Revision as of 07:10, 18 April 2024

There were numerous motherboard revisions of the standalone Genesis and Mega Drive consoles. This page serves to document these board revisions and the technical information for each.

Model 1

837-6656 - "VA0" (1988, Japan)

MDVA0.png

This is the earliest revision of the model 1 and is only found in Japanese Mega Drive consoles. These contain a small daughterboard (part number 839-0231) which has the master oscillator crystal, a 74LS74 D-type flip flop and two 74161 (one 74LS161 and one 74F161) 4-bit counter chips. During manufacturing, Sega discovered that the EDCLK (external dot clock) signal did not work properly in the H40 video mode. Using the master clock frequency and the horizontal sync (HSYNC) signal, the daughterboard generates a proper EDCLK signal which is fed back into the board. Sega also included a discrete 74LS00 logic gate on the motherboard to fix RAM compatibility issues with Master System games.[1]

Although the board is not designated as such, it is commonly referred to as the "VA0" in keeping with Sega's revision naming practices and the fact that it was the first board revision produced.

The VA0 revision also has a significant flaw with its stereo audio output. While the balance between frequencies is excellent, the preamp is far too high, resulting in distortion in games that are too loud. However, this can be fixed by substituting several capacitor and resistor values on the underside of the motherboard. Video quality is also compromised as composite video is afflicted with "rainbow banding" and the RGB output has noticeable jailbars. These would be carried over into later model 1 revisions.

Sega only produced the VA0 for a fairly short period, as it was quickly replaced by the VA1 (detailed below) in order to cut production costs and presumably to reduce the amount of manual labor involved during assembly, since the daughterboard and its connecting wires had to be soldered in by hand.

VA1 (1989, Japan)

JP MDVA1 Recapped2.jpg

Introduced very early in 1989, the VA1 incorporated the EDCLK daughterboard and discrete 74LS00 logic gate into an additional gate array chip, the 315-5339. Like the "VA0" before it, this board is only found in Japanese Mega Drives. It also has the same issue with the overdriven preamp over the headphone output.

VA2 (1989, Japan and North America)

A VA2 board from a Japanese Mega Drive.
A VA2 board from a model 1 Genesis. Note how the audio and video circuitry have been shifted down to make room for the RF modulator.

This is the first board revision for the Genesis and is also found in Japanese Mega Drive consoles. On this revision, the 315-5339 EDCLK generator chip was replaced with the 315-5345, which added an additional bus acknowledgment signal (/BGACK) going to the VDP. It uses this signal to prevent potential glitches related to DMA (direct memory access) refresh timing.[2] No other major changes were made.

Sega made some changes to the board layout for North American models in order to accommodate an internal RF modulator and RF shielding, neither of which the Japanese Mega Drive ever had. The retention of the EXT port also meant that the power input jack had to be moved onto a separate PCB, connected to the motherboard by two wires. These layout differences between Japanese and Western consoles would be carried on into all later model 1 revisions until the VA7.

VA3 (1989-1991, Brazil and North America)

A VA3 board from a model 1 Genesis.

Introduced in late 1989, the VA3 revision was only used in North American and Brazilian consoles (being the first revision for the latter). The 315-5308 bus arbiter and 315-5345 EDCLK generator were consolidated into a single IC, the 315-5364, likely to reduce production cost. A 4.7k ohm pull-up resistor array was also added on the Z80's data bus pins, though it was soldered directly onto the Z80 itself and not placed on the board. The audio circuitry was changed as the overdriven preamp issue from previous revisions was fixed and some low pass filtering was added to the output stage, which slightly muffled the audio. Despite this, the audio quality is generally regarded as being excellent and this variant is often used as a baseline for audio quality in Genesis consoles. Its audio profile is also the basis for the audio portion of the Triple Bypass modification board.

For Brazilian VA3s, some changes were made in order for them to output PAL-M video. A 53.634 MHz master oscillator was fitted and pin 7 of the video encoder was directly connected to GND to force it into PAL mode; this pin is normally controlled by JP3 and JP4, which also set the VDP's video frequency.[3]

VA4 (1989-1992, Asia, Japan, Korea and PAL)

A VA4 board from a PAL Mega Drive. Note that this board has been slightly modified but is largely identical to a stock configuration.

The VA4 is largely identical to the VA3, and was first introduced in late 1989 in Japanese Mega Drive consoles. It was also the first board revision for Asian, PAL and Korean consoles. Asian boards use the same board layout as the PAL models, and have an internal RF modulator. The video encoder was flipped around with the front of the chip pointing towards the front of the console. In addition, the resistor array on the Z80's data bus was given a footprint on the board and was not soldered directly onto the Z80 as before. These are the only significant differences between the VA3 and VA4.

PAL VA4 boards made in 1992 (such as the one pictured above) use a die-shrunk[4] version of the VDP, the 315-5313A (Yamaha FC1001).

SPC-200/201R Rev. A (1992-1993, Korea)

The VA4 variant from a Samsung Super Aladdin Boy.

By 1992, Samsung began manufacturing their own motherboards for the Super Gam-Boy/Super Aladdin Boy, which was the version of the Mega Drive sold in Korea. This board is heavily based on the VA4 and uses the same layout, but does not have the EXT port populated.

VA5 (1990-1991, Japan)

A VA5 board from a Japanese Mega Drive.

Introduced in late 1990, this board design saw further consolidation by combining the 315-5309 I/O chip and 315-5364 bus arbiter into a single ASIC, the 315-5402. Composite video output was moderately improved with the reduction of the "rainbow banding" common on earlier revisions (though it is still present), and the excellent audio quality of the VA3 and VA4 boards was carried over. This particular board revision was only used in Japanese Mega Drives. The region and video frequency jumpers were relocated from the lower center to a new position closer to the expansion port.

One interesting quirk with this board revision is the presence of three jumper wires which are sometimes mistaken as an aftermarket modification. These jumper wires connect the CE (Chip Enable) and OE (Output Enable) signals of the two 68000 RAM chips to the main system bus; JP5 connects the CE signal of both chips, JP6 the OE signal of IC2, and JP7 the OE signal of IC3. It is unknown why these signals were connected using jumper wires and not properly routed on the board itself as on previous revisions. The jumper wires are also present on the VA6 and its variants (detailed below).

Two different versions of the 315-5402 I/O chip exist, one which completely lacks TMSS, and another which will display the TMSS screen only if the system's region setting is changed from Japanese to English/export, which is accomplished by cutting the trace on jumper JP1 and connecting JP2 in its place.

This was the last board revision where Sega used 68000 CPUs made by Signetics, who had been Sega's sole supplier of this component for the Mega Drive since its launch. During 1991, Sega began sourcing CPUs from Motorola and Hitachi instead, and VA5s manufactured in 1991 (denoted with a "1" as the second digit of the serial number) have Motorola CPUs.

VA6 (1991-1992, Asia, Brazil, Japan and North America)

A VA6 board as found in a Genesis.
A VA6 board from a Japanese Mega Drive. It is completely identical to the VA5 board above but with a different I/O controller.

Completely identical to the VA5 but substitutes the I/O chip with the 315-5433, which adds the TradeMark Security System (TMSS) lockout screen ("Produced by or Under License by Sega Enterprises, Ltd") that appears for several seconds before playing any game. TMSS checks for the word "SEGA" at two locations in the game code, and if it is not at either one of these memory locations, the game will not run.

In addition, the VA6 was the last board revision in most markets to have the EXT port. Sega had planned to release a modem which used this port, but it ultimately only saw a Japanese release (known there as the Mega Modem), and Sega abandoned their plans to release it elsewhere.

The VA6 first appeared in US Genesis consoles that were sold beginning in September 1991[5], with other regions following later that year. VA6 boards found in later Japanese Mega Drives used the revised 315-5313A VDP. The VA6 did not appear in PAL regions as Sega transitioned directly from the VA4 to the VA6.5 (detailed below) in those markets.

VA6.5 (1992-1993, Asia, North America and PAL)

A VA6.5 board from a PAL Mega Drive.

The VA6.5 is simply a VA6 with the EXT port and its line filters removed, since the EXT port was unused in most markets. No other changes were made, though consoles with this board have a large blank space on the rear where the EXT port once was. The "VA6.5" name originates from Sega service documentation and is not denoted on the board itself. Many of these used the 315-5313A or 315-5313A-01 VDP.

VA6.8 (1993, PAL)

A VA6.8 motherboard. Note the video encoder area, located directly below the RF modulator.

Only found in European Mega Drives, the VA6.8 revision is largely identical to the VA6 but has a surface-mount Fujitsu MB3514 video encoder in place of the DIP Sony CXA1145P found in all other models. No other changes were made.

VA7 (1992-1993, Brazil, Japan, North America and Taiwan)

A VA7 board as found in a Genesis.
A VA7 board from a Mega Drive sold in Taiwan. It uses the same board layout as the Japanese VA7, but the EXT port is not populated.

Introduced in mid-1992, the VA7 was the most drastic revision of the model 1 released. It is the first standalone Genesis/Mega Drive to use the Yamaha FC1004 ASIC found in most model 2 consoles, which combines the 315-5313 VDP, 315-5433 I/O controller, and a modified Yamaha YM3438 audio chip into a single chip. Specifically, the VA7 uses the 315-5487 variant, which first debuted in the Japan-only Wondermega console earlier that year. All VA7s use a surface-mount version of the Sony CXA1145 encoder, a PLCC version of the Motorola 68000 CPU, and only have a single 7805 voltage regulator as opposed to two like on previous board revisions. Both the main RAM and Z80 RAM were also changed to SOP type surface mount chips. These changes also allowed for all of the SMD passive components (resistors, capacitors, etc.) to be moved to the top side of the board.

On non-Japanese consoles, the ports on the back have been shifted significantly towards the center, having the AV out occupy where the EXT port used to be (except on Japanese VA7s which still have the EXT port present), which leaves a large gap to the right. This was also the first Western revision to have the power jack on the motherboard itself, as the revamped board layout allowed for having both the RF modulator and power jack on the same PCB. VA7s sold in Taiwan use the same board layout as the Japanese VA7, but the EXT port and its line filters are unpopulated, leaving a large blank spot on the rear panel similar to the VA6.5. Taiwan seems to have been the only Asian country outside of Japan where the VA7 was sold.

The sound output is notoriously bad when compared to other revisions; it is muffled, filled with hiss, and possesses a flat frequency range with excessive distortion. This is due to Sega using an inadequate amplifier circuit and can be corrected using a modification board such as the Triple Bypass or Mega Amp. Some of the components in the audio circuitry can also be swapped with different ones to improve the audio quality without these boards. Despite this, video quality is slightly better than previous revisions, with only some minor jailbars on the blue output and the near elimination of the "rainbow banding" on composite.

Although it is possible to region mod these as with any other Genesis/Mega Drive console, switching a VA7 to 50 Hz mode will cause all audio to stop working for many games, and some games will even crash. This is likely due to the use of the 315-5487 ASIC, which was only used in NTSC consoles and has a broken 50 Hz mode. Later versions of this ASIC such as the 315-5660 fixed this problem, but by that time, Sega had discontinued the model 1 in favor of the cost-reduced model 2.

Model 2

VA0 (1993, All Regions)

US VA0 board revision.
JP VA0 board revision.

The first board revision for the model 2, a smaller and greatly cost-reduced redesign of the Genesis/Mega Drive. It is very similar to the VA7 model 1 with many of the same components, though the Z80 CPU was changed to a much smaller QFP (quad flat pack) package in order to save space.

Early NTSC VA0 boards made use of the 315-5487 ASIC from the VA7, which has a broken 50 Hz mode and so this particular chip was not used in the PAL Mega Drive II. Sega would replace it with the 315-5660 early in the model 2's life, and this is the ASIC found in the vast majority of these consoles. The 315-5708 variant was also used, which was a Fujitsu-manufactured version of the 5660. For some unknown reason, Japanese and Asian Mega Drive 2 consoles use a slide switch for power like the model 1 instead of a push button switch, and lack a power LED.

In terms of sound quality, the VA0 is similar to the VA7, though the PSG volume is reduced and so does not distort as much. The FM output is still very distorted and has some hissing.

RGB output is also similar to the VA7, again with only some minor jailbars on the blue output. However, composite video can differ based on what video encoder is used. Most North American consoles with this revision used the Samsung KA2195D, which has fairly blurry video output with noticeable jailbars. Most Japanese and some North American systems used the Sony CXA1145M, which was previously used in the VA7 and has less artifacts than the KA2195D. All PAL Mega Drive IIs with this revision and the later VA1 and 1.8 boards had the Fujitsu MB3514 encoder, which is similar in output to the CXA1145. A few Japanese Mega Drive 2s also used this particular encoder.

All VA0s come with or without HAL (Hot-Air Solder Leveling), although the "HAL" text is not always present on the boards which have it. In addition, it was also manufactured with or without plated through-holes, and the layout is slightly different between these variations. The non-plated variations have extra vias and also have jumper links in place of some power traces.

This and the following two board revisions are among the most difficult to region mod as the two pins on the ASIC responsible for region and video frequency are hardwired to either 5V or GND depending on the console's region. Region modding one of these will involve either very careful trace cutting or lifting these delicate ASIC pins.

Early Brazilian Mega Drive III consoles have a VA0 board but with a small RF modulator board crudely soldered onto the side of the board, with the RF output jack sticking out of the side of the console. In later versions, TecToy used a board revision of their own design, which was heavily based on the VA1/VA1.8. This revision integrated the RF out on the motherboard (with the output jack now at the rear) and featured an internal power supply with switchable input voltage.

VA1 (1993-1994, All Regions)

US VA1 board revision.
JP VA1 board revision.

The VA1 differs from the VA0 mainly in that it consolidates the two VRAM chips and two 68K RAM chips with one chip each - and there are major trace routing changes to accommodate this. Some resistor values were also changed in the audio circuit in an attempt to reduce distortion: R42 through R45 are increased from 4.7 kohm to 7.5 kohm, while R81 is decreased from 7.5k to 6.8k. Like the VA0, some VA1s had plated through-holes while others did not. Video encoders were also the same as before.

This was also the final board revision for Japan.

VA1.8 (1994-1996, Asia, North America and PAL regions)

US VA1.8 board revision.

It is unclear what exactly is different between the VA1 and VA1.8, as the boards appear mostly identical with the exact same components and no documentation of this difference has been found. Some VA1.8 boards have two wires between the positive and negative pins of the power jack and two solder pads on the underside which are not present on earlier boards. In addition, some US VA1.8s had the Fujitsu MB3514 video encoder, making them the only standalone US Genesis consoles to use this particular encoder.

VA2 (1994, North America)

A VA2 motherboard. Note the bodge wires and extra PCB.

Introduced during 1994, the VA2 is the first of two model 2 revisions which use a discrete YM2612 FM chip like most model 1s, as the Toshiba-manufactured ASIC (Sega 315-5786) used in this model does not have a YM3438 core. Sound-wise, it is somewhat better than the previous model 2 revisions, but it is still not perfect. The hissing was removed, but the volume balance was altered with the FM channels being considerably louder than the PSG and external audio from both the cartridge and expansion ports. Sega used the 315-5684 audio amplifier chip as opposed to the dual LM324-based setup used in previous model 2 revisions. Some consoles make use of the Samsung KA2195D video encoder while others have the Sony CXA1645, an improved version of the CXA1145 used in previous consoles. Largely due to the board layout, the RGB output has very noticeable jailbars which cannot be totally eliminated by a Triple Bypass mod.

The VA2 can also be identified by the presence of a small "bodge" PCB with a 74HC14 Schmitt trigger. This fixes an issue with the /ASEL signal on pin B26 of the cartridge port, used by Virtua Racing and the 32X. In addition, there is a shielded wire which routes the clock signal from the ASIC to the CPU, tacked down by either tape (VA2s made in China) or hot glue (VA2s made in Indonesia). These bodges are also visible with the cartridge flaps opened, making this easy to tell apart from other revisions.

Region modding one of these boards is as easy as a model 1, as Sega added jumper traces for both region and video frequency instead of directly wiring the corresponding ASIC pins to 5V.

This revision was only used in North American Genesis consoles.

VA2.3 (1995-1996, North America)

A VA2.3 motherboard with the 315-5786 ASIC.

During 1995, Sega introduced a revised version of the VA2 known as the VA2.3. Much of the board layout was changed on this revision, with many major components being shuffled around and the Schmitt trigger circuit now being integrated into the board. As a result of the changed layout, the RGB output from this board is much cleaner than the VA2 and is similar to previous model 2 revisions. The different layout also eliminated the separate clock signal wire for the 68000 since it was positioned much closer to the main ASIC.

Sega used two different ASICs made by Toshiba in the VA2.3, the 315-5685 (TC6158AF) and 315-5786 (T9N13BF), with the older 5685 oddly being used in later-built VA2.3s. The 5685 has a flawed VDP core with a broken shadow and highlights mode and glitchy raster effects. These issues were fixed in the 5786, which had been used in all VA2s and is found in earlier VA2.3s. More information regarding the issues on the 315-5685 can be found on this page.

VA2.3s manufactured in China (serial numbers beginning with "162" or "752") had the CXA1645 encoder while those manufactured in Malaysia (serial numbers beginning with "452") or Thailand (serial numbers beginning with "252") had the KA2195D.

As before, the VA2.3 is only found in North American Genesis consoles. Most VA2.3 consoles can be identified by their different model number, MK-1631A; consoles with this model number also have the 315-5685 chip.

VA3 (1994-1996, North America)

A VA3 motherboard as found in a model 2 Genesis.

The first of the short board revisions. It uses a variant of the FC1004 ASIC as used in the VA0 through 1.8, but has a simplified audio section based on the 315-5684 amplifier chip instead of two separate LM324 op-amp chips. This in addition to some layout changes allowed Sega to reduce the size of the motherboard, cutting down production costs in the process.

Identifying these can be done by looking at the RF shielding through the bottom vents. If the shielding doesn't extend all the way across the width of the console, and does not have rectangular holes, it will be this revision. Sega used three different video encoders on the VA3, the CXA1145M, CXA1645 and the KA2195D. The encoder used can be roughly determined by the serial number; if it starts with "163" or "253", it is almost guaranteed to have a Samsung encoder. Serials starting with "743", "753" or "R43" are extremely likely to have a Sony encoder, with the latter two being more likely to have a CXA1645. Sound is universally improved on the VA3, but the PSG volume is reduced and the frequency response is lower at higher frequencies.

Most VA3s use the 315-5660 ASIC, but a few made use of the 315-5700 (Yamaha FF1004) which was also used in the Nomad handheld. As with the VA0 through VA1.8 boards, the ASIC pins for selecting region and video frequency are hardwired to 5V. However, region modding a VA3 is somewhat easier as Sega added an exposed pad for each of these pins, meaning a simple trace cut is all that is needed to isolate these pins.

Like the VA2 and VA2.3 consoles, this board revision is only found in US Genesis consoles.

VA4 (1996-1998, Asia, Brazil, Korea, North America and PAL)

US VA4 board revision.
Underside of a US VA4.

This was the final board revision for the model 2. It is especially notable in that it is the first Genesis console to use a GOAC (Genesis On A Chip) ASIC, specifically the Sega 315-5960 (Yamaha FJ3002). This incorporates all of the major components of the console (68000, Z80, VDP, YM3438, I/O controller, Z80 RAM) into a single chip, save for the main RAM, video RAM, video encoder and audio amplifier. Due to this change, the VA4 also consumes far less power than any other model 2. This is reflected by the position of the 7805 voltage regulator, which is now mounted horizontally on the motherboard and partially uses the top RF shield as a heat sink.

To identify one of these, look for two square cutouts on the shielding visible through the bottom vents. These will also have compatibility issues with both revisions of the 32X, which can be bypassed by loading 32X games from a flash cart instead of using original cartridges.

All US VA4s use the Sony CXA1645 encoder, which provides slightly improved composite video with less artifacts than its implementation on the VA3. RGB output has some noticeable jailbars. Most Asian and PAL VA4 boards instead feature a ROHM BH7236AF encoder, which is pin-compatible with the CXA1645. Curiously, Sega added the footprints for an internal RF modulator and its support circuitry, though no VA4 ever had these populated.

Both region and video frequency are set by 0 ohm jumpers for all VA4s. Region is selected by R31 (JP) or R32 (EN), and video frequency is controlled by R33 (50 Hz) or R34 (60 Hz).

Later VA4s in North America were sold under license by Majesco in 1998, who marketed the Genesis as a budget console. The build quality is largely identical to the earlier Sega-built examples.

Genesis 3

VA1 (1998)

VA1 motherboard

Essentially a highly condensed VA4 model 2 without an expansion port. Although it is not compatible with the 32X or Master System games, it can be modified for these due to it using the same 315-5960 GOAC as the VA4 model 2. One other major change is that stereo sound was omitted, and all audio goes through a single LM324 op-amp. The Sony CXA1645 video encoder was also carried over, providing decent quality composite video output and RGB.

VA2 (1998-1999)

VA2 motherboard

The VA2 uses a different GOAC, the 315-6123 (Yamaha FQ8002). This board also combined both the main RAM and video RAM into a single 128 KB SDRAM chip, leaving it, the video encoder and audio amplifier as the only remaining discrete ICs. 32X and Master System compatibility cannot be restored on this revision; only Game Genie and Virtua Racing compatibility issues are fixable on the VA2. The TAS instruction in the 68000 CPU was also fixed, which breaks compatibility with the game Gargoyles. Video output quality was not changed, but the audio is a little better in low frequencies, and does not hiss.

References

  1. ↑ http://gendev.spritesmind.net/forum/viewtopic.php?t=2922
  2. ↑ https://twitter.com/nukeykt/status/1718625118031839563#m
  3. ↑ https://www.youtube.com/watch?v=NQeTLohYzTg
  4. ↑ https://github.com/emu-russia/SEGAChips/tree/main/VDP
  5. ↑ https://law.justia.com/cases/federal/district-courts/FSupp/785/1392/2144879/
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