Yo yo yo HXC
About Six years ago I picked up a brand new boxed Japanese Sega MegaCD2 when I was in South Africa on a business trip.
Bouyed by the successful (relatively) switch mod of my Megadrive2 I've decided to bite the bullet and do the multibios mod on the MegaCD to enhance my setup.
If I had any other region CD2 then I wouldn't need to bother as the scdconv utility that is floating around the internet would allow me to rip my PAL games to ISOs and twiddle the header to change the region. Unfortunately there's no way to go from Euro to Japanese in the current version of scdconv, so that leaves me with the multibios mod as the only option.
Using the rather excellent guides provided by mmmonkey (http://www.mmmonkey.co.uk/console/sega/megacd.htm) and Arakon (http://arakon.dyndns.org/tutorial/bios.html) the mod should be fairly straight forward.
Bits
Couple of eproms ordered off of ebay $10
2 x SPDT sub-miniture slide switches from Maplin £1.24 each
1 x 40 pin DIL IC socket
Soldering equipment
Dremel Dril or equivalent
Part share in the Madbury family Willem EPROM blower and 16bit adapter currently with my dad 
Problems
Getting the case open was no problem at all and I was surprised to see my MegaCD2 did not feature a huge metal shield. I was expecting this as it was mentioned by the guide. Nevertheless I've had it up and running before and not had any problems, so maybe this was a cost cutting effort towards the end of the machine's lifespan?
Case open, it wasn't long before I hit my first snag...
The next step was to work out how to remove the existing bios chip from the PCB. There's probably a slick and professional tool for this job, but I don't have one of those. Arakon suggests cutting one row of legs with a dremmel and then working the chip up and down until the other side breaks off. After a lot (and I mean A LOT) of deliberation I eventually decided that I should try this approach. In retrospect I probably should have tried to snip the legs with some cutters as one slip of the cut off wheel later I'd taken a chunk out of the surface of the PCB. A quick check with the multimeter and an eye glass confirmed the worst. In my haste I'd sliced through one of the tiny tracks right next to the bios chip. The good news is that it's only one track and effecting a repair should be fairly straight forward; the track connects a via to one of the bios pins.
Once the pins were cut, working the chip up and down snapped the opposite legs (which are inaccesible as they sit under the socket that connects to the Megadrive). Removing the remains of the IC legs from the PCB will be a fairly simple affair using the soldering iron, pliers and a solder pump.
This is as far as I have got at the moment. I'll bore you even more as and when I get around to the next stage of the mod. Whether all this is worth it to play my massive library of 2 MegaCD games is questionable, but it's all part of the learning experience
I don't have a Mega CD, but always quite fancied trying this out.
I wouldn't use pliers to pull the remaining legs out, if they get jammed or the solder doesn't melt completely you might end up pulling some of the track away from the PCB. Arakon's site is down at the moment, but looking at mmmonkey's guide he bungs the IC socket on top of the existing bios, lifting one pin - is the MCD2 different to that?
Also, this tool should be able to convert Eur to Jpn MegaCD images:
http://www.retrodev.com/convscd.html
but the bios hack is more fun, and less hassle
Also sounds like you'd have grief with a lot of 50Hz optimised stuff, FMV mainly, if converting iso images and running them on an unmodded Jpn MegaCD. You would need the MD 50/60Hz and language switches to get some stuff working it sounds like. As the MCD checks console region that might not be possible without the MCD hack, although whether you could switch to 50Hz once a game has booted I'm not sure.
What MCD games you got?
This also looks really interesting:
http://www.retrodev.com/transfer.html
Homebrew uploader for a MCD equipped machine.
Hmm that software tool might do what I wanted to do. However it seems that it's based on scdconv, which couldn't convert from EU to JPN. I guess that's a conversion that no normal person would want to do right?
Too late now anyway, the BIOS is off the board! And you're right it's more fun (and ultimately more convenient).
mmmonkey's solution is to ram a socket over the old bios chip. I fancied something a bit neater than that. I think the primary reason he elected for that method is that the MegaCD1 has connections to the bios on both sides of the board, which makes soldering and desoldering a ball ache.
Incidentally I'm going to have a spare eprom at the end of this if anyone else is interested in trying this mod. I'll happily pop the multibios onto it too
The objective is to end up with a nice compact Megadrive + MegaCD combo that can play everything I can throw at it. The jewel (ahem) in the crown will be a 32X. I'll be covered for Sega 16bit gaming then
This also looks really interesting:
http://www.retrodev.com/transfer.html
Homebrew uploader for a MCD equipped machine.
That sounds really interesting and pretty cool
I have in my collection
Sylpheed (sp?) PAL
KEOI Flying Squadron (sp?) PAL
Frankly it's the latter game that I'd quite like to play. It's supposed to be an alright shooter
Update.
EPROM is programmed (Paper I'll give you my spare multibios next time we hook up). Took a couple of minutes using my dad's Willem programmer
Switches are mounted too. All that remains is to wire up the IC socket and solder it to the board. Probably get around to it this week. If it all goes to plan then I should have a multiregion Megadrive2 and MegaCD2 combo.
Still need to do the slot mod on the MD2. I've picked up a Japanese version of Super Monaco GP from a local indie for sizing. Doesn't look like you have to remove a lot of material from the slot to get the JPN carts to fit.
That just leaves the 32x to sort out. There's an interesting mod where you fit the innards of the 32x into the MD2 case. This seems to make a lot of sense, but I'm sure there must be some drawback that I've not considered. Maybe some of the MD carts won't work via the 32x (I remember reading something about virtua racing somewhere).
Well there's good news and then there's bad news.
The good news is that the machine boots and I can select the 3 different bios regions and the built in game (which is Columns) works flawlessly. What doesn't work is the bloody CD-ROM drive. The head moves in and out, so that seems to be working, but the disk simply won't spin up. It sounds like it's trying to spin, but refuses to work. Most frustrating
I will check my solder connections although thinking about it if the machine is booting and displaying the Bios graphics and sound correctly it should follow that it is working. Afterall the Bios is just a bunch of instructions and doesn't directly control the CD-ROM hardware (I guess).
The other possibility is that I'm not using a pukka power supply with it (old multivoltage thing). Not sure how sensitive the electronics are to voltage and current draw, but it's possible that my dodgy powersupply isn't helping matters.
This is all very frustrating. I could test the CD-ROM drive in another MegaCD if I can lay my hands on one. It's possible that it is simply faulty.
My Pc Engine CD-Rom is very twitchy about what power supply you use, it might well be that. Be worth trying the original with a step down.
What were the symptoms? Did the drive refuse to spin up or was it simply a case of read errors?
Digging out the psu means a trip into the loft. TBH I don't recall the PSU or what it looks like. I must still have it somewhere. It may even be in my Aunts loft still inside the MegaCD2 Box
. Annoyingly the JPN MegaCD2 has a different dc in jack to the PAL MD2, so I can't simply use the MD2 PSU with the MegaCD.
This is bloody irritating, you can imagine my dissapointment at not being able to play Silpheed and KEOI Flying Squadron, games I've had for about 5 years sitting on a shelf. So damn close to the perfect setup too
Read errors, though I think the Mega CD 2 would normally be more solid.