Recently I got a Mega CD 2 unit which had it's BIOS ROM violently removed by the previous owner in an attempt to install a Region Free BIOS. The damage is pretty bad, many pads and traces have been ripped out. At first I tried to recreate all the missing connections after soldering in a socket for the BIOS. By using the service manual and a second MCD2 to double check I tried my best to get everything right. However all I got when starting the MCD2 is the static background image, no Sega logo, no sign of life from the CD drive. From what I know, this means that the MCD2 is not booting properly. At this point I figured that there's probably a dodgy connection somewhere among those I didn't touch and started from scratch and rewired the entire ROM. But unfortunately that changed nothing, same static image. The second image is a drawing on how I wired the connector-side of the BIOS to the individual resistors. I am certain that all the connections I made are good, I tested each of them multiple times by now (MD connector to ROM, ROM to IC101, MD connector to IC101). The data lines VD0 - VD15 are on IC101 Pin 191 - 208 VA1 - VA16 on IC101 Pin 171 - 188 BROM on IC101 Pin 4 I am out of ideas on this one. Hopefully somebody with better understanding of the Mega CD (and it's schematics) can point me in the right direction and maybe have a look at the schematics to see if I'm missing anything. Maybe the BIOS is fine but there's something else wrong with this unit? The fuse was missing when I got it. But the board looks clean, I can't see any other damage (for example there's no corrosion on the IC101 pins which can cause the same problem).
it's generally a big gamble if you get anything that someone has already butchered if they have had it open and had no idea what they were doing they could have done pretty much anything to the internals.....good luck though and your work so far looks nice so hopefully it all pays off for you
The MEGA-CD will do that (boot but stop before showing the logo animation) if it has problem accessing the CD-ROM drive micro controller (CDC). It will only display the logo animation if all steps of initialization are cleared. Under normal circumstances, the CD drive CPU should start automatically (aka try to read discs) as long the CD unit has power. It doesn't require any initialization from the MEGA-CD 68k to do that. So make sure you can check if the drive has reset signal, if the SUB CPU can read the CDC status and if the CDC is trying to detect a disc (laser will do lens movement and turn laser LD on) if you have lid switch on closed position.
Thanks for your input, l_oliveira! I've managed to fix the MCD2 today, but there was a lot of luck involved. While checking the connections from CPU to DRAM again, I noticed that a small movement of the multimeter probe was enough to cut the connection of Pin 60 on the CPU (Data bus, D13). I've probed that Pin 4 or 5 times before but never noticed it. As it turns out, the previous owner put a lot of force on the CPU to lever out the BIOS. I've resoldered all the pins by that edge and it's back to life
