Minimum Boot (Dreamcast)
In order for the dreamcast to boot, it requires only the motherboard, a working and spinning fan, and the 3.3V / 5V rails connected.
No front board, disc drive or 12V rail are required (the 12V rail is only needed for the disc drive to work, but not required to get you to BIOS menu).
The following is a list of things that need to be functional in general for your console to get to the boot menu.
- Working power board able to supply required current on 3.3V and 5V rail
- Connection between power supply and motherboard must be clean and solid
- The 3.3V rail should draw between 2.0A to 2.7A
- The 5V rail should draw 0.6A
- BIOS must be fully working, soldered well, and no torn traces
- CPU (SH-4) and PowerVR (GPU) must be fully working
Now power on the console, make sure the fan spins, and your TV should turn solid black. After about 5 seconds you should get as static Dreamcast logo. If so, all is good, the BIOS has loaded, and your GPU and CPU are ok.
If you get nothing (no black screen)
- Check AV cable works
- Power supply is outputting 3.3V and 5V still while connected and powered on
- Fan is connected and spinning
- AV connector port pins are solid (they can fracture off easily with pressure so perhaps reflow them)
- Check BIOS is being read (with oscilloscope on Ax and Dx lines)
If you get to static Dreamcast logo screen, it's time to test the rest of the console.
Re-install the disc drive and all components and test again. You should get full boot. If not, check for problems with the disc drive (hot swap between consoles perhaps) or check your connections.
A black screen, or intermittent boot to menu (sometimes boots, sometimes doesn't) is often the disc drive connection is poor. This is a very weak connector and even removing the disc drive a few times during service can be enough to prevent good contact and failed boot to menu.
The best test for that is to simply remove the disc drive.
If you get black screen only (no static Dreamcast logo) without a disc drive installed, check your BIOS is ok, and your CPU (SH-4) could be bad also. This is not common.