I just recently bought a Dreamcast with some extra for $15. The Dreamcast fires up, and the disk spins. It starts loading the menu, but never goes past setting the date before the screen goes black, and there is no longer video. The power stays on, and sometimes when I open it the disk is still spinning. Not sure if it's junk or a simple fix. I just wanted to test a couple games on it.
Either the thermal fuse is faunty (heatpipes model) or Fan is not spinning (newer model with no heatpipes). There's a protection mechanism that cuts the clock frequency generator if either the thermal fuse blows or the fan is not spinning. The reason for the fault varies with the model but the result is the same:
CPU and GPU stops processing, making the video disappear from the screen.