I've recently gotten a second model 1 Sega CD as a fun project. It had, what I thought, some simple issues. Didn't read any discs, bad tray and crackling sound. Crackling sound was easily fixed by recapping the sub and main boards, disk tray is just a new belt (well I boiled the old one as a temporary fix, works fine till the new ones arrive).
However I can't seem to fix the not reading discs issues. I did check the laser, which is fine in my good system. Same with the mainboard. Only when I switch the sub-boards the known good system suddenly can't read discs and the new one suddenly can. So I vaguely remembered I had an old sub-board from fixing the known good system more than a decade ago. (apparently TR4 was broken and back than couldn't find a replacement so replaced to board) Just replaced TR4 and it worked, kind of. It had the exact same reading disk issues as well not giving PCM sound, weirdly enough only in the Japanese BIOS, but not in the American as known good one had an american board and the new one is Japanese. Maybe because the one giving PCM issues on the Japanese BIOS came from an American system as it works fine with the American BIOS, IDK.
To be more specific about the disc reading, the disc gets spun up and is attempted to be read only for it to fail and stop spinning and seem to not recognize the disc.
So anyone know what could cause laser issues caused by the sub-board? And to fix the other one also, PCM issues on the sub-board?
Edit: Found a clue, probably something in the power circuit. Using a different PSU instead of the trident style psu for the sega CD one board powers the system and reads discs (and thus fully functions), the other doesn't boot at all. Power going into the system with the trident looks fine however with a ~4.8V
Edit 2: for anyone in the future. Don't use a 7805 to replace the 5V regulator instead use the L4940V5, or some other some other low-dropout regulator.