Hello...

The original Bios in the STV is a not eraseable "Maskrom".
So you can be lucky to have an eprom with maskrom compatible pinout on your board. What type (numbers) is it.??
Organisation is 256k x 16 (4 megabits)

I donīt know if i am allowed to send you a download link for a japanese bios, but you can also find it yourself by using any search engine with "Sega STV bios" .

regards
Andreas