dark screen, but can hear it whirring and working

Hi!

I bought my husband a Macintosh Color Classic with tube and analog board, but no logic board.  He bought an “working” LC 575 Mystic Mac board off eBay. He installed it and turned it on, with the screen flashing an error sign, then it went dark.  You can hear it booting up, it whirls and beeps, but no screen.  He bought an original “working” recapped color classic logic board, but no luck.  Lots of info on recapping, etc, but no symptoms like his.  Do you know what problem this might be or whom we could ask?  Or where to send it for repair? Thanks