Is there a modem or an ADB Startup card installed in the modem bay and is it seated properly?

Have you checked that all the internal ribbon cables are seated properly, if not, you might try the unplugging/replugging routine.

Have you got an older Duo so you can test it with a SCSI HDD, a different LCD & related Hardware?

Dunno, maybe there's a catalog of "chimes of death" online as reference material . . . sorry to hear about your troubles.