All the PS voltages check ok in operation (idling). No beep, but I’ve not yet tried disconnecting everything reasonably disconnectable. (HD, all but 1 RAM stick, etc.)
If no keyboard, should the machine still “beep”? I really wonder about this combination keyboard + mouse socket.
Check the net for beep codes. It varies from bios to bios. Ancient stuff is difficult to deal with, that is why I flamed you with my initial response.(sorry about that). In a previous lifetime, I supported research equipment. Sometimes the vendor went out of business and the vendors that swooped in failed as well. We did HD imaging and scoured the internet for identical PCs as backups. I wish you luck on solving your problem..