OK.
A bit more experimentation and another observation (but no real progress).
With no keyboard, just a mouse, the startup goes all the way to the normal screen with the menu bar and hard drive and Trash icons. However, the screen is locked up and the mouse will not move the cursor. The same result with several keyboards and mice which work just fine with any other ADB-capable computer I have. I thought the ADB drivers were in ROM.
I don't know if it makes any difference for the troubleshooting but this logic board has a socket-mount CPU and the mount looks to be original. I didn't know such an arrangement existed.
AichEss
A bit more experimentation and another observation (but no real progress).
With no keyboard, just a mouse, the startup goes all the way to the normal screen with the menu bar and hard drive and Trash icons. However, the screen is locked up and the mouse will not move the cursor. The same result with several keyboards and mice which work just fine with any other ADB-capable computer I have. I thought the ADB drivers were in ROM.
I don't know if it makes any difference for the troubleshooting but this logic board has a socket-mount CPU and the mount looks to be original. I didn't know such an arrangement existed.
AichEss