I have an Apple //c ROM3 (which I upgraded just for the diagnostic ability).
I started out with a bad RAM indicated in position low 7, replaced it, then next was position low 0, and then high 6.
After replacing high 6 RAM, now diagnostics runs continous loop with small color blocks showing, and then a brief flicker just when I am eagerly hoping for the message System OK. But instead it seems to start over.
What is this telling me?
Is the keyboard connected?
That was it! I had the computer disassembled on the bench and as you surmised, the keyboard was not attached. Fully assembled, it responds with System OK.
Thanks!