DG Nova repair etc.

What do you mean? I think i did not get your answer. out to be a missing clock.Of course, yours may be soemthing different but you aregoing to need some schematics and a scope to track down.( I did mine without a schematic but I was lucky ).Dwight We already sighted most of the pdfs and schematics at bitsavers and simh. We have a scope and logic analyzer and some experience in component level repairs of electronics. This machine is just a bit harder to diagnose than for say our old VT100...the documentation is overwhelming and my brain has not adapted to it well atm ;) so i just hope to get somebody that has intimate knowledge of the machine.... The problem of the machine only running on single step and not on "run" may well be related to a missing clock, although i wonder if even single instruction stepping would work without clock since it also needs some form of clock to single step through one asm instruction...although i might be on the wrong train since there may well be more than one clock inside the machine.