It looks to me like one or more of the stepper motor phases are shorted (or their drivers are shorted) by garbage signals that come over the 50-pin Shugart interface from the IMSAI when it's powered on. Garbage signals are a fact of life, and the circuitry on the drives shouldn't respond in such an awful way to them. Two things to do: 1. Ohm out the stepper motors looking for shorted phases. 2. Check the driver transistors on the 851's. If any are hot when the problem occurs, it's almost certainly related. It's not impossible that the circuitry that drives the driver transistors is screwed up, too. It wouldn't surprise me if the foldback protection in your floppy power supply folds all the outputs back simultaneously. This certainly seems likely based on the voltages you see. If you can unhook the 24V output from the power supply (this is what runs the stepper motors) and try again, this might be useful. -- Tim Shoppa Email: shoppa(a)trailing-edge.com Trailing Edge Technology WWW: http://www.trailing-edge.com/ 7328 Bradley Blvd Voice: 301-767-5917 Bethesda, MD, USA 20817 Fax: 301-767-5927