I picked up two ProFiles from an ex-Apple employee in San Jose today. Both required hearty thwacking to get them spinnings, but once they spun up they seemed to work on my Apple ///. They are not NOS like the ///, but I can copy files to them and they pass the "Verify Volume" test in System Utilities.
Well, OK, one passed. The other copied a bunch of files and then made a series of popping noises and emitted great clouds of acrid smoke. Fortunately it didn't kill anything in the ///; the computer's fine and the other ProFile still works.
I guess I need to open up the power supply and see what exploded. The controller board looks OK. I suppose I should also check the power supply in the ProFile that still works...
Turns out it was a 0.1 uf capacitor, looks like a Mylar type. Should be an easy fix if it didn't take out anything else.
Sounds like the filter capacitor - a common failure on power supplies from that era. Easy enough to replace.