Well I identified the failure, fixed it, and the disk pack is
formatting right now. Whew!
Another crapacitor -- looks like a tantalum, but it's only .01uF,
a dipped-looking bright blue axial part, about the size of a 1/8W
resistor. I think these were discussed recently; not necessarily
tants, but some short-lived process.
Anyways, the problem cap was on the read/write amp board. The bad
cap was on the -15V line. The shorted cap took out the 1-ohm, 1/4W
series resistor on the logic board (very obvious once I looked at
it; all the smoke what went out of it, and all the color-code
bands went brown).
There's only one chip (LM311) using -15, took that out, nope.
Started following copper, removing caps, finally saw one blue cap
that was... brown in the middle! Yup, shorted.
I replaced the bad one, and all the lifted caps, with ceramic
disks since I had 'em partly out anyways.
Had to take the head assembly dust cover off, to get the head
connectors plugged on, which mean clean-room again. Unnerving.
Still running diags, but no errors so far.
Man, there's hundreds of those little blue bastards in this
machine. I hope future shorts are so benign. Gulp.