Sequential circuits samplers

>>Any other suggestions?<<

yeah- the series regulators that provide the 5V & 12V rails into the
rest of the instrument. 78xx & 79xx devices- these are the things you
see fastened to the back of the case inside, probably with what looks
like toothpaste (thermally conductive paste, in fact) around them.
these things (78xx, 79xx) frequently fail in older gear.

you need to get a tech in there, with a voltmeter. your p2k may be
perfectly alright once these are replaced & working again. but you've
reached, with the greatest of respect, the limit of what you can do
without a voltmeter & some component-replacement chops.

hth-
duncan.