I got the SDE working Saturday morning. The 16v on the output of the 6v regulator was caused by the trace for its ground connection breaking off. The traces on this board were very delicate, I had to rebuild 3 of them. I also replaced the 3 tantalum caps just to be safe.
Thankfully, the problem in the power supply did not cause any other apparent faults in the rest of the machine. The regulators and the transformer don't get very hot now either.
It works perfectly and is just as quirky and strange as I remember 10 years ago. I've since lost one of the screws for the rack ears, and a red plastic panel that goes over the display... there also seems to be an acoustically audible buzzing sound (the box itself makes a noise) that appeared after the machine was left on for a few hours. Is this a cause for concern?
Next up on the bench I have a Korg Poly Six with a bad voice, but I'll give you all a break from me for a month or so...
Thank you all for your help.
Christian