I have built this supply long ago, it sits in old AT case, but today I found problem, that on negative (-) supply I am having -40V DC against wall plug ground. It's not problem in general, but if you want to supply something that is for example connected to computer through USB it'll blow the fuse if not USB port of computer..
What could be causing this problem?
Serial communications between external devices often have these same sort of grounding problems which are overlooked mainly because the data is transfered reliably. Your better off using an opto-isolator.