VAX 11/750 2.5V supply problems

I have been trying for some time to repair the 2.5V supply on an 11/750, without success. The symptoms are loud squealing which changes back and forth in pitch (in fact it sounds a bit like RTTY, only over a couple of pitches), OVERCURRENT and REG FAIL indicators on the status panel on the back of the machine. All the boards are pulled from the backplane and I?m using dummy loads on the supply. There?s no evidence of shorts on the chassis or on any of the supply outputs. I?m testing with the voltage sense wiring in place. The output electrolytics have been replaced on the 2 regulator boards and the big electrolytics on the primary side check out OK. I?ve measured 300V DC across the switching transistors on the primary side. I?ve not detected any problems with the electrolytics on the 30V supply and 2.5V supply but I have ordered replacements just to be on the safe side. I?ve checked over all the diodes and transistors with the Huntron tracker with no obvious evidence of any failures. So far all I?ve really been able to establish is that there?s no output on the ?secondary? side of the supply, that is, the 30V supply which drives the +/-5VB and 12V regulators, and the 2.5V main output of the supply. I think the REG FAIL lamp is a red herring because without the 30V supply active, REG FAIL would have to be lit if I understand the schematic correctly. I?m posting on the off-chance that someone has had this problem and has dealt with it. I?ve been looking at this on and off since July but haven?t made any headway. Thanks for any suggestions the group may be able to offer. By the way, the story of the machine and work done to date is at http://comley.us/browse.php?&action=show&artefactID=1051 -Dave