PDP-11/23 H786 PSU Fault Finding

Can anyone offer any guidance on an 11/23 switchmode power supply I'm attempting to repair ? It failed in use and started blowing fuses. I diagnosed a failed bridge rectifier in the primary side and replaced it. That resolved the problem with the fuses blowing. However, when I monitor the +12V line, I can see it peak around +11.5V on power up and then it slowly sags (off-load), all the way down to +4.5V over a period of a few minutes. With a small dummy load connected the 12V line drops down to 4.5V in a matter of less than a second. I can see why it stops at 4.5V; there's a diode, normally reverse biased when the voltages are correct, that's connected between +12 and +5. The +5V line is rock solid at 5.1V. The power supply has a single pair of transistors driving the primary side of T2, and +5 works so I am inclined to think that the problem is somewhere on the secondary side. I pulled the pass transistor (Q3) for the 12V line and checked it with the Huntron Tracker; it appears to be serviceable. The schematic for this is on p75 of http://bitsavers.org/pdf/dec/qbus/MP00740_1123_schem_Oct81.pdf. Is there a common cause for 'sag' in output voltages in switchmode supplies ? I am wondering if this is regulation issue or a problem with drive to the pass transistor ? Thanks, -Dave