[cctalk] Rainbow H7842 PSU Fault

As some of you may be aware I am trying to find a fault in a Rainbow H7842 PSU. I am using Tony Duell's schematic from here http://www.bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf. I have been testing the Control Module by using a bench PSU to supply 15VDC to the input of the 7812 regulator (p2, PSU Sheet 1). My diagnosis shows that the control module is shutting down the PWM (p6, Control Module Sheet 2) because it is detecting an overcurrent in the -12V side (E3d on Control Module sheet 1, although I have determined that it is actually E3c). This seems to be because I measure a steady 0.6V on pin 6 of the transformer (p4, PSU Sheet 3). I just can't imagine where it might be coming from as the chopper won't be running. I had previously removed the transformer and there are no shorts between the pin 5-pin 6 winding and any of the other pins on the transformer. I checked all the DC outputs of the PSU when powering the 7812 from the bench, both on a working PSU and the non-working one. They are all at zero except the -12V output on the non-working PSU, which is +0.6V. But the voltage can't come out of nowhere. I am stumped and would appreciate any suggestions. Thanks Rob