Testing H7864 (MicroVAX II) PSU With No Load

Some of you may recall I have a faulty H7864 PSU, which failed a while ago with a loud pop, but no obvious physical damage. I replaced the blown transistor (on the primary side of the large transformer), but when I power it on, the transistor does not switch and there is no output, so clearly there is still a problem somewhere. I have been spending some time drawing schematics for almost the whole thing. I am now at the point where I intend to compare it with a working one, probing each one side by side, to see up to where it appears to be working. It would be awkward to have two dummy loads, just for lack of suitable equipment. Does anyone know if it is safe to run these PSUs with no load? Would my testing be valid without a load? Thanks Rob