With the power control, main transformer and fan assembly operational the next module in the power supply is the 15VDC regulator. This mounts on the other side of the power control box and is powered from the 28VAC output of the main transformer.
As well as 15VDC, the module also provides AC and DC Low signals and a 50hz signal for the real time clock on the DL-11W serial board.
There are two electrolytic capacitors on the board that needed testing before I was willing to connect it to the main transformer.
I built the capacitor test jig described by on http://dustyoldcomputers.com/pdp12/pdp12/restore1.html using junkbox parts.
Combined with 3 batteries saved from the UPS at work I was able to test and both capacitors on the 15VDC regulator board and they seem to be serviceable.