-11/40's in general only have one of those large H742x suppplies in a rack. The documentation and prints all show only a single one - in fact, the 11/40 power harness (which is specific to the KD11-A backplane, at the CPU end) can only attach to one. The KB11 machines (-11/45 and /70) use two, but their harness has provision for two. I don't know of a DEC document that lists the difference between the H742 and H7420, but my CHWiki page for them: https://gunkies.org/wiki/H742_Power_Supply gives what I think is a pretty good list; any errors or missing details would be appreciated. That's going to be a hassle, replacing the main harness! Especially since production of the 8-pin MATE-N-LOK connector shells used to interface to the H744/etc 'bricks' - part numbers here: https://gunkies.org/wiki/DEC_standard_modular_regulators are out of production, although some vendors have residual stocks. Hoard them while they last! The "PDP-11/40, -11/35 (21 inch chassis) system manual" (EK-11040-TM-002): http://www.bitsavers.org/pdf/dec/pdp11/1140/1140_SystemManual.pdf has pretty good coverage of the harness; the back end of Chapter 6 covers it in detail. That's a lot easier to understand than the FMPS: http://www.bitsavers.org/pdf/dec/pdp11/1140/PDP-1140_System_Engr_Drawings_R… so read that before you tackle the prints. Note that there are two different kinds of harness, depending on whether the machine has MM11-L (-15V) or MM11-U (+20V) core memory. Unless you're planning on using one of those, you can probably ignore that, though. Any questions, ask here right off; we have a lot of expertise! :-) ('My' first -11 - as in, one I was in charge of - was a -11/40. Fond mempries!) Noel