PDP 11/84 PSU wiring / differences

We were trying to find out why the left half of our PDP 11/84's backplane is dead earlier. Seems like it's not getting any power at all supplied to it. The system's got two power supplies in it - an H7202-KA which runs the right (working) side of the backplane, and an H7202-KB which runs the left side (and seems to be dead) Now, the left hand power supply is fed AC from a cable that plugs into a large mains filter on the left side of the cabinet, halfway up and midway between front and back of the chassis. AC is getting as far as the PSU, but there's nothing on the +5V output. There seem to be no fuses in the PSU itself, and no obvious sign of problem. Initial questions: 1) It seems that AC input is always available to the PSU with the power switches on though, so the PSU must be started via a signal over the ribbon cable that plugs into the PSU? 2) As well as AC input to the left hand PSU, there's also a pair of wires (yellow and green) which terminate in a plug up by the mains filter. There's nothing which obviously plugs into this - any ideas what it does? 3) What's the difference between the two supply types? The rightmost one is identical to the left, but with the addition of a second vertical board inside, and also runs the blower fan at the front of the machine. Is it just that it provides voltages other than +5V to the system? It's possible that we can source another supply in the near future, but I'm not yet convinced that the supply is actually dead - has anyone got a list of what the various lines connected to the supply actually do? cheers, Jules