After further tests of the power supply to make sure that the 12v and 5v supplies are stable, I decided to install the CPU card and try to fire up the machine.  I'm pretty sure that I have got all the cables in the right places and the right ways round.  I originally used an old RJ11 telephone cable to attach the console panel of the PDP to my MMJ to DB9 adaptor, and then to my PC.  The RJ11 cable I have only contains 4 wires, so the 2 outer wires of the MMJ connector (which has 6 connections) are not being used, but they are the RTS and CTS signals and won't be needed for a simple console.  The RJ11 cable I've used is crossed over so the TX and RX lines are crossed, which is useful.

The first time I switched the power on, nothing happened.  So I flipped dip switch 1 on the front control panel, which I think is for the Line Time Clock.  It was just a hunch.  However, when I powered it on again I got this message:

KDJ11-D/S   4.55
Error, see troubleshooting section in Owner's manual for assistance
RAM    VPC=024454  PA=17604454  00000000/124776 <> 177776
 
KDJ11-D/S>

Pretty much what I expected, since the machine does not have any drives to boot from.  This is an encouraging start.  I'm on my way...