H7140 PDP11 PSU

I took the plunge and put in the CPU modules (M7133 and M7134) into the PDP11/24. The machine powered up and the DC ON light came on, suggesting voltages were in tolerance. So I connected a terminal and got an ODT prompt. I then tried to run a basic diagnostic (using the ODT command 165000G). It came back to the prompt and I was just entering another command when I heard a vague click and the machine powered off. I cannot power it on now. Previously after connecting it to the mains I would hear a slight click, but I don't even hear that now and turning the key will not even start the fans running. However power is getting to the big capacitors because I can see that with my multimeter, so it is not the external fuse or the circuit breaker I think. Presumably some internal fuse has gone, I will leave it to discharge and take a look tomorrow to see if I can find an internal fuse. Any other ideas as to what this might mean? It was going so well.. Regards Rob