The power supply of my vax 3900 is making a weak squeaking sound

You turn it on, and it goes "squeak squeak squeak squeak"? Maybe the LED on the supply is faintly blinking at the same rate? This is the symptom if there isn't the required minimum power load in the Q-bus backplane. The power supply begins to start up, doesn't see enough load, and shuts down, and this repeats every second or so. For minimally configured systems there was a Q-bus card with a bunch of resistors to suck up some current. I think in the box, that one of the two power supplies does the odd-numbered slots, and the other does the even numbered slots. Loads must be distributed to cover both supplies and both supplies need their minimum load met. Sometimes even if you have the required minimum load the power supply will not start up with this symptom. Adding more load seems to help! Tim.