Replacing failed powersupplies on qbus PDP-11s

Chuck Guzis wrote: I believe in that discussion he's referring to objects in their pure form: when he says SMPS he means just the DC-to-DC converter; not what we colloquially refer to as a SMPS, which is really a conventional AC-DC converter (rectifier & filter) followed by a switch-mode DC-DC converter. As I would understand it, the problem with adding the EMI filter to the front of the SM DC-DC converter is that the converter actually draws current from the DC source at a frequency corresponding to the switch-mode frequency, so the current through an input EMI filter is nothing like DC, and you can get AC interaction between the filter and the DC-DC converter. The EMI filter becomes the resonant circuit, and negative resistance characteristics appearing at the the input of the DC-DC converter turn the combination into an oscillator. I believe his discussion is intended to be more of pertinence to DC distribution-bus systems such as in aircraft or telecom (note he refers in a couple of places to 28V distribution). I don't believe he is discussing the situation of adding an EMI filter to the front of a typical 120/240VAC input SMPS. In such supplies the 'lytic filter caps of the AC-DC converter at the beginning of the supply reduce the current-variation frequency to 50/100/60/120Hz (unless there is inadequate filtering), which is not a problem for the EMI filters. (Of course, these supplies generally do include built-in EMI filters at the front).