Replacing failed powersupplies on qbus PDP-11s

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.