VT100 PSU smelling.

Yes of course. If I have a PSU where I have doubts about the function and there is no schematic I spend a few hours trace out the relevant portions. Normally this is not a big chore. Then there are of course complex PSU and simple ones. I would not like the idea of tracing out the VAX-11/750 PSU, but a VT100 PSU is very small circuit on the primary side. OK. No I am not using onIy a variac. That wouldn't work very well. I usually try to identify the control circuit and power it independently of the switch circuit. That is usually no problem. Some PSU has a simple voltage divider from mains input other like the VT100 has a normal small iron core transformer to supply the control circuit supply. Many of the PSUs I work with have been not been powered up for decades. I think it is a good practice to take it slowly in the beginning. Observing it so that everything behaves well in the circuit using a very light load. Verify that the switch control circuit and regulation circuit behaves well and that the switching transistor waveforms looks good. Nothing is getting abnormally hot or so. Also be a little bit kind to all the electrolytics in the circuit etc. That is why I do this and I think that this method have served me well. Then of course I also use a protection transformer and very often some kind of input current limitation, not just a variac and a bench supply. /Mattis