This afternoon I took the old HP-4191A apart. Yesterday in the middle of
some measurements it went pop-pop-pop, loud, like firecrackers. I ran
over and turned it off, upon which the popping stopped. An immense
amount of light-gray smoke wafted out and it had an odd stench to it.
The stench lingered for hours.
I can't find any source and upon powering it up the analyzer worked
fine. That was before I changed anything. As if it had repaired itself.
The only thing I could see is a crack in the plastic of the X-capacitor.
Snipped the thing out, measured, has almost 0.28uF capacitance (27% more
than stated) and no leakage current. There is another X-cap and two
Y-caps inside a canned IEC receptacle but those can't be the culprits
because that is on the line side of the power switch.
Is it possible that an X-cap self-heals to the point where you can
barely see a thing, yet release a serious plume of smoke? It is a Rifa
GPF-series film cap 0.22uF/250VAC with all kinds of agency logos on there.
Those foil capacitors can blow bits of confetti out the sides and still
work fairly well afterwards. The metal is so thin that it vaporizes
more than the insulator. You can call it self-healing but I wouldn't
trust it to last long with the innards exposed to air.