I'm preparing to briefly touch upon the ANALOG vs. DIGITAL debate (oh so tired a discourse), guess which side I'm going to take.
So, as determined last time I had a look at the Tek 556, it would behoove me to focus only on circuit elements that are upstream of the power switch, narrowing my attention to the mains filter. Easiest test was to read for continuity across the AC legs of the power jack, at the highest range I watched my DMM read lower and lower resistances. Digital meters send a pulse out one probe and weigh the voltage drop vs current delivered into the completed circuit to determine resistance between the leads. In this case the pulse was charging the capacitor meant to shunt high frequency line noise, so as the cap slowly charged the perceived voltage drop was less and less, delivering the false reading of a lower resistance. Clearly, sometimes easiest is not best, or even good.
Time to start sniffing around for a decent analog meter.
While it has been over a week, this hole retains traces of the odor of failure.
The inlet cavity of the Sprague filter also appears to have seen heat in the past. While this could be patina that evolved over the past 45 years, I don't trust this part, so it will go.
I highly doubt I can find another one of these with ease, and if I did I'm betting it would be of roughly the same age, meaning it would be "vintage" and there'd likely be some sort of premium on top of the decrepit part. No thanks.
Seems the easiest path before me is to build in a modern power conditioner in the space previously occupied by the large can of the Sprague. I toyed, for some fraction of a moment, with the idea of building the circuit inside the Sprague can, but the PITA factor coupled with not knowing what's inside the can (1966 was within the polychlorinated biphenyl era) sobered me up.
Not reusing the Sprague component will demand I change my hardware a little in order to mount the voltage badge, I'm thinking nuts.
Spot the high voltage wire!