John,
Thanks for the information. I had not had the CRT problem - yet. There's always plenty of time for additional problems to accrue!
Not to be snarky, but did your filament transformer have at least a 5,000 Volt rating? I know that the CRT heater voltage is elevated quite a bit to prevent H-K breakdown, and that is why I have been saving just such a 6.3 Volt transformer for over thirty years.
As for my 310A troubles, I had wondered if the insulating varnish might be breaking down, although I have no transformer information or specifications that could confirm this by DC resistance readings of the windings.
The confounding thing is that this came on so suddenly and to both of my 310As to the point that I would put the heat theory in second place more because of the infrequency of use of both of these 'scopes. Perhaps, though, infrequent use was the cause of the trouble by leading to a high inrush current to form those damned electrolytic capacitors (I am gradually switching to film types where possible. ). I had blown a metal film 10 ohm resistor, R601, at the input of the -150 volt power supply two or three years ago and had attributed it to a poor choice of resistor type in that position in the circuit, but perhaps it was more than just the type. The carbon film types that I put in are still working.
PMF