Thank you for your input. I have a Voltcraft VC870 DMM rated 750V for both AC and DC. For more accurate component testing, I have a DE-5000 LCR meter. Given the unclear state of the power supply modifications by the previous owner, my current plan is to check the power supply on PC149 first and make sure the +24V, -24V, +30V and +105V produce a clean output. As can be seen in
this post, the +24 had some suspicious ripple on it, the last time I checked. Then, when all power rails are OK, I will advance to the HV section.
One further thing I noticed while examining the scope today was that the power lamp is lighting up again. I find this strange, since the power lamp has its own separate 6.5V transformer winding. The only other thing connected there is the 50Hz line trigger input of the horizontal section. So how could it stop working in connection with this fault and then lit up again? It should be always on as soon as the transformer gets power. It does not seem to be a case of randomly connecting dangling filament in the lamp, since it does not react to the orientation I hold the scope.