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Mike, I cannot help thinking that the problems exist in the power supplies somehow.
It may be worth carefully checking over the wiring of all supplies and making sure the pre-regulator has sufficient headroom on its input. If this supply is not right it will affect the Anode and Screen test voltages. Also check that all 0V Ground connections are all linked up, the presence of a 50Hz waveform is strange and seems to suggest a missing ground somewhere.