Since you DO have the tech info . .don't berate your capabilities . . .you 'da man !
Upon perusing the units design we see no basic DC power supply and its electrolytic filter bank, since the unit is using the positive nodes of the power supply.
But on going with your good analytical info provided, do note that the accuracy of the meter circuitry is dependent upon the conversion of its incoming AC signal level ito a stable DC level to operate the meter proper.
I would say that after umpteen . .(to the power of 2) . . . years, that the C3 marked up electrolytic has diminished down into the few farads catagory.
With you then getting an erroneous meter reading you were adjusting the R16 AC line series dropping pot to the erroneous reading that you were actually getting.
Confirm if this is the situation, and while you gots the hood up, see if [Aqua-blue] marked up fuse lamp might have also been damaged.
Another confirming procedure would be to use AC meter to monitor the 5Y3 and 83's filaments supplys to see how close they are to 5 VAC WHEN the line set is at its mark . . .(not your MARK, its mark). hi hi
Thassitt . . . . .
Pee Ess . . .Be real caredfull with that thar' tube tester of yours . . . its rocket powered!
Notice at the VERY TOP-LEFT corner of the schematic page , theres a BLAST OFF !
73's de Edd