Maybe this is better in the tech forum. This forum did have a discussion of 69-72 Zenith sets with a bad VDR in the HV pulse regulator circuit.
This 1971 Sylvania 25V set is low hours and ready to look great but the 6LR6 grid measures -30 volts, Sams shows -70. The HV control varies the G1 from -19 to -30. Scoping the sawtooth at HOT grid, it looks good but only 120v p-p instead of 220 v. G2 measures 230vdc when it needs to be 160v. HOT Plate glows a bit but not blazing orange
This set was "serviced" before and the VDR was replaced, along with a new fly, tripler, tubes and some wiring mistakes I corrected. I thought that was it but I have no way of knowing what VDR is in there now, Sams lists only an original Sylvania Part # 38-29959-1 and describes the VDR rating as 1ma@1050 volts. No Workman # was listed.
I will scan a schematic later but the VDR "limits to 1300v p-p) a pulse fed to it from flyback winding via a 560 pf/2kV capacitor. I measure 25v P-p there.
This limits drive to the HOT so much that its in conduction.