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Hi Keith,

Obviously my suggestions are too cryptically for you. I hesitate to continue since in my opinion you should understand the purpose of test measurements, to learn something and also to protect the equipment and yourself from damage. But let me try to clarify my previous suggestions.
Q1418 is the heart of the oscillator. Oscillation occurs because of the feedback from collector to base provided by the windings 6-7 and 8-9 of the transformer. The voltage and current amplitude in these windings is determined by average base current which has to come from the regulator circuit, Q1416 emitter. The oscillator produces an up-transformed alternating voltage in the HV winding between pins 23 and 5 which is half-wave rectified by CR1421 to give a negative HV at C1421 to C1424 and CRT cathode.
A fault could be that one of these HV caps (or even C1488) starts to leak at say 70 V. That would put extra load on the oscillator in its start-up phase and and prevent the amplitude to rise further.
Another fault could be that CR1421 starts to leak, with the same consequence.
Both these fault would show up of you externally feed TP1423 with a high enough negative voltage (scope disconnected from the power inlet of course). I simply use my 576 curve tracer for this purpose but you have to improvise something. A DC supply, preferably variable, would be needed, with + to scope ground and – to TP via a DMM. The DMM at say 200 V DC range. The DMM is 10 M (usually), the load on TP is about 30 M. So the DMM reading should be about 25% of the supply voltage and maintain that percentage when the supply voltage is increased. When the percentage starts to increase it indicates leakage somewhere, and you can also try to estimate how heavy that leakage is.
Of course this would eliminate only a leakage fault in the branch from CR1421.
A fault in the branche from pin 3 is not very likely, it’s low voltage.
That leaves a fault in the transformer itself (any winding could be involved) or in the HV multiplier.

Suppose you disconnect P1400. Then you can supply an AC voltage across the collector winding between pin 1 of P1400 socket and the fuse holder terminal. I use a sine wave function generator for this. The generator output is w.r.t. generator ground, so I connect that side to the fuse and the live side to mentioned pin 1. (Additionally the fuse side can be shorted to scope ground to prevent any floating voltage levels inside the scope.) Some people use an audio amplifier.
This way I can control the input frequency an voltage amplitude. I also monitor the generator output current and the TP voltage. With all this information I can check that the resonance frequency is about what it should be (maybe 50 kHz for a 465? I didn’t look it up) and also see if stange things happen if I increase the amplitude.

L1419 and C1418/C1419 mainly serve to prevent oscillator signals to enter 15 V elsewhere. I think that even without these C’s the oscillator should still work.

Albert