[Nuvistor]

Is this the 900 volt line that is dropping? The 900v circuit uses a voltage doubler, the centre tap of the transformer winding is not connected to anything.
If there is no overheating when the voltage drops it would indicate a high impeadance supply, the 900 volt line is on the Rec A board on the circuit I have downloaded. There are very few components in the 900v doubler circuit and a diode and capacitor located off the board, circuit designation 200.

I have never worked on one of theses, just looking at the circuit.

The voltages are very high, extreme caution is required in servicing this rig.