[morzh]

Switch. Have you checked the switch?

There are very few weak links:
1. The Tube (checked)
2. The tube panel.
3. The coils.
4. The switch that commutates the coils.


Now, to check the switch you do not need to trace it: simply measure appopriate coils's continuities from the beginning of the circuit to the very end with the switch in a partiular position. If the switch is not closing properly you will see the open or large resistance where it should be a low coil's resistance.