[Radio1950]

This normally happens because the modulator or RF PA (OSC in this case) has gone high source impedance, and the plate voltage of the OSC cannot be pushed high enough on the positive part of the modulating waveform.

Ensure heater volts are correct to all valves, and all are glowing merrily on high.

Try replacing the 6X5, or with two 1N4007s etc and a 330R 5W resistor.
This will remove doubt about the emission, and whether it was half or full wave rectification due to possible fault.

Replace 6J5s one by one.
The emission of V3 is high suspect.

You don't say which resistors you have tested or replaced.
There's only a few components.
So, ... test ALL resistors R1, R2, R3 etc.

Have a look for crook earth connection on OSC valve cathode.

If all tests OK, consider if the Phantom Repairer has been here, and wired a component incorrectly.

Wiggle all valve in their sockets whilst probing RF OP with CRO.

Do you have a service manual with expected volts on each valve pin?

Good luck.