[Mondial]

Consider that there should never be any positive voltage to ground on the AVC tube plate. I doubt there is anything wrong with C20 as it is a mica with very little voltage across it

The AVC tube cathode is always negative with respect to ground and the plate will vary from zero to some negative voltage depending on the volume control setting. Measuring with respect to chassis ground, the cathode will always be more or less negative. Unfortunately the schematic does not tell what the negative voltage should be but I would take a guess it would be around - 30 or -40 V at the choke L17.