According to the schematic - with the volume control all the way up(volume control center wiper at bottom end of antenna coil), you should basically be reading 6 ohms across your antenna terminal to ground(DC resistance of the lower half of the antenna transformer in parallel with 400 ohm volume control). With the volume all the way down, the volume controls middle wiper should be shorting the antenna post directly to ground. So if you're measuring 100 ohms at some point, that would seem suspect. Have you taken resistance measurements of the two windings of the antenna transformer? It would be completely reasonable for that ant coil and/or volume control to have been zapped by lightning at some point, although that would normally leave something open circuit, it could also leave a carbon track. You can try hooking your antenna directly to the top half of the antenna coil(1st RF grid). At that point the volume should get much louder.
