how come my am / fm tube radio works on am side but not fm side?
i have an Arvin 480 tfm am/fm tube radio from 1950 that the am side works perfectly with the built in am antenna and has no hum. but when i switch it to fm mode it has a faint hum coming from the speaker and doesnt change with volume, no static, music or anything, just a hum and it just stays the same. all the tubes light up and i even switched around the ones with the same rating in the radio but it still does the same hum. i connected a wire to the fm antenna screw to make sure it wasnt just not recieving anything but it didnt change. im just confused why the radio works perfectly on the am side and the fm side has nothing but a faint hum. any ideas?