21-08-2019, 12:26 PM
Not sure I can agree with Lawrence here. Ignore the FM bit and think in terms of a simple crystal set.
Normally, one side of the tuned circuit and one headphone lead go to earth and the other side of the tuned circuit goes through the diode to the other side of the headphones. It must still work if you transpose the diode and the tuned circuit and that is what is happening here, it is just that it uses a thermionic diode in place of a germanium diode (or crystal and cat's whisker).
There will be no AM IF signals when the set is switched to FM, so the diode will not be conducting so, therefore, the AM circuitry will not load the FM detector and the 300k resistor is sufficiently high enough to ensure there is no undue loading from the ratio detector when the set is on AM.
Normally, one side of the tuned circuit and one headphone lead go to earth and the other side of the tuned circuit goes through the diode to the other side of the headphones. It must still work if you transpose the diode and the tuned circuit and that is what is happening here, it is just that it uses a thermionic diode in place of a germanium diode (or crystal and cat's whisker).
There will be no AM IF signals when the set is switched to FM, so the diode will not be conducting so, therefore, the AM circuitry will not load the FM detector and the 300k resistor is sufficiently high enough to ensure there is no undue loading from the ratio detector when the set is on AM.
