Before jumping into the FM a couple of points to make.
I let the post speak for itself.
The ratio detector is also used in commercial radios. We can look at them more but a search should find the theory behind them.
The set passes AM and FM through the same amp. The top transformer feeds a ratio detector. The lower transformer feeds the AM detector.
Notice the bottom secondary feeds the two diodes in the EBF80. The line marked Audio is going to the volume pot. The FM audio comes off the top secondary and down to the same line. Since AM works and FM does not we could do some theoretical analysis about the common components.
I'll save that until some test are run.
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