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The Dynaco FM-3 tuner uses what appears to be a Foster-Seeley discriminator with one diode reversed, or, if you prefer, a ratio detector without the third coil and stabilizing capacitor. The Dynaco marketing material and user manual uses the term 'discriminator' consistently throughout.

I don't find this variant discussed in the RDH, or Cook & Liffey, which are my only FM texts of enough depth to cover this sort of thing. Does it have a name? How does it work? And what are its salient properties?

Schematic (also here):

enter image description here

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Answering my own question here.

It is indeed a Foster-Seeley discriminator, by the arrangement of the coils and the inductive and capacitative coupling between them, which produces the characteristic voltage-difference AM, but the method of demodulating the AM is different by the reversal of one of the diodes.

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  • \$\begingroup\$ I don't really believe in doing this in general, but I'm marking this answer as correct in default of a better one. \$\endgroup\$ – user207421 Sep 24 '14 at 1:36
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Looks pretty similar to a later (postwar) version developed by Seeley, which he called a ratio detector:

enter image description here

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  • \$\begingroup\$ Except that it doesn't have C4. I did mention that in the question. C4 is a fundamental feature of the ratio detector. Without it there is no ratio to detect as far as I can see. \$\endgroup\$ – user207421 Jul 8 '14 at 6:16
  • \$\begingroup\$ @EJP couldn't C4 be regarded as part of the two series capacitors junctioning at point P. \$\endgroup\$ – Andy aka Jul 8 '14 at 7:29
  • \$\begingroup\$ @Andyaka Not unless they were a million times the value. C4 is typically 25uF and does two things: provides a constant DC voltage so that a voltage ratio can be detected when the phase shifts, and the time constant is chosen to also provide AM rejection. The two you mention are 47pF each. They are a capacitative centre-tap to ground the balanced discriminator. \$\endgroup\$ – user207421 Jul 8 '14 at 9:30
  • \$\begingroup\$ @EJP I see your point..the cutoff of the RC is 65kHz. \$\endgroup\$ – Spehro Pefhany Jul 8 '14 at 13:14

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