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Add AFC to FM table radio?

T

thomash85715

AK member
Banned
If my Zenith H724 is any indication, it's no myth that early FM sets drifted off frequency as they warmed up. Has anyone ever succeeded in adding a generic AFC circuit to one using a handful of resistors or caps and a switch? At first I thought it was just the dial string expanding as it got warm, but the AM is rock steady. My RCA tube manual has a generic ACDC AMFM radio circuit with switchable AFC that uses almost identical tubes as the zenith but I don't have zenith schematic to compare. Thank you. --Tom
 
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If your FM detector develops a positive voltage per frequency at its output then feed it through a pi-network of resistance of about 47K-100K-ohms in series with a 47K ohm resistor towards the oscillator tank circuit, with a capacitor to ground at the junction of those two resistors. For such capacitor arrange a trimmer cap of b/n 3 and 47 pF depending on the situation; you need to do a bit of empirical work--thus the importance of adjustability of capacitance here. Arrange a 1n34 diode to receive the detector voltage at its anode with the cathode going to RF ground. Hitch the return (normally going to ground) end of the trimmer cap to that junction. The other end of that trimmer capacitor should go to the LO tank circuit. If your discrim seems to operate in the opposite direction of V-vs-F then you simply reverse the polarity of that control diode.

The "capacitor" I have emboldened in the previous paragraph I had at a value of about 1-5 MFD, and started to decrease it, and noted a decrease in the typical distortion noted of FM on weak stations...Which propmpted me to experiment with this feeding back of some audio into the AFC circuit in an effort to reduce distortion. A kind of audio feedback loop--as applied to AFC... It worked well in many of my earlier FM tuners I built.... FWIW...

I did experiment with this principle of operation with my tube-type tuners, and not with later squalid-state ones, where I never got around to it.... :o
 
WOW! Thank you for proving that "ask and you shall receive" still works. All I have to do now is correlate the various circuit points you describe against the actual radio itself. I appreciate the concrete and clear instructions. I might be the only person with a working KARG tuner; it also drifts and will benefit from this.
--Tom
 
Try to find some schematics of similar radios with AFC. This should give you some ideas. I also have a Zenith radio that I have thought about adding AFC to. It's a nice bakelite clock radio with FM, but it drifts in the first 5-10 minutes.
 
Is your clock radio dark brown with a kind of square on each side that spills out past the main box? Mine is like that and it drifts just like the non-clock zenith table radio referenced in my original post. So if I succeed with one, I shall post results and kill many birds with one stone. I guess if anyone has a generic zenith 50s tube schematic to post, it would help a lot. Thank you.
--Tom
 
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