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 More mods to my radio, an AM RF stage tuned input and tuned output
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 Return to top of page · Post #: 1 · Written at 5:31:11 AM on 2 October 2019.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 724

Yet more mods to that AM/FM radio I've modded recently. Once I added a second AM/FM tuning cap to my AM/FM radio (the one with the new tuned FM RF input), I figured, as it has AM sections, I could put its AM antenna variable cap to use. I built an RF amp to go in front of the AM converter stage. The existing AM ferrite rod antenna L5 LC circuit would stay as is, but its secondary winding would be disconnected from the AM converter transistor. And now it feeds the new AM RF amp stage. The AM RF transistor's collector feeds a new RF transformer, and its secondary now feeds the AM converter. I took an AM 455KHz IF transformer and removed its capacitor, to create the tuned transformer that will be placed between the RF amp and the converter. . The ferrite rod antenna's inductance measured to be 590uH, and I was able to adjust the full primary of the transformer to the same inductance. I also checked the AM antenna tuning cap C1A, it maxes at 150pF, and the new tuning cap C1R also is the same. So this new transformer and the tuning cap C1R should resonate on AM stations, once the trimmer C1T and transformer slug are tweaked. AM RF amp stages can be hard to tame, I ended up selecting a rather small emitter bypass cap of 100pF. I used small coax cables to feed this AM RF amp from the ferrite rod antenna, and another coax cable to feed the AM converter the AM RF amp's output. And I used one shield to deliver the AM switched positive supply to this amp, and the other shield the negative ground return. Another short wire connects this stage's RF ground to the tuning cap's RF ground, via a coupling cap, to keep the LC resonant currents local.



AM MW propagation was good last night, but I did hear a station from Syracuse (my college town!) WFBL 1390KHz before sunrise today. Never heard any station from Syracuse in New Jersey before!

Though I get birdies worse on the 2nd and third harmonics of the IF frequency (from the detector). Not surprising as the radio is more sensitive to signals and noise and stray from the detector.


 
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