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 Post subject: More mods to my radio, an AM RF stage tuned input and output
PostPosted: Oct Tue 01, 2019 9:42 pm 

Joined: Aug Mon 16, 2010 7:01 pm
Posts: 276
Location: USA
Now that I have a second AMFM tuning cap in this radio, I figured I could build an AM RF amp 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. 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 transfornmer 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.


This diagram also shows the JFET AVC assist and received bandwidth circuits. These help keep the strong stations from distorting.

AM MW propagation was good just before sunrise today (Oct 1, 2019), but I didn't expect to hear a station from Syracuse (my college town!) WFBL 1390KHz, but I did! "The Dinosaur". 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.

 Post subject: Re: More mods to my radio, an AM RF stage tuned input and ou
PostPosted: Oct Wed 02, 2019 4:33 am 
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Joined: Feb Sun 05, 2012 5:12 pm
Posts: 1028
Location: North Syracuse NY 13212
WFBL used to be a powerhouse, but they moved and downsized their power.
I used to have a crystal set driving a speaker.
When I was stationed in Fort Devens MA, I could get WFBL and WWVA in Wheeling West Virginia.


Life is too short to be serious all the time.

 Post subject: Re: More mods to my radio, an AM RF stage tuned input and ou
PostPosted: Oct Wed 02, 2019 1:16 pm 

Joined: Jan Thu 01, 1970 1:00 am
Posts: 18047
Location: Utica, NY 13502 (USA)
WFBL is still 5 kW but their directional antenna patterns may be different than they were at their original location.


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