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 Adding a tuned LC circuit to the input side of the FM RF amp in a solid state table mantle radio
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 Return to top of page · Post #: 1 · Written at 9:38:50 AM on 27 September 2019.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 759

Many solid state AM-FM radios and cheaper stereo sets did not use a tuned LC circuit on the input from the antenna to the FM RF amp stage. Many such sets used those plastic cube miniature 4 gang tuning caps (2 for FM, 2 for AM). And as far as I can tell, none were made with 3 FM. But I figured out how to add a tuned circuit to the FM input. Find a 2 gang FM only plastic cube tuning cap (rare, so maybe just get another AM FM one and ignore the FM osc and the AMs). And build or modify a coil with a tap to be the L of the new LC circuit. Okay, now you have to do a mechanical linkage of some sort from the new cap to the existing tuning cap. 2 methods shown in the picture below:

The modified schematic: (do a right click on it and select "view image" to see it big)

The main difficulty in adding a tuned LC circuit on the input to the FM RF amp is to mechanically ouple it to the rest of the radio's tuning mechanism. Initially, I mounted the new cap coaxially with the xisting tuning cap. And I used a long screw to act as a pin attached to the new cap that in turn fits into a new hole in the tuning wheel mounted to the existing tuning cap. So when the radio is tuned, this pin also makes the new cap also get tuned. A kludge, but it worked reasonably well. I then decided on a ore elegant method, mounting the new cap to the existing piece of circuit board I mounted the tuning nob to. And using a pair of same size tuning pulleys and dial string to couple it to the main existing uning cap.

Looking at the bottom of the "new RF amp" schematic above, you can see the new FM front end RF amp input LC circuit. With the new variable tuning cap "C1K" (with its own trimmer C1M, and the coil LN are adjusted using a grid dip oscillator (GDO) to have it track the existing tuning cap. Tune in a station near the bottom of the band, tune the grid dip oscillator to "jam" that station, and adjust rimmer C1M to get the new LC circuit to resonate (which is when the GDO dips). Do that with another station midway in the band, and another near the top of the band. and iterate a couple of times. And adjust the inductance of coil LN a little, by spreading or squeezing the turns. The GDO's dial may be off, but jamming a station tells you you have the desired frequency dialed up on the GDO

FM station signal strengths increased on the FM band after I was done.

 Return to top of page · Post #: 2 · Written at 1:13:54 PM on 27 September 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 746

How cool!
Next you need to couple up another gang and then insert some triode valves between the gangs!
Ah, bugger, that's been done before!!
Sort of back to the future.
Anyway crummy jokes aside, there is a good lesson in all that.
I wish more members would post up what may seem to be trivial mods to things.
That's getting back to the basics of how things work.
Good stuff.

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