Additional AVC circuit for Solid State AM radio

[wa2ise]

Had a cheapie AM/FM clock radio, the kind made in Hong Kong "Lloyds", that tended to distort on very strong AM stations. Not worth much, so I decided to hack the AM circuit some. I decided to try using a JFET in its "triode" low level ohmic region, to vary its resistance via the voltage on the gate. And I connected this JFET across the secondary winding on the AM ferrite rod antenna, right in front of the AM converter circuit. One end of this winding is grounded to the radio ground, and the other end feeds a coupling cap to the converter transistor base. The JFET connected to ground and this winding, and when the resistance of the FET drops enough, I get attenuation of the station tuned in. And I do that by getting a gate control voltage from an AVC level shifting transistor that is fed by the set's AVC line. In this case I had to invert the direction of the AVC line voltage, to get attenuation when a strong station is tuned in. And to level shift to get the sweet spot of attenuation to less attenuation to none in the range the radio AVC works in. A useful side effect of this additional attenuation via resistive loading of the antenna circuit is to reduce the antenna's LC circuit Q, thus making the radio's passband a little wider on strong stations, so you'll hear higher frequency audio from the station. The below schematic shows an example radio without the FM circuits, my mods don't involve the FM anyway:



Yeah, I realize that this isn't just a "plug and play" schematic, you'll have to play around with whatever JFETs you can lay hands on. I got mine out of a junked VGA monitor circuit board. A small one, not the one in the monitor's power supply (which is likely to have too much capacitance).