The AM Forum

I have modded my valiant with this circuit: http://amfone.net/ECSound/Valiant%20Audio%20Mods%205-2003.jpg  It works great on 75 and 40.  I was having a QSO with a W4 last night on 20M and I was getting RF feedback.  Any suggestions to stop it without killing the audio quality.  I also get feedback on 15 and 10.  I can hear a squeel from the mod tranny when it does it

thanks

Tom - AB3FL

Tom,

I am not sure if your using external audio processing driving the 600 ohm input or not.  If you are unplug it and see if you still get RF into the audio when you turn the AF gain pot up. 

Assuming that your just plugging a mic into the 600 ohm input, then I would suggest some RF protection on the input.  Some small ferrite beads slid on the input line right inside the mic jack is a good place to kill it.  You should also place 100 pf or so to ground on the tube side of the RF choke.  This will form an LC filter to kill the RF getting into your rig.  On high impedance inputs you can get away with a series resistor around 4.7K or so in place of the choke but the resistor approach does not work well with a 600 ohm input impedance. 

I am not sure anything about your antenna for 20M.  If your using balanced feeders, you may want to check the RF currents in your feeders to see if they are equal.  If they are not equal you will have feed line radiation since the line does not appear balanced.  Unfortunately these balance issues get tougher as you go up in frequency.  As always its good to eliminate the source of the problem if your able to. 

Joe, W3GMS

I think I found it.  The trigger switch I made to key transmitter is not bypassed/choked properly and when I bring it close to the mic, I get the feedback.    :o


Tom - AB3FL

I usually bypass Mike PTT lines with a .1 Disc ceramic just inside the RF box.  John, K1DEU

Then don't do that, Tom!!! ;D ;D

John has the right idea!
Shielded twisted pair on my button helped greatly too!